LZW compression
You are encouraged to solve this task according to the task description, using any language you may know.
The Lempel-Ziv-Welch (LZW) algorithm provides loss-less data compression.
You can read a complete description of it in the Wikipedia article on the subject. It was patented, but it entered the public domain in 2004.
11l
F compress(uncompressed)
V dict_size = 256
V dictionary = Dict((0 .< dict_size).map(i -> (String(Char(code' i)), i)))
V w = ‘’
[Int] result
L(c) uncompressed
V wc = w‘’c
I wc C dictionary
w = wc
E
result.append(dictionary[w])
dictionary[wc] = dict_size
dict_size++
w = c
I !w.empty
result.append(dictionary[w])
R result
F decompress([Int] &compressed)
V dict_size = 256
V dictionary = Dict((0 .< dict_size).map(i -> (i, String(Char(code' i)))))
V result = ‘’
V w = String(Char(code' compressed.pop(0)))
result ‘’= w
L(k) compressed
V entry = ‘’
I k C dictionary
entry = dictionary[k]
E I k == dict_size
entry = w‘’w[0]
E
exit(‘Bad compressed k: ’k)
result ‘’= entry
dictionary[dict_size] = w‘’entry[0]
dict_size++
w = entry
R result
V compressed = compress(‘TOBEORNOTTOBEORTOBEORNOT’)
print(compressed)
print(decompress(&compressed))Ada
lzw.ads:
package LZW is
MAX_CODE : constant := 4095;
type Codes is new Natural range 0 .. MAX_CODE;
type Compressed_Data is array (Positive range <>) of Codes;
function Compress (Cleartext : in String) return Compressed_Data;
function Decompress (Data : in Compressed_Data) return String;
end LZW;
lzw.adb:
with Ada.Containers.Ordered_Maps;
with Ada.Strings.Unbounded;
package body LZW is
package UStrings renames Ada.Strings.Unbounded;
use type UStrings.Unbounded_String;
--------------
-- Compress --
--------------
function Compress (Cleartext : in String) return Compressed_Data is
-- translate String to Code-ID
package String_To_Code is new Ada.Containers.Ordered_Maps (
Key_Type => UStrings.Unbounded_String,
Element_Type => Codes);
Dictionary : String_To_Code.Map;
-- Next unused Code-ID
Next_Entry : Codes := 256;
-- maximum same length as input, compression ratio always >=1.0
Result : Compressed_Data (1 .. Cleartext'Length);
-- position for next Code-ID
Result_Index : Natural := 1;
-- current and next input string
Current_Word : UStrings.Unbounded_String :=
UStrings.Null_Unbounded_String;
Next_Word : UStrings.Unbounded_String :=
UStrings.Null_Unbounded_String;
begin
-- initialize Dictionary
for C in Character loop
String_To_Code.Insert
(Dictionary,
UStrings.Null_Unbounded_String & C,
Character'Pos (C));
end loop;
for Index in Cleartext'Range loop
-- add character to current word
Next_Word := Current_Word & Cleartext (Index);
if String_To_Code.Contains (Dictionary, Next_Word) then
-- already in dictionary, continue with next character
Current_Word := Next_Word;
else
-- insert code for current word to result
Result (Result_Index) :=
String_To_Code.Element (Dictionary, Current_Word);
Result_Index := Result_Index + 1;
-- add new Code to Dictionary
String_To_Code.Insert (Dictionary, Next_Word, Next_Entry);
Next_Entry := Next_Entry + 1;
-- reset current word to one character
Current_Word := UStrings.Null_Unbounded_String &
Cleartext (Index);
end if;
end loop;
-- Last word was not entered
Result (Result_Index) :=
String_To_Code.Element (Dictionary, Current_Word);
-- return correct array size
return Result (1 .. Result_Index);
end Compress;
----------------
-- Decompress --
----------------
function Decompress (Data : in Compressed_Data) return String is
-- translate Code-ID to String
type Code_To_String is array (Codes) of UStrings.Unbounded_String;
Dictionary : Code_To_String;
-- next unused Code-ID
Next_Entry : Codes := 256;
-- initialize resulting string as empty string
Result : UStrings.Unbounded_String := UStrings.Null_Unbounded_String;
Next_Code : Codes;
-- first code has to be in dictionary
Last_Code : Codes := Data (1);
-- suffix appended to last string for new dictionary entry
Suffix : Character;
begin
-- initialize Dictionary
for C in Character loop
Dictionary (Codes (Character'Pos (C))) :=
UStrings.Null_Unbounded_String & C;
end loop;
-- output first Code-ID
UStrings.Append (Result, Dictionary (Last_Code));
for Index in 2 .. Data'Last loop
Next_Code := Data (Index);
if Next_Code <= Next_Entry then
-- next Code-ID already in dictionary -> append first char
Suffix := UStrings.Element (Dictionary (Next_Code), 1);
else
-- next Code-ID not in dictionary -> use char from last ID
Suffix := UStrings.Element (Dictionary (Last_Code), 1);
end if;
-- expand the dictionary
Dictionary (Next_Entry) := Dictionary (Last_Code) & Suffix;
Next_Entry := Next_Entry + 1;
-- output the current Code-ID to result
UStrings.Append (Result, Dictionary (Next_Code));
Last_Code := Next_Code;
end loop;
-- return String
return UStrings.To_String (Result);
end Decompress;
end LZW;
test.adb:
with LZW;
with Ada.Text_IO;
procedure Test is
package Text_IO renames Ada.Text_IO;
package Code_IO is new Ada.Text_IO.Integer_IO (LZW.Codes);
Test_Data : constant LZW.Compressed_Data :=
LZW.Compress ("TOBEORNOTTOBEORTOBEORNOT");
begin
for Index in Test_Data'Range loop
Code_IO.Put (Test_Data (Index), 0);
Text_IO.Put (" ");
end loop;
Text_IO.New_Line;
declare
Cleartext : constant String := LZW.Decompress (Test_Data);
begin
Text_IO.Put_Line (Cleartext);
end;
end Test;
Applesoft BASIC
0 PLAIN$ = "TOBEORNOTTOBEORTOBEORNOT"
10 GOSUB 200"ENCODE PLAIN$
20 FOR I = 1 TO LEN (ENCODE$) STEP 2
30 PRINT S$ ASC ( MID$ (ENCODE$,I)) + 256 * ASC ( MID$ (ENCODE$,I + 1));
40 LET S$ = " "
50 NEXT
60 PRINT
70 GOSUB 300"DECODE ENCODE$
80 PRINT PLAIN$;
90 END
100 FOR C = 0 TO 1E9 STEP 0
110 IF I > S THEN RETURN
120 FOR D = 1 TO L - 1
130 IF W$ < > DICT$(D) THEN NEXT D
140 IF D > = L THEN RETURN
150 LET I = I + 1
160 LET W$ = W$ + MID$ (PLAIN$,I,1)
170 LET C = D
180 NEXT C
190 RETURN
REM ENCODE PLAIN$ RETURN ENCODE$
200 IF NOT DI THEN DIM DICT$(4095)
210 FOR I = 0 TO 255:DICT$(I) = CHR$ (I): NEXT
220 LET DI = 1 : L = I : S = LEN (PLAIN$):ENCODE$ = ""
230 LET W$ = LEFT$ (PLAIN$,1)
240 FOR I = 1 TO 1E9 STEP 0
250 GOSUB 100
260 LET DICT$(L) = W$:L = L + 1:W$ = RIGHT$ (W$,1)
270 LET C% = C / 256:ENCODE$ = ENCODE$ + CHR$ (C - C% * 256) + CHR$ (C%)
280 IF I < = S THEN NEXT I
290 RETURN
REM DECODE ENCODE$ RETURN PLAIN$
300 IF NOT DI THEN DIM DICT$(4095)
310 FOR I = 0 TO 255:DICT$(I) = CHR$ (I): NEXT
320 LET DI = 1 : L = I
330 FOR I = L TO 4095:DICT$(I) = "": NEXT
340 LET C = ASC (ENCODE$) + 256 * ASC ( MID$ (ENCODE$,2))
350 LET W$ = DICT$(C)
360 LET PLAIN$ = W$
370 LET S = LEN (ENCODE$)
380 IF S < 4 THEN RETURN
400 FOR I = 3 TO S STEP 2
410 LET C = ASC ( MID$ (ENCODE$,I)) + 256 * ASC ( MID$ (ENCODE$,I + 1))
420 IF C < L THEN T$ = DICT$(C)
430 IF C > = L THEN T$ = W$ + LEFT$ (W$,1)
440 LET PLAIN$ = PLAIN$ + T$
450 LET DICT$(L) = W$ + LEFT$ (T$,1)
460 LET L = L + 1
470 LET W$ = T$
480 NEXT
490 RETURN
- Output:
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
AWK
copy the following to standard input
==comp
TOBEORNOTTOBEORTOBEORNOT
To be or not to be that is the question!
"There is nothing permanent except change." --- Heraclitus [540 -- 475 BCE]
==decomp
84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263
84,111,32,98,101,32,111,114,32,110,111,116,32,116,257,259,268,104,97,267,105,115,272,260,113,117,101,115,116,105,111,110,33
34,84,104,101,114,101,32,105,115,32,110,111,116,104,105,110,103,32,112,259,109,97,110,101,110,116,32,101,120,99,101,112,281,99,104,277,103,101,46,34,32,296,45,298,296,32,72,259,97,99,108,105,116,117,264,32,91,53,52,48,32,299,52,55,53,32,66,67,69,93# ported from Python
BEGIN {
is_comp = 0
is_decomp = 0
}
{
if ($0 == "") next
if ($1 == "==comp") {
is_comp = 1
is_decomp = 0
print "\ncompressing..."
next
}
if ($1 == "==decomp") {
is_comp = 0
is_decomp = 1
print "\ndecompressing..."
next
}
if (is_comp) print compress($0)
if (is_decomp) print decompress($0)
}
function compress(str, dict_size, i, dictionary, w, result, len, uncompressed, c, wc ) {
dict_size = 256
for (i = 0; i <= dict_size; i++) dictionary[chr(i)] = i
w = ""
result = ""
len = split(str, uncompressed, "")
for (i = 1; i <= len; i++) {
c = uncompressed[i]
wc = w c
if (wc in dictionary) w = wc
else {
result = result "," dictionary[w]
dictionary[wc] = dict_size++
w = c
}
}
if (length(w)) result = result "," dictionary[w]
return substr(result,2)
return "[" substr(result,2) "]"
}
function decompress(str) {
dict_size = 256
for (i = 0; i <= dict_size; i++) dictionary[i] = chr(i)
result = ""
len = split(str, compressed, ",")
w = chr(compressed[1])
result = result w
for (i = 2; i <= len; i++) {
k = compressed[i]
if (k in dictionary) entry = dictionary[k]
else if (k == dict_size) entry = w substr(w,1,1)
else { entry = "*" }
result = result entry
dictionary[dict_size++] = w substr(entry,1,1)
w = entry
}
return result
}
function chr(c) {
return sprintf("%c", c + 0)
}
Arturo
compress: function [str][
dict: #[]
loop 0..255 'i -> dict\[to :char i]: i
w: ""
result: new []
loop str 'c [
wc: w ++ c
if? key? dict wc -> w: wc
else [
'result ++ dict\[w]
dict\[wc]: size dict
w: to :string c
]
]
if 0 < size w -> 'result ++ dict\[w]
return result
]
decompress: function [compressed][
dict: #[]
arr: new compressed
loop 0..255 'i -> dict\[i]: to :string to :char i
w: dict\[first arr]
remove 'arr .index 0
result: w
loop arr 'k [
entry: ""
if? key? dict k -> entry: dict\[k]
else [
if? k = size dict -> entry: w ++ first w
else -> panic ~"Error with compressed: |k|"
]
'result ++ entry
dict\[size dict]: w ++ first entry
w: entry
]
return result
]
compressed: compress "TOBEORNOTTOBEORTOBEORNOT"
print "Compressed:"
print compressed
print ""
decompressed: decompress compressed
print "Decompressed:"
print decompressed
- Output:
Compressed: 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 Decompressed: TOBEORNOTOBEORNOTOOBEORTOOBEORNOT
BaCon
CONST lzw_data$ = "TOBEORNOTTOBEORTOBEORNOT"
PRINT "LZWData: ", lzw_data$
encoded$ = Encode_LZW$(lzw_data$)
PRINT "Encoded: ", encoded$
PRINT "Decoded: ", Decode_LZW$(encoded$)
'----------------------------------------------------------
FUNCTION Encode_LZW$(sample$)
LOCAL dict ASSOC int
LOCAL ch$, buf$, result$
LOCAL nr, x
FOR nr = 0 TO 255
dict(CHR$(nr)) = nr
NEXT
FOR x = 1 TO LEN(sample$)
ch$ = MID$(sample$, x, 1)
IF dict(buf$ & ch$) THEN
buf$ = buf$ & ch$
ELSE
result$ = APPEND$(result$, 0, STR$(dict(buf$)))
dict(buf$ & ch$) = nr
INCR nr
buf$ = ch$
END IF
NEXT
result$ = APPEND$(result$, 0, STR$(dict(buf$)))
RETURN result$
END FUNCTION
'----------------------------------------------------------
FUNCTION Decode_LZW$(sample$)
LOCAL list$ ASSOC STRING
LOCAL old$, ch$, x$, out$, result$
LOCAL nr
FOR nr = 0 TO 255
list$(STR$(nr)) = CHR$(nr)
NEXT
old$ = TOKEN$(sample$, 1)
ch$ = list$(old$)
result$ = ch$
FOR x$ IN LAST$(sample$, 1)
IF NOT(LEN(list$(x$))) THEN
out$ = list$(old$)
out$ = out$ & ch$
ELSE
out$ = list$(x$)
END IF
result$ = result$ & out$
ch$ = LEFT$(out$, 1)
list$(STR$(nr)) = list$(old$) & ch$
INCR nr
old$ = x$
NEXT
RETURN result$
END FUNCTION- Output:
LZWData: TOBEORNOTTOBEORTOBEORNOT Encoded: 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 Decoded: TOBEORNOTTOBEORTOBEORNOT
BBC BASIC
Uses fixed bit-width (16 bits) and initial dictionary size = 256.
plaintext$ = "TOBEORNOTTOBEORTOBEORNOT"
encodeLZW$ = FNencodeLZW(plaintext$)
FOR i% = 1 TO LEN(encodeLZW$) STEP 2
PRINT ; ASCMID$(encodeLZW$,i%) + 256*ASCMID$(encodeLZW$,i%+1) " " ;
NEXT
PRINT ' FNdecodeLZW(encodeLZW$)
END
DEF FNencodeLZW(i$)
LOCAL c%, d%, i%, l%, o$, w$, dict$()
DIM dict$(4095)
FOR i% = 0 TO 255 : dict$(i%) = CHR$(i%) : NEXT
l% = i%
i% = 1
w$ = LEFT$(i$,1)
REPEAT
d% = 0
REPEAT
c% = d%
IF i% > LEN(i$) EXIT REPEAT
FOR d% = 1 TO l%-1
IF w$ = dict$(d%) EXIT FOR
NEXT d%
IF d% < l% i% += 1 : w$ += MID$(i$, i%, 1)
UNTIL d% >= l%
dict$(l%) = w$ : l% += 1 : w$ = RIGHT$(w$)
o$ += CHR$(c% MOD 256) + CHR$(c% DIV 256)
UNTIL i% >= LEN(i$)
= o$
DEF FNdecodeLZW(i$)
LOCAL c%, i%, l%, o$, t$, w$, dict$()
DIM dict$(4095)
FOR i% = 0 TO 255 : dict$(i%) = CHR$(i%) : NEXT
l% = i%
c% = ASC(i$) + 256*ASCMID$(i$,2)
w$ = dict$(c%)
o$ = w$
IF LEN(i$) < 4 THEN = o$
FOR i% = 3 TO LEN(i$) STEP 2
c% = ASCMID$(i$,i%) + 256*ASCMID$(i$,i%+1)
IF c% < l% t$ = dict$(c%) ELSE t$ = w$ + LEFT$(w$,1)
o$ += t$
dict$(l%) = w$ + LEFT$(t$,1)
l% += 1
w$ = t$
NEXT
= o$
- Output:
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
C
LZW encoder/decoder. Using variable bit length from 9 to up to 15. Encoder needs to know max allow bits, decoder doesn't. Code 256 for clear table, 257 for end of data, everything else are either byte values (<256) or code values.
WARNING: This code appears to have come from a GIF codec that has been modified to meet the requirements of this page, provided that the decoder works with the encoder to produce correct output. For writing GIF files the write_bits subroutine is wrong for Little Endian systems (it may be wrong for Big Endian as well.) The encoder also increases the number of bits in the variable length GIF-LZW after the N-2 code, whereas this must be done after N-1 to produce a working GIF file (just looking at the encoder, it's easy to see how this mistake could be made.)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
/* -------- aux stuff ---------- */
void* mem_alloc(size_t item_size, size_t n_item)
{
size_t *x = calloc(1, sizeof(size_t)*2 + n_item * item_size);
x[0] = item_size;
x[1] = n_item;
return x + 2;
}
void* mem_extend(void *m, size_t new_n)
{
size_t *x = (size_t*)m - 2;
x = realloc(x, sizeof(size_t) * 2 + *x * new_n);
if (new_n > x[1])
memset((char*)(x + 2) + x[0] * x[1], 0, x[0] * (new_n - x[1]));
x[1] = new_n;
return x + 2;
}
inline void _clear(void *m)
{
size_t *x = (size_t*)m - 2;
memset(m, 0, x[0] * x[1]);
}
#define _new(type, n) mem_alloc(sizeof(type), n)
#define _del(m) { free((size_t*)(m) - 2); m = 0; }
#define _len(m) *((size_t*)m - 1)
#define _setsize(m, n) m = mem_extend(m, n)
#define _extend(m) m = mem_extend(m, _len(m) * 2)
/* ----------- LZW stuff -------------- */
typedef uint8_t byte;
typedef uint16_t ushort;
#define M_CLR 256 /* clear table marker */
#define M_EOD 257 /* end-of-data marker */
#define M_NEW 258 /* new code index */
/* encode and decode dictionary structures.
for encoding, entry at code index is a list of indices that follow current one,
i.e. if code 97 is 'a', code 387 is 'ab', and code 1022 is 'abc',
then dict[97].next['b'] = 387, dict[387].next['c'] = 1022, etc. */
typedef struct {
ushort next[256];
} lzw_enc_t;
/* for decoding, dictionary contains index of whatever prefix index plus trailing
byte. i.e. like previous example,
dict[1022] = { c: 'c', prev: 387 },
dict[387] = { c: 'b', prev: 97 },
dict[97] = { c: 'a', prev: 0 }
the "back" element is used for temporarily chaining indices when resolving
a code to bytes
*/
typedef struct {
ushort prev, back;
byte c;
} lzw_dec_t;
byte* lzw_encode(byte *in, int max_bits)
{
int len = _len(in), bits = 9, next_shift = 512;
ushort code, c, nc, next_code = M_NEW;
lzw_enc_t *d = _new(lzw_enc_t, 512);
if (max_bits > 15) max_bits = 15;
if (max_bits < 9 ) max_bits = 12;
byte *out = _new(ushort, 4);
int out_len = 0, o_bits = 0;
uint32_t tmp = 0;
inline void write_bits(ushort x) {
tmp = (tmp << bits) | x;
o_bits += bits;
if (_len(out) <= out_len) _extend(out);
while (o_bits >= 8) {
o_bits -= 8;
out[out_len++] = tmp >> o_bits;
tmp &= (1 << o_bits) - 1;
}
}
//write_bits(M_CLR);
for (code = *(in++); --len; ) {
c = *(in++);
if ((nc = d[code].next[c]))
code = nc;
else {
write_bits(code);
nc = d[code].next[c] = next_code++;
code = c;
}
/* next new code would be too long for current table */
if (next_code == next_shift) {
/* either reset table back to 9 bits */
if (++bits > max_bits) {
/* table clear marker must occur before bit reset */
write_bits(M_CLR);
bits = 9;
next_shift = 512;
next_code = M_NEW;
_clear(d);
} else /* or extend table */
_setsize(d, next_shift *= 2);
}
}
write_bits(code);
write_bits(M_EOD);
if (tmp) write_bits(tmp);
_del(d);
_setsize(out, out_len);
return out;
}
byte* lzw_decode(byte *in)
{
byte *out = _new(byte, 4);
int out_len = 0;
inline void write_out(byte c)
{
while (out_len >= _len(out)) _extend(out);
out[out_len++] = c;
}
lzw_dec_t *d = _new(lzw_dec_t, 512);
int len, j, next_shift = 512, bits = 9, n_bits = 0;
ushort code, c, t, next_code = M_NEW;
uint32_t tmp = 0;
inline void get_code() {
while(n_bits < bits) {
if (len > 0) {
len --;
tmp = (tmp << 8) | *(in++);
n_bits += 8;
} else {
tmp = tmp << (bits - n_bits);
n_bits = bits;
}
}
n_bits -= bits;
code = tmp >> n_bits;
tmp &= (1 << n_bits) - 1;
}
inline void clear_table() {
_clear(d);
for (j = 0; j < 256; j++) d[j].c = j;
next_code = M_NEW;
next_shift = 512;
bits = 9;
};
clear_table(); /* in case encoded bits didn't start with M_CLR */
for (len = _len(in); len;) {
get_code();
if (code == M_EOD) break;
if (code == M_CLR) {
clear_table();
continue;
}
if (code >= next_code) {
fprintf(stderr, "Bad sequence\n");
_del(out);
goto bail;
}
d[next_code].prev = c = code;
while (c > 255) {
t = d[c].prev; d[t].back = c; c = t;
}
d[next_code - 1].c = c;
while (d[c].back) {
write_out(d[c].c);
t = d[c].back; d[c].back = 0; c = t;
}
write_out(d[c].c);
if (++next_code >= next_shift) {
if (++bits > 16) {
/* if input was correct, we'd have hit M_CLR before this */
fprintf(stderr, "Too many bits\n");
_del(out);
goto bail;
}
_setsize(d, next_shift *= 2);
}
}
/* might be ok, so just whine, don't be drastic */
if (code != M_EOD) fputs("Bits did not end in EOD\n", stderr);
_setsize(out, out_len);
bail: _del(d);
return out;
}
int main()
{
int i, fd = open("unixdict.txt", O_RDONLY);
if (fd == -1) {
fprintf(stderr, "Can't read file\n");
return 1;
};
struct stat st;
fstat(fd, &st);
byte *in = _new(char, st.st_size);
read(fd, in, st.st_size);
_setsize(in, st.st_size);
close(fd);
printf("input size: %d\n", _len(in));
byte *enc = lzw_encode(in, 9);
printf("encoded size: %d\n", _len(enc));
byte *dec = lzw_decode(enc);
printf("decoded size: %d\n", _len(dec));
for (i = 0; i < _len(dec); i++)
if (dec[i] != in[i]) {
printf("bad decode at %d\n", i);
break;
}
if (i == _len(dec)) printf("Decoded ok\n");
_del(in);
_del(enc);
_del(dec);
return 0;
}
C#
using System;
using System.Collections.Generic;
using System.Text;
namespace LZW
{
public class Program
{
public static void Main(string[] args)
{
List<int> compressed = Compress("TOBEORNOTTOBEORTOBEORNOT");
Console.WriteLine(string.Join(", ", compressed));
string decompressed = Decompress(compressed);
Console.WriteLine(decompressed);
}
public static List<int> Compress(string uncompressed)
{
// build the dictionary
Dictionary<string, int> dictionary = new Dictionary<string, int>();
for (int i = 0; i < 256; i++)
dictionary.Add(((char)i).ToString(), i);
string w = string.Empty;
List<int> compressed = new List<int>();
foreach (char c in uncompressed)
{
string wc = w + c;
if (dictionary.ContainsKey(wc))
{
w = wc;
}
else
{
// write w to output
compressed.Add(dictionary[w]);
// wc is a new sequence; add it to the dictionary
dictionary.Add(wc, dictionary.Count);
w = c.ToString();
}
}
// write remaining output if necessary
if (!string.IsNullOrEmpty(w))
compressed.Add(dictionary[w]);
return compressed;
}
public static string Decompress(List<int> compressed)
{
// build the dictionary
Dictionary<int, string> dictionary = new Dictionary<int, string>();
for (int i = 0; i < 256; i++)
dictionary.Add(i, ((char)i).ToString());
string w = dictionary[compressed[0]];
compressed.RemoveAt(0);
StringBuilder decompressed = new StringBuilder(w);
foreach (int k in compressed)
{
string entry = null;
if (dictionary.ContainsKey(k))
entry = dictionary[k];
else if (k == dictionary.Count)
entry = w + w[0];
decompressed.Append(entry);
// new sequence; add it to the dictionary
dictionary.Add(dictionary.Count, w + entry[0]);
w = entry;
}
return decompressed.ToString();
}
}
}- Output:
84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263 TOBEORNOTTOBEORTOBEORNOT
C++
#include <string>
#include <map>
// Compress a string to a list of output symbols.
// The result will be written to the output iterator
// starting at "result"; the final iterator is returned.
template <typename Iterator>
Iterator compress(const std::string &uncompressed, Iterator result) {
// Build the dictionary.
int dictSize = 256;
std::map<std::string,int> dictionary;
for (int i = 0; i < 256; i++)
dictionary[std::string(1, i)] = i;
std::string w;
for (std::string::const_iterator it = uncompressed.begin();
it != uncompressed.end(); ++it) {
char c = *it;
std::string wc = w + c;
if (dictionary.count(wc))
w = wc;
else {
*result++ = dictionary[w];
// Add wc to the dictionary.
dictionary[wc] = dictSize++;
w = std::string(1, c);
}
}
// Output the code for w.
if (!w.empty())
*result++ = dictionary[w];
return result;
}
// Decompress a list of output ks to a string.
// "begin" and "end" must form a valid range of ints
template <typename Iterator>
std::string decompress(Iterator begin, Iterator end) {
// Build the dictionary.
int dictSize = 256;
std::map<int,std::string> dictionary;
for (int i = 0; i < 256; i++)
dictionary[i] = std::string(1, i);
std::string w(1, *begin++);
std::string result = w;
std::string entry;
for ( ; begin != end; begin++) {
int k = *begin;
if (dictionary.count(k))
entry = dictionary[k];
else if (k == dictSize)
entry = w + w[0];
else
throw "Bad compressed k";
result += entry;
// Add w+entry[0] to the dictionary.
dictionary[dictSize++] = w + entry[0];
w = entry;
}
return result;
}
#include <iostream>
#include <iterator>
#include <vector>
int main() {
std::vector<int> compressed;
compress("TOBEORNOTTOBEORTOBEORNOT", std::back_inserter(compressed));
copy(compressed.begin(), compressed.end(), std::ostream_iterator<int>(std::cout, ", "));
std::cout << std::endl;
std::string decompressed = decompress(compressed.begin(), compressed.end());
std::cout << decompressed << std::endl;
return 0;
}
Clojure
(defn make-dict []
(let [vals (range 0 256)]
(zipmap (map (comp #'list #'char) vals) vals)))
(defn compress [#^String text]
(loop [t (seq text)
r '()
w '()
dict (make-dict)
s 256]
(let [c (first t)]
(if c
(let [wc (cons c w)]
(if (get dict wc)
(recur (rest t) r wc dict s)
(recur (rest t) (cons (get dict w) r) (list c) (assoc dict wc s) (inc s))))
(reverse (if w (cons (get dict w) r) r))))))
(compress "TOBEORNOTTOBEORTOBEORNOT")
- Output:
(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)
CoffeeScript
This only does the encoding step for now.
lzw = (s) ->
dct = {} # map substrings to codes between 256 and 4096
stream = [] # array of compression results
# initialize basic ASCII characters
for code_num in [0..255]
c = String.fromCharCode(code_num)
dct[c] = code_num
code_num = 256
i = 0
while i < s.length
# Find word and new_word
# word = longest substr already encountered, or next character
# new_word = word plus next character, a new substr to encode
word = ''
j = i
while j < s.length
new_word = word + s[j]
break if !dct[new_word]
word = new_word
j += 1
# stream out the code for the substring
stream.push dct[word]
# build up our encoding dictionary
if code_num < 4096
dct[new_word] = code_num
code_num += 1
# advance thru the string
i += word.length
stream
console.log lzw "TOBEORNOTTOBEORTOBEORNOT"
- Output:
> coffee lzw.coffee [ 84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263 ]
Common Lisp
This version is based upon the Perl one. It doesn't contain mixed type data at the cost of being more consy. It includes vector operation routines, since using VECTOR-PUSH-APPEND reallocates the whole vector with each call.
The Babel library is required to convert octet vectors to strings.
Lisp strings can contain characters out of the ASCII/latin1 character set, including the whole Unicode range in them.
The exact encoding used is dependent upon the user's locale (LC_CTYPE on Unix).
(declaim (ftype (function (vector vector &optional fixnum fixnum) vector)
vector-append))
(defun vector-append (old new &optional (start2 0) end2)
(declare (optimize (speed 3) (safety 0) (debug 0)))
(prog1 old
(let* ((old-fill (fill-pointer old))
(new-fill (+ old-fill (length new))))
(when (> new-fill (array-dimension old 0))
(adjust-array old (* 4 new-fill)))
(setf (fill-pointer old) new-fill)
(replace old new :start1 old-fill :start2 start2 :end2 end2))))
(declaim (ftype (function (vector t) vector) vector-append1))
(defun vector-append1 (old new)
(prog1 old
(let* ((old-fill (fill-pointer old))
(new-fill (1+ old-fill)))
(when (> new-fill (array-dimension old 0))
(adjust-array old (* 4 new-fill)))
(setf (fill-pointer old) new-fill)
(setf (aref old old-fill) new))))
(declaim (ftype (function (&optional t) vector) make-empty-vector))
(defun make-empty-vector (&optional (element-type t))
(make-array 0 :element-type element-type :fill-pointer 0 :adjustable t))
(declaim (ftype (function (t &optional t) vector) make-vector-with-elt))
(defun make-vector-with-elt (elt &optional (element-type t))
(make-array 1 :element-type element-type
:fill-pointer 1
:adjustable t
:initial-element elt))
(declaim (ftype (function (vector t) vector) vector-append1-new))
(defun vector-append1-new (old new)
(vector-append1 (vector-append (make-empty-vector 'octet) old)
new))
(declaim (ftype (function (vector vector) vector) vector-append-new))
(defun vector-append-new (old new)
(vector-append (vector-append (make-empty-vector 'octet) old)
new))
(deftype octet () '(unsigned-byte 8))
(declaim (ftype (function () hash-table) build-dictionary))
(defun build-dictionary ()
(let ((dictionary (make-hash-table :test #'equalp)))
(loop for i below 256
do (let ((vec (make-vector-with-elt i 'octet)))
(setf (gethash vec dictionary) vec)))
dictionary))
(declaim (ftype (function ((vector octet)) (vector octet))
lzw-compress-octets))
(defun lzw-compress-octets (octets)
(declare (optimize (speed 3) (safety 0) (debug 0)))
(loop with dictionary-size of-type fixnum = 256
with w = (make-empty-vector 'octet)
with result = (make-empty-vector 't)
with dictionary = (build-dictionary)
for c across octets
for wc = (vector-append1-new w c)
if (gethash wc dictionary) do (setq w wc)
else do
(vector-append result (gethash w dictionary))
(setf (gethash wc dictionary)
(make-vector-with-elt dictionary-size))
(incf dictionary-size)
(setq w (make-vector-with-elt c 'octet))
finally (unless (zerop (length (the (vector octet) w)))
(vector-append result (gethash w dictionary)))
(return result)))
(declaim (ftype (function (vector) (vector octet)) lzw-decompress))
(defun #1=lzw-decompress (octets)
(declare (optimize (speed 3) (safety 0) (debug 0)))
(when (zerop (length octets))
(return-from #1# (make-empty-vector 'octet)))
(loop with dictionary-size = 256
with dictionary = (build-dictionary)
with result = (make-vector-with-elt (aref octets 0) 'octet)
with w = (copy-seq result)
for i from 1 below (length octets)
for k = (make-vector-with-elt (aref octets i) 't)
for entry = (or (gethash k dictionary)
(if (equalp k dictionary-size)
(vector-append1-new w (aref w 0))
(error "bad compresed entry at pos ~S" i)))
do (vector-append result entry)
(setf (gethash (make-vector-with-elt dictionary-size) dictionary)
(vector-append1-new w (aref entry 0)))
(incf dictionary-size)
(setq w entry)
finally (return result)))
(defgeneric lzw-compress (datum)
(:method ((string string))
(lzw-compress (babel:string-to-octets string)))
(:method ((octets vector))
(lzw-compress-octets octets)))
(defun lzw-decompress-to-string (octets)
(babel:octets-to-string (lzw-decompress octets)))
(defun test (string)
(assert (equal #2=(lzw-decompress-to-string (lzw-compress string)) string) ()
"Can't compress ~S properly, got ~S instead" string #2#)
t)
And the format used:
CL-USER> (test "TOBEORNOTTOBEORTOBEORNOT")
T
CL-USER> (lzw-compress "TOBEORNOTTOBEORTOBEORNOT")
#(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)
CL-USER> (lzw-decompress-to-string *)
"TOBEORNOTTOBEORTOBEORNOT"
D
Simpler Version
import std.stdio, std.array;
auto compress(in string original) pure nothrow {
int[string] dict;
foreach (immutable char c; char.min .. char.max + 1)
dict[[c]] = c;
string w;
int[] result;
foreach (immutable ch; original)
if (w ~ ch in dict)
w = w ~ ch;
else {
result ~= dict[w];
dict[w ~ ch] = dict.length;
w = [ch];
}
return w.empty ? result : (result ~ dict[w]);
}
auto decompress(in int[] compressed) pure nothrow {
auto dict = new string[char.max - char.min + 1];
foreach (immutable char c; char.min .. char.max + 1)
dict[c] = [c];
auto w = dict[compressed[0]];
auto result = w;
foreach (immutable k; compressed[1 .. $]) {
auto entry = (k < dict.length) ? dict[k] : w ~ w[0];
result ~= entry;
dict ~= w ~ entry[0];
w = entry;
}
return result;
}
void main() {
auto comp = "TOBEORNOTTOBEORTOBEORNOT".compress;
writeln(comp, "\n", comp.decompress);
}
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
More Refined Version
This longer version is a little more efficient and it uses stronger static typing.
struct LZW {
import std.array: empty;
// T is ubyte instead of char because D strings are UTF-8.
alias T = ubyte;
alias Tcomp = ushort;
static assert(Tcomp.sizeof > 1);
alias Ta = immutable(T)[];
enum int initDictSize = 256;
static immutable ubyte[initDictSize] bytes;
static this() {
foreach (immutable T i; 0 .. initDictSize)
bytes[i] = i;
}
static Tcomp[] compress(immutable scope T[] original) pure nothrow @safe
out(result) {
if (!original.empty)
assert(result[0] < initDictSize);
} body {
if (original.empty)
return [];
Tcomp[Ta] dict;
foreach (immutable b; bytes)
dict[[b]] = b;
// Here built-in slices give lower efficiency.
struct Slice {
size_t start, end;
@property opSlice() const pure nothrow @safe @nogc {
return original[start .. end];
}
alias opSlice this;
}
Slice w;
Tcomp[] result;
foreach (immutable i; 0 .. original.length) {
auto wc = Slice(w.start, w.end + 1); // Extend slice.
if (wc in dict) {
w = wc;
} else {
result ~= dict[w];
assert(dict.length < Tcomp.max); // Overflow guard.
dict[wc] = cast(Tcomp)dict.length;
w = Slice(i, i + 1);
}
}
if (!w.empty)
result ~= dict[w];
return result;
}
static Ta decompress(in Tcomp[] compressed) pure @safe
in {
if (!compressed.empty)
assert(compressed[0] < initDictSize, "Bad compressed");
} body {
if (compressed.empty)
return [];
auto dict = new Ta[initDictSize];
foreach (immutable b; bytes)
dict[b] = [b];
auto w = dict[compressed[0]];
auto result = w;
foreach (immutable k; compressed[1 .. $]) {
Ta entry;
if (k < dict.length)
entry = dict[k];
else if (k == dict.length)
entry = w ~ w[0];
else
throw new Exception("Bad compressed k.");
result ~= entry;
dict ~= w ~ entry[0];
w = entry;
}
return result;
}
}
void main() {
import std.stdio, std.string;
immutable txt = "TOBEORNOTTOBEORTOBEORNOT";
immutable compressed = LZW.compress(txt.representation);
compressed.writeln;
LZW.decompress(compressed).assumeUTF.writeln;
}
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
More Efficient Version
This code retains part of the style of the original C code.
enum Marker: ushort {
CLR = 256, // Clear table marker.
EOD = 257, // End-of-data marker.
NEW = 258 // New code index.
}
ubyte[] lzwEncode(scope const(ubyte)[] inp, in uint maxBits) pure nothrow
in {
assert(maxBits >= 9 && maxBits <= 16);
} body {
// Encode dictionary array. For encoding, entry at
// code index is a list of indices that follow current one,
// i.e. if code 97 is 'a', code 387 is 'ab', and code 1022 is 'abc',
// then dict[97].next['b'] = 387, dict[387].next['c'] = 1022, etc.
alias LZWenc = ushort[256];
auto len = inp.length;
uint bits = 9;
auto d = new LZWenc[512];
auto result = new ubyte[16];
size_t outLen = 0;
size_t oBits = 0;
uint tmp = 0;
void writeBits(in ushort x) nothrow {
tmp = (tmp << bits) | x;
oBits += bits;
if (result.length / 2 <= outLen)
result.length *= 2;
while (oBits >= 8) {
oBits -= 8;
assert(tmp >> oBits <= ubyte.max);
result[outLen] = cast(ubyte)(tmp >> oBits);
outLen++;
tmp &= (1 << oBits) - 1;
}
}
// writeBits(Marker.CLR);
ushort nextCode = Marker.NEW;
uint nextShift = 512;
ushort code = inp[0];
inp = inp[1 .. $]; // popFront.
while (--len) {
ushort c = inp[0];
inp = inp[1 .. $]; // popFront.
ushort nc = d[code][c];
if (nc) {
code = nc;
} else {
writeBits(code);
nc = d[code][c] = nextCode;
nextCode++;
code = c;
}
// Next new code would be too long for current table.
if (nextCode == nextShift) {
// Either reset table back to 9 bits.
bits++;
if (bits > maxBits) {
// Table clear marker must occur before bit reset.
writeBits(Marker.CLR);
bits = 9;
nextShift = 512;
nextCode = Marker.NEW;
d[] = LZWenc.init;
} else { // Or extend table.
nextShift *= 2;
d.length = nextShift;
}
}
}
writeBits(code);
writeBits(Marker.EOD);
if (tmp) {
assert(tmp <= ushort.max);
writeBits(cast(ushort)tmp);
}
return result[0 .. outLen];
}
ubyte[] lzwDecode(scope const(ubyte)[] inp) pure {
// For decoding, dictionary contains index of whatever prefix
// index plus trailing ubyte. i.e. like previous example,
// dict[1022] = { c: 'c', prev: 387 },
// dict[387] = { c: 'b', prev: 97 },
// dict[97] = { c: 'a', prev: 0 }
// the "back" element is used for temporarily chaining indices
// when resolving a code to bytes.
static struct LZWdec {
ushort prev, back;
ubyte c;
}
auto result = new ubyte[4];
uint outLen = 0;
void writeOut(in ubyte c) nothrow {
while (outLen >= result.length)
result.length *= 2;
result[outLen] = c;
outLen++;
}
auto d = new LZWdec[512];
ushort code = 0;
uint bits = 9;
uint len = 0;
uint nBits = 0;
uint tmp = 0;
void getCode() nothrow {
while (nBits < bits) {
if (len > 0) {
len--;
tmp = (tmp << 8) | inp[0];
inp = inp[1 .. $]; // popFront.
nBits += 8;
} else {
tmp = tmp << (bits - nBits);
nBits = bits;
}
}
nBits -= bits;
assert(tmp >> nBits <= ushort.max);
code = cast(ushort)(tmp >> nBits);
tmp &= (1 << nBits) - 1;
}
uint nextShift = 512;
ushort nextCode = Marker.NEW;
void clearTable() nothrow {
d[] = LZWdec.init;
foreach (immutable ubyte j; 0 .. 256)
d[j].c = j;
nextCode = Marker.NEW;
nextShift = 512;
bits = 9;
}
clearTable(); // In case encoded bits didn't start with Marker.CLR.
for (len = inp.length; len;) {
getCode();
if (code == Marker.EOD)
break;
if (code == Marker.CLR) {
clearTable();
continue;
}
if (code >= nextCode)
throw new Error("Bad sequence.");
auto c = code;
d[nextCode].prev = c;
while (c > 255) {
immutable t = d[c].prev;
d[t].back = c;
c = t;
}
assert(c <= ubyte.max);
d[nextCode - 1].c = cast(ubyte)c;
while (d[c].back) {
writeOut(d[c].c);
immutable t = d[c].back;
d[c].back = 0;
c = t;
}
writeOut(d[c].c);
nextCode++;
if (nextCode >= nextShift) {
bits++;
if (bits > 16) {
// If input was correct, we'd have hit Marker.CLR before this.
throw new Error("Too many bits.");
}
nextShift *= 2;
d.length = nextShift;
}
}
// Might be OK, so throw just an exception.
if (code != Marker.EOD)
throw new Exception("Bits did not end in EOD");
return result[0 .. outLen];
}
void main() {
import std.stdio, std.file;
const inputData = cast(ubyte[])read("unixdict.txt");
writeln("Input size: ", inputData.length);
immutable encoded = lzwEncode(inputData, 12);
writeln("Encoded size: ", encoded.length);
immutable decoded = lzwDecode(encoded);
writeln("Decoded size: ", decoded.length);
if (inputData.length != decoded.length)
return writeln("Error: decoded size differs");
foreach (immutable i, immutable x; inputData)
if (x != decoded[i])
return writeln("Bad decode at ", i);
"Decoded OK.".writeln;
}
- Output:
Input size: 206403 Encoded size: 97633 Decoded size: 206403 Decoded OK.
Dylan
Module: LZW
Synopsis: LZW implementation for Rosetta code
define method output(n :: <integer>)
format-out("%d ", n);
end;
define method contains?(dict, var)
let x = element(dict, var, default: #f);
x ~= #f;
end;
define method byte->string(c)
add("", as(<character>, c));
end;
define method compress(input :: <string>) => <vector>;
let result = make(<vector>);
let dict = make(<string-table>);
for (x from 0 to 255)
dict[byte->string(x)] := x;
end;
let next-code = 256;
let cur-seq = "";
for (c in input)
let wc = add(cur-seq, c);
if (contains?(dict, wc))
cur-seq := wc;
else
result := add(result, dict[cur-seq]);
dict[wc] := next-code;
next-code := next-code + 1;
cur-seq := add("", c);
end
end;
unless (empty?(cur-seq))
result := add(result, dict[cur-seq]);
end;
result
end;
format-out("%=\n", compress("TOBEORNOTTOBEORTOBEORNOT"))
Eiffel
class
APPLICATION
create
make
feature {NONE}
make
local
test: LINKED_LIST [INTEGER]
do
create test.make
test := compress ("TOBEORNOTTOBEORTOBEORNOT")
across
test as t
loop
io.put_string (t.item.out + " ")
end
io.new_line
io.put_string (decompress (test))
end
decompress (compressed: LINKED_LIST [INTEGER]): STRING
--Decompressed version of 'compressed'.
local
dictsize, i, k: INTEGER
dictionary: HASH_TABLE [STRING, INTEGER]
w, entry: STRING
char: CHARACTER_8
do
dictsize := 256
create dictionary.make (300)
create entry.make_empty
create Result.make_empty
from
i := 0
until
i > 256
loop
char := i.to_character_8
dictionary.put (char.out, i)
i := i + 1
end
w := compressed.first.to_character_8.out
compressed.go_i_th (1)
compressed.remove
Result := w
from
k := 1
until
k > compressed.count
loop
if attached dictionary.at (compressed [k]) as ata then
entry := ata
elseif compressed [k] = dictsize then
entry := w + w.at (1).out
else
io.put_string ("EXEPTION")
end
Result := Result + entry
dictsize := dictsize + 1
dictionary.put (w + entry.at (1).out, dictsize)
w := entry
k := k + 1
end
end
compress (uncompressed: STRING): LINKED_LIST [INTEGER]
-- Compressed version of 'uncompressed'.
local
dictsize: INTEGER
dictionary: HASH_TABLE [INTEGER, STRING]
i: INTEGER
w, wc: STRING
char: CHARACTER_8
do
dictsize := 256
create dictionary.make (256)
create w.make_empty
from
i := 0
until
i > 256
loop
char := i.to_character_8
dictionary.put (i, char.out)
i := i + 1
end
create Result.make
from
i := 1
until
i > uncompressed.count
loop
wc := w + uncompressed [i].out
if dictionary.has (wc) then
w := wc
else
Result.extend (dictionary.at (w))
dictSize := dictSize + 1
dictionary.put (dictSize, wc)
w := "" + uncompressed [i].out
end
i := i + 1
end
if w.count > 0 then
Result.extend (dictionary.at (w))
end
end
end
- Output:
84 79 66 69 79 82 78 79 84 257 259 261 266 260 262 264 TOBEORNOTTOBEORTOBEORNOT
Elixir
defmodule LZW do
@encode_map Enum.into(0..255, Map.new, &{[&1],&1})
@decode_map Enum.into(0..255, Map.new, &{&1,[&1]})
def encode(str), do: encode(to_char_list(str), @encode_map, 256, [])
defp encode([h], d, _, out), do: Enum.reverse([d[[h]] | out])
defp encode([h|t], d, free, out) do
val = d[[h]]
find_match(t, [h], val, d, free, out)
end
defp find_match([h|t], l, lastval, d, free, out) do
case Map.fetch(d, [h|l]) do
{:ok, val} -> find_match(t, [h|l], val, d, free, out)
:error -> d1 = Map.put(d, [h|l], free)
encode([h|t], d1, free+1, [lastval | out])
end
end
defp find_match([], _, lastval, _, _, out), do: Enum.reverse([lastval | out])
def decode([h|t]) do
val = @decode_map[h]
decode(t, val, 256, @decode_map, val)
end
defp decode([], _, _, _, l), do: Enum.reverse(l) |> to_string
defp decode([h|t], old, free, d, l) do
val = if h == free, do: old ++ [List.first(old)], else: d[h]
add = [List.last(val) | old]
d1 = Map.put(d, free, add)
decode(t, val, free+1, d1, val++l)
end
end
str = "TOBEORNOTTOBEORTOBEORNOT"
IO.inspect enc = LZW.encode(str)
IO.inspect dec = LZW.decode(enc)
IO.inspect str == dec
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] "TOBEORNOTTOBEORTOBEORNOT" true
EMal
type LzwCompression
fun compress ← List by text uncompressed
List output ← int[]
text working ← Text.EMPTY
Map symbolTable ← text%int[].with(256, <int i|text%int(chr(i) => i))
for each text c in uncompressed
text augmented ← working + c
if symbolTable.has(augmented)
working ← augmented
else
symbolTable.insert(augmented, symbolTable.length)
int i ← symbolTable[working]
output.append(i)
working ← c
end
end
if not working.isEmpty()
int i ← symbolTable[working]
output.append(i)
end
return output
end
fun decompress ← text by List compressed
Map symbolTable ← int%text[].with(256, <int i|int%text(i => chr(i)))
text working ← symbolTable[compressed[0]]
text output ← *working
for each int i in compressed.extract(1)
text s
if symbolTable.has(i)
s ← symbolTable[i]
else if i æ symbolTable.length # cScSc problem
s ← working + working[0]
else
error(65, "Error decompressing")
end
output.append(s)
symbolTable.insert(symbolTable.length, working + s[0])
working ← s
end
return output
end
List compressed = compress("TOBEORNOTTOBEORTOBEORNOT")
writeLine(compressed)
text decompressed = decompress(compressed)
writeLine(decompressed)- Output:
[84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263] TOBEORNOTTOBEORTOBEORNOT
Erlang
-module(lzw).
-export([test/0, encode/1, decode/1]).
-import(lists, [reverse/1, reverse/2]).
test() ->
Str = "TOBEORNOTTOBEORTOBEORNOT",
[84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263] =
encode(Str),
Str = decode(encode(Str)),
ok.
encode(Str) ->
D = init(dict:new()),
encode(Str, D, 256, []).
encode([H], D, _, Out) ->
Val = dict:fetch([H], D),
reverse([Val|Out]);
encode([H|T], D, Free, Out) ->
Val = dict:fetch([H], D),
find_match(T, [H], Val, D, Free, Out).
find_match([H|T], L, LastVal, D, Free, Out) ->
case dict:find([H|L], D) of
{ok, Val} ->
find_match(T, [H|L], Val, D, Free, Out);
error ->
D1 = dict:store([H|L], Free, D),
encode([H|T], D1, Free+1, [LastVal|Out])
end;
find_match([], _, LastVal, _, _, Out) ->
reverse([LastVal|Out]).
decode([H|T]) ->
D = init1(dict:new()),
Val = dict:fetch(H, D),
decode(T, Val, 256, D, Val).
decode([], _, _, _, L) ->
reverse(L);
decode([H|T], Old, Free, D, L) ->
Val = dict:fetch(H, D),
Add = [lists:last(Val)|Old],
D1 = dict:store(Free, Add, D),
decode(T, Val, Free+1, D1, Val ++ L).
init(D) -> init(255, D).
init(0, D) -> D;
init(N, D) -> D1 = dict:store([N],N,D), init(N-1, D1).
init1(D) -> init1(255, D).
init1(0, D) -> D;
init1(N, D) -> D1 = dict:store(N,[N],D), init1(N-1, D1).
Forth
256 value next-symbol
\ current string fragment
create w 256 allot \ counted string
: w=c ( c -- ) w 1+ c! 1 w c! ;
: w+c ( c -- ) w count + c! w c@ 1+ w c! ;
\ Compression
\ dictionary of strings to symbols
0 value dict
: init-dict table to dict 256 to next-symbol dict set-current ;
: free-dict forth-wordlist set-current ;
: in-dict? ( key len -- ? ) \ can assume len > 1
dict search-wordlist dup if nip then ;
: lookup-dict ( key len -- symbol )
dup 1 = if drop c@ exit then
dict search-wordlist if >body @ else abort" bad-dict!" then ;
: put-dict ( data key len -- )
nextname create , ;
\ output buffer of symbols
\ in real life, these symbols would be packed into octets
variable out-size
create out 256 cells allot
: output ( symbol -- )
dup out out-size @ cells + ! 1 out-size +!
dup 256 < if emit space else . then ;
: compress ( addr len -- )
init-dict 0 out-size !
over c@ w=c 1 /string
bounds do
i c@ w+c
w count in-dict? 0= if
w count 1- lookup-dict output
next-symbol dup w count put-dict
1+ to next-symbol
i c@ w=c
then
loop
w count lookup-dict output
free-dict ;
\ Decompression
\ array of symbols to strings (in real code this would need to be growable)
\ next-symbol is reused for the size of this table
create symtab 256 cells allot
0 value start
: init-symtab 256 to next-symbol here to start ;
: free-symtab start here - allot ;
: get-symbol ( symbol -- addr len )
dup 256 < if pad c! pad 1 exit then
256 - cells symtab + @ count ;
: add-symbol ( addr len -- )
here symtab next-symbol 256 - cells + !
s,
next-symbol 1+ to next-symbol ;
create entry 256 allot
: decompress ( addr len -- )
init-symtab
over @ dup emit w=c
cells bounds cell+ do
i @ next-symbol < if
i @ get-symbol entry place
else i @ next-symbol = if
w 1+ c@ w count + c! w count 1+ entry place
else
abort" bad symbol!"
then then
entry count type \ output
entry 1+ c@ w+c
w count add-symbol
entry count w place
1 cells +loop
free-symtab ;
\ Testing
s" TOBEORNOTTOBEORTOBEORNOT" compress cr
\ T O B E O R N O T 256 258 260 265 259 261 263
out out-size @ decompress cr
\ TOBEORNOTTOBEORTOBEORNOT
Fortran
!
! lzw_shared_parameters.f90
!
! LZW Common Variables Used by Coder and Decoder
!
! Author: Pedro Garcia Freitas <sawp@sawp.com.br>
! May, 2011
!
! License: Creative Commons http://creativecommons.org/licenses/by-nc-nd/3.0/
!
MODULE LZW_SHARED_PARAMETERS
IMPLICIT NONE
!
! PARAMETER definitions
!
INTEGER , PARAMETER :: COMPILER_INTEGER_SIZE = 32 , BITS = 12 , FILEIN = 66 , &
& FILEOUT = 99 , MAX_VALUE = (2**BITS) - 1 , &
& MAX_CODE = MAX_VALUE - 1 , MAX_DICTIONARY_SIZE = 5021 , &
& SYMBOL_SIZE = 8 , MISSING_BITS = COMPILER_INTEGER_SIZE - &
& SYMBOL_SIZE
!
! Local variables
!
INTEGER , DIMENSION(0:MAX_DICTIONARY_SIZE) :: concatenatedsymbols
INTEGER , DIMENSION(0:MAX_DICTIONARY_SIZE) :: prefixcodes
INTEGER :: the_status = 0
! change this if compiler dont use 32 bits for integer
END MODULE LZW_SHARED_PARAMETERS
!
! codecIO.f90
!
! bit IO routines for coder and encoder.
!
! Author: Pedro Garcia Freitas <sawp@sawp.com.br>
! May, 2011
!
! License: Creative Commons http://creativecommons.org/licenses/by-nc-nd/3.0/
!
MODULE CODECIO
USE LZW_SHARED_PARAMETERS
IMPLICIT NONE
!
CONTAINS
SUBROUTINE SETOUTPUTCODE(Code)
IMPLICIT NONE
!
! Dummy arguments
!
INTEGER :: Code
INTENT (IN) Code
!
! Local variables
!
INTEGER :: buffer
INTEGER :: outputbitbuffer = 0
INTEGER :: outputbitcount = 0
INTEGER :: shift
INTEGER :: shiftedsymbol
!
shift = COMPILER_INTEGER_SIZE - BITS - outputbitcount
shiftedsymbol = ISHFT(Code , shift)
outputbitbuffer = IOR(outputbitbuffer , shiftedsymbol)
outputbitcount = outputbitcount + BITS
DO WHILE(outputbitcount >= SYMBOL_SIZE)
! IF( outputbitcount<SYMBOL_SIZE )EXIT
buffer = ISHFT(outputbitbuffer , -MISSING_BITS)
CALL SETRAWBYTE(buffer)
outputbitbuffer = ISHFT(outputbitbuffer , SYMBOL_SIZE)
outputbitcount = outputbitcount - SYMBOL_SIZE
END DO
RETURN
END SUBROUTINE SETOUTPUTCODE
SUBROUTINE SETRAWBYTE(Symbol)
IMPLICIT NONE
!
! Dummy arguments
!
INTEGER :: Symbol
INTENT (IN) Symbol
!
CALL FPUTC(FILEOUT , ACHAR(Symbol))
END SUBROUTINE SETRAWBYTE
FUNCTION GETRAWBYTE()
IMPLICIT NONE
!
! Dummy arguments
!
INTEGER :: GETRAWBYTE
!
! Local variables
!
CHARACTER :: bufferedbyte
!
CALL FGETC(FILEIN , bufferedbyte , THE_status)
GETRAWBYTE = IACHAR(bufferedbyte)
END FUNCTION GETRAWBYTE
FUNCTION GETINPUTCODE()
IMPLICIT NONE
!
! Dummy arguments
!
INTEGER :: GETINPUTCODE
!
! Local variables
!
INTEGER :: inputbitbuffer = 0
INTEGER :: inputbitcounter = 0
INTEGER :: integerinputbuff
INTEGER :: returnn
INTEGER :: shiftedbit
!
DO WHILE( inputbitcounter <= MISSING_BITS )
! IF( inputbitcounter>MISSING_BITS )EXIT
integerinputbuff = GETRAWBYTE()
shiftedbit = ISHFT(integerinputbuff , MISSING_BITS - inputbitcounter)
inputbitbuffer = IOR(inputbitbuffer , shiftedbit)
inputbitcounter = inputbitcounter + SYMBOL_SIZE
END DO
returnn = ISHFT(inputbitbuffer , BITS - COMPILER_INTEGER_SIZE)
inputbitbuffer = ISHFT(inputbitbuffer , BITS)
inputbitcounter = inputbitcounter - BITS
GETINPUTCODE = returnn
RETURN
END FUNCTION GETINPUTCODE
end module codecIO
! lzw_encoder.f90
!
! LZW Coder (Compressor)
!
! Author: Pedro Garcia Freitas <sawp@sawp.com.br>
! May, 2011
!
! License: Creative Commons http://creativecommons.org/licenses/by-nc-nd/3.0/
!
MODULE LZW_ENCODER
USE LZW_SHARED_PARAMETERS
USE CODECIO
IMPLICIT NONE
!
! PARAMETER definitions
!
INTEGER , PARAMETER :: HASH_SHIFT = BITS - SYMBOL_SIZE
!
! Local variables
!
INTEGER , DIMENSION(0:MAX_DICTIONARY_SIZE) :: symbolvalues
CONTAINS
SUBROUTINE COMPRESS()
IMPLICIT NONE
!
! Local variables
!
INTEGER :: codedsymbol
INTEGER :: my_index
INTEGER :: nextsymbol
INTEGER :: symbol
CHARACTER :: bufferedbyte
!
nextsymbol = COMPILER_INTEGER_SIZE*SYMBOL_SIZE
SYMbolvalues(:) = -1
! codedsymbol = GETRAWBYTE()
CALL FGETC(FILEIN , bufferedbyte , THE_status)
codedsymbol = IACHAR(bufferedbyte)
!Can be hand optimized - optimization
DO WHILE(THE_status == 0)
! symbol = GETRAWBYTE() ! Manual inline of function
CALL FGETC(FILEIN , bufferedbyte , THE_status)
symbol = IACHAR(bufferedbyte)
IF( THE_status/=0 )CYCLE
my_index = GETPOSITIONONDICTIONARY(codedsymbol , symbol)
IF( SYMbolvalues(my_index)/= - 1 )THEN
codedsymbol = SYMbolvalues(my_index)
ELSE
IF( nextsymbol<=MAX_CODE )THEN
SYMbolvalues(my_index) = nextsymbol
nextsymbol = nextsymbol + 1
PREfixcodes(my_index) = codedsymbol
CONcatenatedsymbols(my_index) = symbol
END IF
CALL SETOUTPUTCODE(codedsymbol)
codedsymbol = symbol
END IF
END DO
CALL SETOUTPUTCODE(codedsymbol)
CALL SETOUTPUTCODE(MAX_VALUE)
CALL SETOUTPUTCODE(0)
END SUBROUTINE COMPRESS
function getPositionOnDictionary(hashPrefix, hashSymbol)
integer, intent(in) :: hashPrefix
integer, intent(in) :: hashSymbol
integer :: getPositionOnDictionary
integer :: index
integer :: offset
index = ishft(hashSymbol, HASH_SHIFT)
index = ieor(index, hashPrefix)
if (index == 0) then
offset = 1
else
offset = MAX_DICTIONARY_SIZE - index
endif
do
if (symbolValues(index) == -1) then
getPositionOnDictionary = index
exit
endif
if (prefixCodes(index) == hashPrefix .and. &
& concatenatedSymbols(index) == hashSymbol) then
getPositionOnDictionary = index
exit
endif
index = index - offset
if (index < 0) then
index = index + MAX_DICTIONARY_SIZE
endif
end do
return
end function
end module LZW_Encoder
! lzw_decoder.f90
!
! LZW Decoder (Expanssor)
!
! Author: Pedro Garcia Freitas <sawp@sawp.com.br>
! May, 2011
!
! License: Creative Commons http://creativecommons.org/licenses/by-nc-nd/3.0/
!
MODULE LZW_DECODER
USE LZW_SHARED_PARAMETERS
USE CODECIO
IMPLICIT NONE
!
! Derived Type definitions
!
TYPE :: DECODE_BUFFER_STACK
INTEGER , DIMENSION(0:MAX_DICTIONARY_SIZE) :: DECODERSTACK
INTEGER :: TOP
END TYPE DECODE_BUFFER_STACK
!
! Local variables
!
TYPE(DECODE_BUFFER_STACK) :: stack
CONTAINS
!
! Can be hand optimized - optimization
SUBROUTINE DECOMPRESS()
IMPLICIT NONE
!
! Local variables
!
INTEGER :: newsymbol
INTEGER :: nextsymbol
INTEGER :: oldsymbol
INTEGER :: popedsymbol
INTEGER :: symbol
!
nextsymbol = COMPILER_INTEGER_SIZE*SYMBOL_SIZE
oldsymbol = GETINPUTCODE()
symbol = oldsymbol
CALL SETRAWBYTE(oldsymbol)
DO
newsymbol = GETINPUTCODE()
IF( newsymbol==MAX_VALUE )RETURN
IF( newsymbol>=nextsymbol )THEN
STAck%DECODERSTACK(0) = symbol
CALL DECODESYMBOL(STAck%DECODERSTACK(1:) , oldsymbol)
ELSE
CALL DECODESYMBOL(STAck%DECODERSTACK(:) , newsymbol)
END IF
symbol = STAck%DECODERSTACK(STAck%TOP)
DO WHILE ( STAck%TOP>=0 )
popedsymbol = STAck%DECODERSTACK(STAck%TOP)
CALL SETRAWBYTE(popedsymbol)
STAck%TOP = STAck%TOP - 1
END DO
IF( nextsymbol<=MAX_CODE )THEN
PREfixcodes(nextsymbol) = oldsymbol
CONcatenatedsymbols(nextsymbol) = symbol
nextsymbol = nextsymbol + 1
END IF
oldsymbol = newsymbol
END DO
RETURN
END SUBROUTINE DECOMPRESS
SUBROUTINE DECODESYMBOL(Buffer , Code)
IMPLICIT NONE
!
! Dummy arguments
!
INTEGER :: Code
INTEGER , DIMENSION(:) :: Buffer
INTENT (IN) Code
INTENT (INOUT) Buffer
!
! Local variables
!
INTEGER :: i
INTEGER :: j
INTEGER :: symbol
!
j = 0
symbol = Code
STAck%TOP = 0
DO WHILE ( symbol>=COMPILER_INTEGER_SIZE*SYMBOL_SIZE )
! IF( symbol<COMPILER_INTEGER_SIZE*SYMBOL_SIZE )EXIT
IF( j>=MAX_CODE )THEN
PRINT * , "Decoding error"
STOP
END IF
i = STAck%TOP + 1
Buffer(i) = CONcatenatedsymbols(symbol)
symbol = PREfixcodes(symbol)
STAck%TOP = STAck%TOP + 1
j = j + 1
END DO
i = j + 1
Buffer(i) = symbol
END SUBROUTINE DECODESYMBOL
end module LZW_Decoder
! lzw.f90
!
! LZW Coder and Decoder
!
! Author: Pedro Garcia Freitas <sawp@sawp.com.br>
! May, 2011
!
! License: Creative Commons http://creativecommons.org/licenses/by-nc-nd/3.0/
!
MODULE LZW
USE LZW_SHARED_PARAMETERS
USE LZW_ENCODER
USE LZW_DECODER
IMPLICIT NONE
CONTAINS
SUBROUTINE INIT(Input , Output , Operation , Filename)
IMPLICIT NONE
!
! Dummy arguments
!
CHARACTER(100) :: Filename
CHARACTER(100) :: Input
CHARACTER(1) :: Operation
CHARACTER(100) :: Output
INTENT (IN) Filename , Input , Operation , Output
!
IF( Operation/='d' .AND. Operation/='e' )THEN
PRINT * , "Usage: " // TRIM(Filename) // " <operation> input output"
PRINT * , "Possible operations: "
PRINT * , " e -> encode (compress)"
PRINT * , " d -> decode (inflate)"
STOP
END IF
OPEN(UNIT = FILEIN , FILE = Input , ACTION = "read" , STATUS = "old" , &
&ACCESS = 'stream' , FORM = "formatted")
OPEN(UNIT = FILEOUT , FILE = Output , ACTION = "write" , STATUS = "replace" , &
& ACCESS = 'stream' , FORM = "formatted")
IF( Operation=='d' )THEN
PRINT * , "Decoding..."
CALL DECOMPRESS()
ELSE
PRINT * , "Encoding..."
CALL COMPRESS()
END IF
CLOSE(UNIT = FILEIN)
CLOSE(UNIT = FILEOUT)
END SUBROUTINE INIT
end module LZW
!
PROGRAM MAIN
USE LZW
IMPLICIT NONE
!
! Local variables
!
CHARACTER(100) :: filename
REAL :: finish
CHARACTER(100) :: input
CHARACTER(1) :: operation
CHARACTER(100) :: output
REAL :: start
!
CALL GETARG(0 , filename)
CALL GETARG(1 , operation)
CALL GETARG(2 , input)
CALL GETARG(3 , output)
CALL CPU_TIME(start)
CALL INIT(input , output , operation , filename)
CALL CPU_TIME(finish)
PRINT '("Time = ",f6.3," seconds.")' , finish - start
END PROGRAM MAIN
FreeBASIC
' version 22-02-2019
' compile with: fbc -s console
Type dict
prefix As Integer
B As String
End Type
Sub init(dictionary() As dict, ByRef last As ULong)
For i As ULong = 0 To 255
dictionary(i).prefix = -1
dictionary(i).B = Chr(i)
Next
last = 255
End Sub
Function encode_LZW(dictionary() As dict, last_entry As ULong, input_str As String) As String
If Len(input_str) < 2 Then
Print "input string is to short"
Return ""
End If
Dim As String word, output_str, char
Dim As ULong i = 1, index, j, len_input = Len(input_str)
Do
If i > len_input Then
output_str = output_str + " " + Str(index)
Return output_str ' no more chars to process. we are done
End If
char = Mid(Input_str, i, 1)
i += 1
For j = 0 To last_entry
If dictionary(j).B = word + char Then
word += char
index = j
Continue Do
End If
Next
output_str = output_str + " " + Str(index)
last_entry = last_entry +1
dictionary(last_entry).B = word + char
dictionary(last_entry).prefix = index
word = char : index = Asc(char)
Loop
End Function
Function decode_LZW(dictionary() As dict, last_entry As ULong, input_str As String) As String
Dim As String temp, word, output_str
Dim As ULong i, i1 = 1, j, index
input_str = Trim(input_str)
Dim As ULong len_input = Len(input_str)
input_str = input_str + " "
i = InStr(i1, input_str, " ")
index = Val(Mid(Input_str, i1, i - i1))
word = dictionary(index).B
output_str = word
i1 = i +1
Do
i = InStr(i1, input_str, " ")
If i >= len_input Then
index = Val(Mid(input_str, i1))
output_str = output_str + dictionary(index).B
Return output_str
End If
index = Val(Mid(Input_str, i1, i - i1))
i1 = i +1
If index <= last_entry Then
temp = dictionary(index).B
Else
temp = word + Left(word, 1)
End If
output_str = output_str + temp
last_entry = last_entry +1
dictionary(last_entry).B = word + Left(temp, 1)
word = temp
Loop
End Function
' ------=< MAIN >=------
Dim As ULong last_entry, max_bit = 9
Dim As ULong dict_max = 1 Shl max_bit -1
Dim As String output_str, input_str = "TOBEORNOTTOBEORTOBEORNOT"
Dim As dict dictionary()
Print " input str: ";input_str
ReDim dictionary(dict_max +1)
init(dictionary(), last_entry)
output_str = encode_LZW(dictionary(), last_entry, input_str)
Print "encoded str: ";output_str
ReDim dictionary(dict_max +1)
init(dictionary(), last_entry)
output_str = decode_LZW(dictionary(), last_entry, output_str)
Print "decoded str: "; output_str
' empty keyboard buffer
While InKey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End- Output:
input str: TOBEORNOTTOBEORTOBEORNOT encoded str: 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 decoded str: TOBEORNOTTOBEORTOBEORNOT
Go
Go also has the
compress/lzw
package in the standard library.
This handles any series of bytes in the input string, not just ASCII or valid UTF8 encoding (tested with go-fuzz).
package main
import (
"fmt"
"log"
"strings"
)
// Compress a string to a list of output symbols.
func compress(uncompressed string) []int {
// Build the dictionary.
dictSize := 256
// We actually want a map of []byte -> int but
// slices are not acceptable map key types.
dictionary := make(map[string]int, dictSize)
for i := 0; i < dictSize; i++ {
// Ugly mess to work around not having a []byte key type.
// Using `string(i)` would do utf8 encoding for i>127.
dictionary[string([]byte{byte(i)})] = i
}
var result []int
var w []byte
for i := 0; i < len(uncompressed); i++ {
c := uncompressed[i]
wc := append(w, c)
if _, ok := dictionary[string(wc)]; ok {
w = wc
} else {
result = append(result, dictionary[string(w)])
// Add wc to the dictionary.
dictionary[string(wc)] = dictSize
dictSize++
//w = []byte{c}, but re-using wc
wc[0] = c
w = wc[:1]
}
}
if len(w) > 0 {
// Output the code for w.
result = append(result, dictionary[string(w)])
}
return result
}
type BadSymbolError int
func (e BadSymbolError) Error() string {
return fmt.Sprint("Bad compressed symbol ", int(e))
}
// Decompress a list of output symbols to a string.
func decompress(compressed []int) (string, error) {
// Build the dictionary.
dictSize := 256
dictionary := make(map[int][]byte, dictSize)
for i := 0; i < dictSize; i++ {
dictionary[i] = []byte{byte(i)}
}
var result strings.Builder
var w []byte
for _, k := range compressed {
var entry []byte
if x, ok := dictionary[k]; ok {
//entry = x, but ensuring any append will make a copy
entry = x[:len(x):len(x)]
} else if k == dictSize && len(w) > 0 {
entry = append(w, w[0])
} else {
return result.String(), BadSymbolError(k)
}
result.Write(entry)
if len(w) > 0 {
// Add w+entry[0] to the dictionary.
w = append(w, entry[0])
dictionary[dictSize] = w
dictSize++
}
w = entry
}
return result.String(), nil
}
func main() {
compressed := compress("TOBEORNOTTOBEORTOBEORNOT")
fmt.Println(compressed)
decompressed, err := decompress(compressed)
if err != nil {
log.Fatal(err)
}
fmt.Println(decompressed)
}
- Output:
[84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263] TOBEORNOTTOBEORTOBEORNOT
Groovy
def compress = { text ->
def dictionary = (0..<256).inject([:]) { map, ch -> map."${(char)ch}" = ch; map }
def w = '', compressed = []
text.each { ch ->
def wc = "$w$ch"
if (dictionary[wc]) {
w = wc
} else {
compressed << dictionary[w]
dictionary[wc] = dictionary.size()
w = "$ch"
}
}
if (w) { compressed << dictionary[w] }
compressed
}
def decompress = { compressed ->
def dictionary = (0..<256).inject([:]) { map, ch -> map[ch] = "${(char)ch}"; map }
int dictSize = 128;
String w = "${(char)compressed[0]}"
StringBuffer result = new StringBuffer(w)
compressed.drop(1).each { k ->
String entry = dictionary[k]
if (!entry) {
if (k != dictionary.size()) throw new IllegalArgumentException("Bad compressed k $k")
entry = "$w${w[0]}"
}
result << entry
dictionary[dictionary.size()] = "$w${entry[0]}"
w = entry
}
result.toString()
}
Testing:
def plaintext = 'TOBEORNOTTOBEORTOBEORNOT'
def compressed = compress(plaintext)
def result = decompress(compressed)
println """\
Plaintext: '$plaintext'
Compressed: $compressed
Uncompressed: '$result'""".stripIndent()
- Output:
Plaintext: 'TOBEORNOTTOBEORTOBEORNOT' Compressed: [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] Uncompressed: 'TOBEORNOTTOBEORTOBEORNOT'
Haskell
import Data.List (elemIndex, tails)
import Data.Maybe (fromJust)
doLZW :: Eq a => [a] -> [a] -> [Int]
doLZW _ [] = []
doLZW as (x:xs) = lzw (return <$> as) [x] xs
where
lzw a w [] = [fromJust $ elemIndex w a]
lzw a w (x:xs)
| w_ `elem` a = lzw a w_ xs
| otherwise = fromJust (elemIndex w a) : lzw (a ++ [w_]) [x] xs
where
w_ = w ++ [x]
undoLZW :: [a] -> [Int] -> [a]
undoLZW _ [] = []
undoLZW a cs =
cs >>=
(!!)
(foldl
((.) <$> (++) <*>
(\x xs -> return (((++) <$> head <*> take 1 . last) ((x !!) <$> xs))))
(return <$> a)
(take2 cs))
take2 :: [a] -> [[a]]
take2 xs = filter ((2 ==) . length) (take 2 <$> tails xs)
main :: IO ()
main = do
print $ doLZW ['\0' .. '\255'] "TOBEORNOTTOBEORTOBEORNOT"
print $
undoLZW
['\0' .. '\255']
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
print $
((==) <*> ((.) <$> undoLZW <*> doLZW) ['\NUL' .. '\255'])
"TOBEORNOTTOBEORTOBEORNOT"
- Output:
[84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263] "TOBEORNOTTOBEORTOBEORNOT" True
Other (elegant) code can be found at Haskell wiki Toy compression
J
Straightforward implementations of encoding and decoding:
encodeLZW =: 4 : 0
d=. ;/x
r=.0$0
wc=.w=.{.y
for_c. }.y do.
wc=.w,c
if. d e.~ <wc do. w=.wc else.
r=. r, d i.<w
d=.d,<wc
w=.c
end.
end.
r, d i.<w
)
Test:
a. encodeLZW 'TOBEORNOTTOBEORTOBEORNOT' 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
Decoding:
decodeLZW =: 4 : 0
d=.;/x
w=.r=. >d{~{.y
ds=. #d
for_c. }.y do.
select. * c-ds
case. _1 do. r=.r,e=.>c{d
case. 0 do. r=.r,e=.w,{.w
case. do. 'error' return.
end.
d=.d,< w,{.e
w=.e
ds=.ds+1
end.
;r
)
Test:
a. decodeLZW 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
encode --> decode --> compare with original:
a. (] -: [ decodeLZW encodeLZW) 'TOBEORNOTTOBEORTOBEORNOT' 1
Error test:
a. decodeLZW 84 79 66 69 79 82 78 79 84 256 258 456 260 265 259 261 263 error
Tacit J expression for decoding:
decodeLZW=:[:;]{[:;[:(],<@(>@{.,{.@>@{:)@:{)&.>/<@(;/@[),~|.@(2<\])
a. decodeLZW 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Java
import java.util.*;
public class LZW {
/** Compress a string to a list of output symbols. */
public static List<Integer> compress(String uncompressed) {
// Build the dictionary.
int dictSize = 256;
Map<String,Integer> dictionary = new HashMap<String,Integer>();
for (int i = 0; i < 256; i++)
dictionary.put("" + (char)i, i);
String w = "";
List<Integer> result = new ArrayList<Integer>();
for (char c : uncompressed.toCharArray()) {
String wc = w + c;
if (dictionary.containsKey(wc))
w = wc;
else {
result.add(dictionary.get(w));
// Add wc to the dictionary.
dictionary.put(wc, dictSize++);
w = "" + c;
}
}
// Output the code for w.
if (!w.equals(""))
result.add(dictionary.get(w));
return result;
}
/** Decompress a list of output ks to a string. */
public static String decompress(List<Integer> compressed) {
// Build the dictionary.
int dictSize = 256;
Map<Integer,String> dictionary = new HashMap<Integer,String>();
for (int i = 0; i < 256; i++)
dictionary.put(i, "" + (char)i);
String w = "" + (char)(int)compressed.remove(0);
StringBuffer result = new StringBuffer(w);
for (int k : compressed) {
String entry;
if (dictionary.containsKey(k))
entry = dictionary.get(k);
else if (k == dictSize)
entry = w + w.charAt(0);
else
throw new IllegalArgumentException("Bad compressed k: " + k);
result.append(entry);
// Add w+entry[0] to the dictionary.
dictionary.put(dictSize++, w + entry.charAt(0));
w = entry;
}
return result.toString();
}
public static void main(String[] args) {
List<Integer> compressed = compress("TOBEORNOTTOBEORTOBEORNOT");
System.out.println(compressed);
String decompressed = decompress(compressed);
System.out.println(decompressed);
}
}- Output:
(Command Line direct output)
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOTJavaScript
//LZW Compression/Decompression for Strings
var LZW = {
compress: function (uncompressed) {
"use strict";
// Build the dictionary.
var i,
dictionary = {},
c,
wc,
w = "",
result = [],
dictSize = 256;
for (i = 0; i < 256; i += 1) {
dictionary[String.fromCharCode(i)] = i;
}
for (i = 0; i < uncompressed.length; i += 1) {
c = uncompressed.charAt(i);
wc = w + c;
//Do not use dictionary[wc] because javascript arrays
//will return values for array['pop'], array['push'] etc
// if (dictionary[wc]) {
if (dictionary.hasOwnProperty(wc)) {
w = wc;
} else {
result.push(dictionary[w]);
// Add wc to the dictionary.
dictionary[wc] = dictSize++;
w = String(c);
}
}
// Output the code for w.
if (w !== "") {
result.push(dictionary[w]);
}
return result;
},
decompress: function (compressed) {
"use strict";
// Build the dictionary.
var i,
dictionary = [],
w,
result,
k,
entry = "",
dictSize = 256;
for (i = 0; i < 256; i += 1) {
dictionary[i] = String.fromCharCode(i);
}
w = String.fromCharCode(compressed[0]);
result = w;
for (i = 1; i < compressed.length; i += 1) {
k = compressed[i];
if (dictionary[k]) {
entry = dictionary[k];
} else {
if (k === dictSize) {
entry = w + w.charAt(0);
} else {
return null;
}
}
result += entry;
// Add w+entry[0] to the dictionary.
dictionary[dictSize++] = w + entry.charAt(0);
w = entry;
}
return result;
}
}, // For Test Purposes
comp = LZW.compress("TOBEORNOTTOBEORTOBEORNOT"),
decomp = LZW.decompress(comp);
document.write(comp + '<br>' + decomp);
ES6 Version
This is the the same thing, but for ES6. The code has been refactored and cleaned up a bit to look neater.
'use strict';
/**
Namespace for LZW compression and decompression.
Methods:
LZW.compress(uncompressed)
LZW.decompress(compressed)
*/
class LZW
{
/**
Perform the LZW compression
uncompressed - String. The string on which to perform the compression.
*/
static compress(uncompressed)
{
// Initialize dictionary
let dictionary = {};
for (let i = 0; i < 256; i++)
{
dictionary[String.fromCharCode(i)] = i;
}
let word = '';
let result = [];
let dictSize = 256;
for (let i = 0, len = uncompressed.length; i < len; i++)
{
let curChar = uncompressed[i];
let joinedWord = word + curChar;
// Do not use dictionary[joinedWord] because javascript objects
// will return values for myObject['toString']
if (dictionary.hasOwnProperty(joinedWord))
{
word = joinedWord;
}
else
{
result.push(dictionary[word]);
// Add wc to the dictionary.
dictionary[joinedWord] = dictSize++;
word = curChar;
}
}
if (word !== '')
{
result.push(dictionary[word]);
}
return result;
}
/**
Decompress LZW array generated by LZW.compress()
compressed - Array. The array that holds LZW compressed data.
*/
static decompress(compressed)
{
// Initialize Dictionary (inverse of compress)
let dictionary = {};
for (let i = 0; i < 256; i++)
{
dictionary[i] = String.fromCharCode(i);
}
let word = String.fromCharCode(compressed[0]);
let result = word;
let entry = '';
let dictSize = 256;
for (let i = 1, len = compressed.length; i < len; i++)
{
let curNumber = compressed[i];
if (dictionary[curNumber] !== undefined)
{
entry = dictionary[curNumber];
}
else
{
if (curNumber === dictSize)
{
entry = word + word[0];
}
else
{
throw 'Error in processing';
return null;
}
}
result += entry;
// Add word + entry[0] to dictionary
dictionary[dictSize++] = word + entry[0];
word = entry;
}
return result;
}
}
let comp = LZW.compress('TOBEORNOTTOBEORTOBEORNOT');
let decomp = LZW.decompress(comp);
console.log(`${comp}
${decomp}`);
- Output:
84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263 TOBEORNOTTOBEORTOBEORNOT
jq
# LZW compression/decompression for strings
def lzw_compress:
def decode: [.] | implode;
# Build the dictionary:
256 as $dictSize
| (reduce range(0; $dictSize) as $i ({}; .[ $i | decode ] = $i)) as $dictionary
| reduce explode[] as $i
( [$dictionary, $dictSize, "", []]; # state: [dictionary, dictSize, w, result]
.[0] as $dictionary
| .[1] as $dictSize
| .[2] as $w
| ($i | decode) as $c
| ($w + $c ) as $wc
| if $dictionary[$wc] then .[2] = $wc
else
.[2] = $c # w = c
| .[3] += [$dictionary[$w]] # result += dictionary[w]
| .[0][$wc] = $dictSize # Add wc to the dictionary
| .[1] += 1 # dictSize ++
end
)
# Output the code for w unless w == "":
| if .[2] == "" then .[3]
else .[3] + [.[0][.[2]]]
end
;
def lzw_decompress:
def decode: [.] | implode;
# Build the dictionary - an array of strings
256 as $dictSize
| (reduce range(0; $dictSize) as $i ([]; .[ $i ] = ($i|decode))) as $dictionary
| (.[0]|decode) as $w
| reduce .[1:][] as $k
( [ $dictionary, $dictSize, $w, $w]; # state: [dictionary, dictSize, w, result]
.[0][$k] as $entry
| (if $entry then $entry
elif $k == .[1] then .[2] + .[2][0:1]
else error("lzw_decompress: k=\($k)")
end) as $entry
| .[3] += $entry # result += entry
| .[0][.[1]] = .[2] + $entry[0:1] # dictionary[dictSize] = w + entry.charAt(0);
| .[1] += 1 # dictSize++
| .[2] = $entry # w = entry
) | .[3]
;Example:
"TOBEORNOTTOBEORTOBEORNOT" | lzw_compress| lzw_decompress- Output:
$ jq -n -f LZW.jq "TOBEORNOTTOBEORTOBEORNOT"
Julia
function compressLZW(decompressed::String)
dictsize = 256
dict = Dict{String,Int}(string(Char(i)) => i for i in 0:dictsize)
result = Vector{Int}(undef, 0)
w = ""
for c in decompressed
wc = string(w, c)
if haskey(dict, wc)
w = wc
else
push!(result, dict[w])
dict[wc] = dictsize
dictsize += 1
w = string(c)
end
end
if !isempty(w) push!(result, dict[w]) end
return result
end
function decompressLZW(compressed::Vector{Int})
dictsize = 256
dict = Dict{Int,String}(i => string('\0' + i) for i in 0:dictsize)
result = IOBuffer()
w = string(Char(popfirst!(compressed)))
write(result, w)
for k in compressed
if haskey(dict, k)
entry = dict[k]
elseif k == dictsize
entry = string(w, w[1])
else
error("bad compressed k: $k")
end
write(result, entry)
dict[dictsize] = string(w, entry[1])
dictsize += 1
w = entry
end
return String(take!(result))
end
original = ["0123456789", "TOBEORNOTTOBEORTOBEORNOT", "dudidudidudida"]
compressed = compressLZW.(original)
decompressed = decompressLZW.(compressed)
for (word, comp, decomp) in zip(original, compressed, decompressed)
comprate = (length(word) - length(comp)) / length(word) * 100
println("Original: $word\n-> Compressed: $comp (compr.rate: $(round(comprate, digits=2))%)\n-> Decompressed: $decomp")
end
- Output:
Original: 0123456789 -> Compressed: [49, 50, 51, 52, 53, 54, 55, 56, 57] (compr.rate: 10.0%) -> Decompressed: 0123456789 Original: TOBEORNOTTOBEORTOBEORNOT -> Compressed: [79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] (compr.rate: 37.5%) -> Decompressed: TOBEORNOTTOBEORTOBEORNOT Original: dudidudidudida -> Compressed: [117, 100, 105, 256, 258, 260, 259, 97] (compr.rate: 42.86%) -> Decompressed: dudidudidudida
Kotlin
// version 1.1.2
object Lzw {
/** Compress a string to a list of output symbols. */
fun compress(uncompressed: String): MutableList<Int> {
// Build the dictionary.
var dictSize = 256
val dictionary = mutableMapOf<String, Int>()
(0 until dictSize).forEach { dictionary.put(it.toChar().toString(), it)}
var w = ""
val result = mutableListOf<Int>()
for (c in uncompressed) {
val wc = w + c
if (dictionary.containsKey(wc))
w = wc
else {
result.add(dictionary[w]!!)
// Add wc to the dictionary.
dictionary.put(wc, dictSize++)
w = c.toString()
}
}
// Output the code for w
if (!w.isEmpty()) result.add(dictionary[w]!!)
return result
}
/** Decompress a list of output symbols to a string. */
fun decompress(compressed: MutableList<Int>): String {
// Build the dictionary.
var dictSize = 256
val dictionary = mutableMapOf<Int, String>()
(0 until dictSize).forEach { dictionary.put(it, it.toChar().toString())}
var w = compressed.removeAt(0).toChar().toString()
val result = StringBuilder(w)
for (k in compressed) {
var entry: String
if (dictionary.containsKey(k))
entry = dictionary[k]!!
else if (k == dictSize)
entry = w + w[0]
else
throw IllegalArgumentException("Bad compressed k: $k")
result.append(entry)
// Add w + entry[0] to the dictionary.
dictionary.put(dictSize++, w + entry[0])
w = entry
}
return result.toString()
}
}
fun main(args: Array<String>) {
val compressed = Lzw.compress("TOBEORNOTTOBEORTOBEORNOT")
println(compressed)
val decompressed = Lzw.decompress(compressed)
println(decompressed)
}
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Liberty BASIC
The encoder features variable-bit output, a 12 to 21 bit rotating dictionary (that can also be set to "Static"), and an unbalanced binary search tree that assures a worst-case-scenario maximum of 256 searches to find any given index, regardless of the dictionary's size. It uses both read and write buffers so is capable of handling files of any size, and it adds a settings-byte to the beginning of the encoded file to retain the maximum bit-width and rotating status of the dictionary. It also has the option to write the encoding/decoding dictionaries to file so the encoder can be checked for accuracy. This code directly follows the methodology described in an excellent web article by Juha Nieminen entitled "An efficient LZW implementation".
DIM LZW(1, 1)
DIM JDlzw(1)
DIM JDch$(1)
LET maxBits = 20 ' maximum bit width of the dictionary: minimum=12; maximum=21
LET resetDictionary = 1 ' flag to reset the dictionary when it gets full: 1=TRUE; 0=FALSE
LET printDictionary = 0 ' output encoding and decoding dictionaries to files
LET maxChunkSize = 2 ^ 14 ' maximum size of the data buffer
LET dSize = 2 ^ maxBits ' maximum dictionary size
LET JDext$ = ".lzw" ' file extension used for created archives
FILEDIALOG "Select a file to test LZW...", "*.*", inputName$
IF inputName$ = "" THEN END
DO ' get fullPath\ and fileName.ext
P = X
X = INSTR(inputName$, "\", (X + 1))
LOOP UNTIL X = 0
filePath$ = LEFT$(inputName$, P)
fileName$ = MID$(inputName$, (P + 1))
DO ' get fileName and .ext
P = X
X = INSTR(fileName$, ".", (X + 1))
LOOP UNTIL X = 0
fileExt$ = MID$(fileName$, P)
fileName$ = LEFT$(fileName$, (P - 1))
GOSUB [lzwEncode]
GOSUB [lzwDecode]
END
''''''''''''''''''''''''''''''''''''''''
' Start LZW Encoder ''''''''''''''''''''
[lzwEncode]
REDIM LZW(dSize, 4)
LET EMPTY=-1:PREFIX=0:BYTE=1:FIRST=2:LESS=3:MORE=4:bmxCorrect=1
LET bitsRemain=0:remainIndex=0:tagCount=0:currentBitSize=8:fileTag$=""
FOR dNext = 0 TO 255 ' initialize dictionary for LZW
' LZW(dNext, PREFIX) = EMPTY ' prefix index of '<index>' <B>
' LZW(dNext, BYTE) = dNext ' byte value of <index> '<B>'
LZW(dNext, FIRST) = EMPTY ' first index to use <index><B> as prefix
' LZW(dNext, LESS) = EMPTY ' lesser index of binary search tree for <B>
' LZW(dNext, MORE) = EMPTY ' greater index of binary search tree for <B>
NEXT dNext
OPEN inputName$ FOR INPUT AS #lzwIN
IF LOF(#lzwIN) < 2 THEN
CLOSE #lzwIN
END
END IF
OPEN fileName$ + fileExt$ + JDext$ FOR OUTPUT AS #lzwOUT
GOSUB [StartFileChunk]
chnkPoint = 1
IF maxBits < 12 THEN maxBits = 12
IF maxBits > 21 THEN maxBits = 21
settings = maxBits - 12 ' setting for dictionary size; 1st decimal +12
IF resetDictionary THEN settings = settings + 100 ' setting for dictionary type; 2nd decimal even=static, odd=adaptive
#lzwOUT, CHR$(settings); ' save settings as 1st byte of output
orgIndex = ASC(LEFT$(fileChunk$, 1)) ' read 1st byte into <index>
WHILE fileChunk$ <> "" ' while the buffer is not empty
DO ' begin the main encoder loop
chnkPoint = chnkPoint + 1
savIndex = FIRST ' initialize the save-to index
prvIndex = orgIndex ' initialize the previous index in search
newByte = ASC(MID$(fileChunk$, chnkPoint, 1)) ' read <B>
dSearch = LZW(orgIndex, FIRST) ' first search index for this <index> in the dictionary
WHILE (dSearch > EMPTY) ' while <index> is present in the dictionary
IF LZW(dSearch, BYTE) = newByte THEN EXIT WHILE ' if <index><B> is found
IF newByte < LZW(dSearch, BYTE) THEN ' else if new <B> is less than <index><B>
savIndex = LESS ' follow lesser binary tree
ELSE
savIndex = MORE ' else follow greater binary tree
END IF
prvIndex = dSearch ' set previous <index>
dSearch = LZW(dSearch, savIndex) ' read next search <index> from binary tree
WEND
IF dSearch = EMPTY THEN ' if <index><B> was not found in the dictionary
GOSUB [WriteIndex] ' write <index> to the output
IF dNext < dSize THEN ' save <index><B> into the dictionary
LZW(prvIndex, savIndex) = dNext
LZW(dNext, PREFIX) = orgIndex
LZW(dNext, BYTE) = newByte
LZW(dNext, FIRST) = EMPTY
LZW(dNext, LESS) = EMPTY
LZW(dNext, MORE) = EMPTY
IF dNext = (2 ^ currentBitSize) THEN currentBitSize = currentBitSize + 1
dNext = dNext + 1
ELSE ' else reset the dictionary... or maybe not
IF resetDictionary THEN
GOSUB [PrintEncode]
REDIM LZW(dSize, 4)
FOR dNext = 0 TO 255
LZW(dNext, FIRST) = EMPTY
NEXT dNext
currentBitSize = 8
bmxCorrect = 0
END IF
END IF
orgIndex = newByte ' set <index> = <B>
ELSE ' if <index><B> was found in the dictionary,
orgIndex = dSearch ' then set <index> = <index><B>
END IF
LOOP WHILE chnkPoint < chunk ' loop until the chunk has been processed
GOSUB [GetFileChunk] ' refill the buffer
WEND ' loop until the buffer is empty
GOSUB [WriteIndex]
IF bitsRemain > 0 THEN #lzwOUT, CHR$(remainIndex);
CLOSE #lzwOUT
CLOSE #lzwIN
IF bmxCorrect THEN ' correct the settings, if needed
IF (currentBitSize < maxBits) OR resetDictionary THEN
IF currentBitSize < 12 THEN currentBitSize = 12
OPEN fileName$ + fileExt$ + JDext$ FOR BINARY AS #lzwOUT
#lzwOUT, CHR$(currentBitSize - 12);
CLOSE #lzwOUT
END IF
END IF
GOSUB [PrintEncode]
REDIM LZW(1, 1)
RETURN
[WriteIndex]
X = orgIndex ' add remaining bits to input
IF bitsRemain > 0 THEN X = remainIndex + (X * (2 ^ bitsRemain))
bitsRemain = bitsRemain + currentBitSize ' add current bit size to output stack
WHILE bitsRemain > 7 ' if 8 or more bits are to be written
#lzwOUT, CHR$(X MOD 256); ' attatch lower 8 bits to output string
X = INT(X / 256) ' shift input value down by 2^8
bitsRemain = bitsRemain - 8 ' adjust counters
WEND
remainIndex = X ' retain trailing bits for next write
RETURN
' End LZW Encoder ''''''''''''''''''''''
''''''''''''''''''''''''''''''''''''''''
[StartFileChunk]
sizeOfFile = LOF(#lzwIN) ' set EOF marker
bytesRemaining = sizeOfFile ' set EOF counter
chunk = maxChunkSize ' set max buffer size
[GetFileChunk]
fileChunk$ = ""
IF bytesRemaining < 1 THEN RETURN
IF chunk > bytesRemaining THEN chunk = bytesRemaining
bytesRemaining = bytesRemaining - chunk
fileChunk$ = INPUT$(#lzwIN, chunk)
chnkPoint = 0
RETURN
''''''''''''''''''''''''''''''''''''''''
' Start LZW Decoder ''''''''''''''''''''
[lzwDecode]
LET EMPTY=-1:bitsRemain=0:tagCount=0:fileTag$=""
OPEN fileName$ + fileExt$ + JDext$ FOR INPUT AS #lzwIN
OPEN fileName$ + ".Copy" + fileExt$ FOR OUTPUT AS #lzwOUT
GOSUB [StartFileChunk]
chnkPoint = 2
settings = ASC(fileChunk$)
maxBits = VAL(RIGHT$(STR$(settings), 1)) + 12
dSize = 2 ^ maxBits
IF settings > 99 THEN resetDictionary = 1
GOSUB [ResetLZW]
oldIndex = orgIndex
WHILE fileChunk$ <> ""
' decode current index and write to file
GOSUB [GetIndex]
IF JDch$(orgIndex) = "" THEN
tmpIndex = oldIndex
tmp$ = JDch$(tmpIndex)
WHILE JDlzw(tmpIndex) > EMPTY
tmpIndex = JDlzw(tmpIndex)
tmp$ = JDch$(tmpIndex) + tmp$
WEND
tmp$ = tmp$ + LEFT$(tmp$, 1)
ELSE
tmpIndex = orgIndex
tmp$ = JDch$(tmpIndex)
WHILE JDlzw(tmpIndex) > EMPTY
tmpIndex = JDlzw(tmpIndex)
tmp$ = JDch$(tmpIndex) + tmp$
WEND
END IF
#lzwOUT, tmp$;
' add next dictionary entry or reset dictionary
IF dNext < dSize THEN
JDlzw(dNext) = oldIndex
JDch$(dNext) = LEFT$(tmp$, 1)
dNext = dNext + 1
IF dNext = (2 ^ currentBitSize) THEN
IF maxBits > currentBitSize THEN
currentBitSize = currentBitSize + 1
ELSE
IF resetDictionary THEN
GOSUB [PrintDecode]
GOSUB [ResetLZW]
END IF
END IF
END IF
END IF
oldIndex = orgIndex
WEND
CLOSE #lzwOUT
CLOSE #lzwIN
GOSUB [PrintDecode]
REDIM JDlzw(1)
REDIM JDch$(1)
RETURN
[GetIndex]
byteCount = 0:orgIndex = 0
bitsToGrab = currentBitSize - bitsRemain
IF bitsRemain > 0 THEN
orgIndex = lastByte
byteCount = 1
END IF
WHILE bitsToGrab > 0
lastByte = ASC(MID$(fileChunk$, chnkPoint, 1))
orgIndex = orgIndex + (lastByte * (2 ^ (byteCount * 8)))
IF chnkPoint = chunk THEN GOSUB [GetFileChunk]
chnkPoint = chnkPoint + 1
byteCount = byteCount + 1
bitsToGrab = bitsToGrab - 8
WEND
IF bitsRemain > 0 THEN orgIndex = orgIndex / (2 ^ (8 - bitsRemain))
orgIndex = orgIndex AND ((2 ^ currentBitSize) - 1)
bitsRemain = bitsToGrab * (-1)
RETURN
[ResetLZW]
REDIM JDlzw(dSize)
REDIM JDch$(dSize)
FOR dNext = 0 TO 255
JDlzw(dNext) = EMPTY ' Prefix index
JDch$(dNext) = CHR$(dNext) ' New byte value
NEXT dNext
currentBitSize = 8
GOSUB [GetIndex]
#lzwOUT, JDch$(orgIndex);
currentBitSize = 9
RETURN
' End LZW Decoder ''''''''''''''''''''''
''''''''''''''''''''''''''''''''''''''''
''''''''''''''''''''''''''''''''''''''''
[PrintEncode]
IF printDictionary < 1 THEN RETURN
OPEN "Encode_" + fileTag$ + fileName$ + ".txt" FOR OUTPUT AS #dictOUT
FOR X = 0 TO 255
LZW(X, PREFIX) = EMPTY
LZW(X, BYTE) = X
NEXT X
FOR X = dNext TO 0 STEP -1
tmpIndex = X
tmp$ = CHR$(LZW(tmpIndex, BYTE))
WHILE LZW(tmpIndex, PREFIX) > EMPTY
tmpIndex = LZW(tmpIndex, PREFIX)
tmp$ = CHR$(LZW(tmpIndex, BYTE)) + tmp$
WEND
#dictOUT, X; ":"; tmp$
NEXT X
CLOSE #dictOUT
tagCount = tagCount + 1
fileTag$ = STR$(tagCount) + "_"
RETURN
[PrintDecode]
IF printDictionary < 1 THEN RETURN
OPEN "Decode_" + fileTag$ + fileName$ + ".txt" FOR OUTPUT AS #dictOUT
FOR X = dNext TO 0 STEP -1
tmpIndex = X
tmp$ = JDch$(tmpIndex)
WHILE JDlzw(tmpIndex) > EMPTY
tmpIndex = JDlzw(tmpIndex)
tmp$ = JDch$(tmpIndex) + tmp$
WEND
#dictOUT, X; ":"; tmp$
NEXT X
CLOSE #dictOUT
tagCount = tagCount + 1
fileTag$ = STR$(tagCount) + "_"
RETURN
''''''''''''''''''''''''''''''''''''''''
Lua
local function compress(uncompressed) -- string
local dictionary, result, dictSize, w, c = {}, {}, 255, ""
for i = 0, 255 do
dictionary[string.char(i)] = i
end
for i = 1, #uncompressed do
c = string.sub(uncompressed, i, i)
if dictionary[w .. c] then
w = w .. c
else
table.insert(result, dictionary[w])
dictSize = dictSize + 1
dictionary[w .. c] = dictSize
w = c
end
end
if w ~= "" then
table.insert(result, dictionary[w])
end
return result
end
local function decompress(compressed) -- table
local dictionary, dictSize, entry, w, k = {}, 256, "", string.char(compressed[1])
local result = {w}
for i = 0, 255 do
dictionary[i] = string.char(i)
end
for i = 2, #compressed do
k = compressed[i]
if dictionary[k] then
entry = dictionary[k]
elseif k == dictSize then
entry = w .. string.sub(w, 1, 1)
else
return nil, i
end
table.insert(result, entry)
dictionary[dictSize] = w .. string.sub(entry, 1, 1)
dictSize = dictSize + 1
w = entry
end
return table.concat(result)
end
local example = "TOBEORNOTTOBEORTOBEORNOT"
local com = compress(example)
local dec = decompress(com)
print(table.concat(com, ", "))
print(dec)
- Output:
84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263 TOBEORNOTTOBEORTOBEORNOT
M2000 Interpreter
Module BBCtrans {
\\ LZW compression
plaintext$="TOBEORNOTTOBEORTOBEORNOT"
Function encodeLZW$(i$) {
Def long c, d, i, l, o$, w$
DIM dict$(0 to 4095)
FOR i = 0 TO 255 : dict$(i) = CHR$(i) : NEXT i
l = i
i = 1
w$ = LEFT$(i$,1)
REPEAT{
d = 0
REPEAT {
c = d
IF i > LEN(i$) THEN EXIT
FOR d = 1 TO l-1
IF w$ = dict$(d) THEN EXIT
NEXT d
IF d < l Then i += 1 : w$ += MID$(i$, i, 1)
} UNTIL d >= l
dict$(l) = w$ : l += 1 : w$ = RIGHT$(w$, 1)
o$ += CHR$(c MOD 256) + CHR$(c DIV 256)
} UNTIL i > LEN(i$)
= o$
}
encodeLZW$ = encodeLZW$(plaintext$)
FOR i = 1 TO LEN(encodeLZW$) STEP 2
PRINT ASC(MID$(encodeLZW$,i)) + 256*ASC(MID$(encodeLZW$,i+1));" ";
NEXT i
Print
Function decodeLZW$(i$) {
Def c, i, l, o$, t$, w$
DIM dict$(0 to 4095)
FOR i = 0 TO 255 : dict$(i) = CHR$(i) : NEXT i
l = i
c = ASC(i$) + 256*ASC(MID$(i$,2))
w$ = dict$(c)
o$ = w$
IF LEN(i$) < 4 THEN = o$
FOR i = 3 TO LEN(i$) STEP 2
c = ASC(MID$(i$,i)) + 256*ASC(MID$(i$,i+1))
IF c < l Then { t$ = dict$(c) } ELSE t$ = w$ + LEFT$(w$,1)
o$ += t$
dict$(l) = w$ + LEFT$(t$,1)
l += 1
w$ = t$
NEXT i
= o$
}
Print decodeLZW$(encodeLZW$)
}
BBCtransAnd here a change for using Inventories, where we have hash function, and we find entry in O(1).
Module FastM2000 {
plaintext$="TOBEORNOTTOBEORTOBEORNOT"
Function encodeLZW$(i$) {
Def long c, d, i, l, o$, w$
Inventory dict
For i = 0 to 255 {Append dict , Chr$(i):=i}
l = i
i = 1
w$ = LEFT$(i$,1)
REPEAT{
d = 0
Repeat {
c = d
IF i > Len(i$) Then Exit
if exist(dict, w$) Then {
d=eval(dict)
} Else Append dict, w$:=l: Exit
if d<l Then i += 1 : w$ += Mid$(i$, i, 1)
} Until d >= l
l += 1 : w$ = Right$(w$, 1)
o$ += Chr$(c Mod 256) + Chr$(c div 256)
} Until i > Len(i$)
= o$
}
encodeLZW$ = encodeLZW$(plaintext$)
Document Doc$
For i = 1 to Len(encodeLZW$) STEP 2
Doc$= Str$(Asc(Mid$(encodeLZW$,i)) + 256*Asc(Mid$(encodeLZW$,i+1)))
Next i
Doc$={
}
Function decodeLZW$(i$) {
Def c, i, l, o$, t$, w$
Inventory Dict
For i = 0 to 255 {Append dict , i:=chr$(i)}
l = i
c = Asc(i$) + 256*Asc(Mid$(i$,2))
w$ = dict$(c)
o$ = w$
IF Len(i$) < 4 Then = o$
For i = 3 to Len(i$) STEP 2 {
c = Asc(Mid$(i$,i)) + 256*Asc(Mid$(i$,i+1))
IF c < l Then {
t$ = dict$(c)
} Else t$ = w$ + LEFT$(w$,1)
o$ += t$
Append dict, l:=w$ + LEFT$(t$,1)
l += 1 : w$ = t$
}
= o$
}
Doc$=decodeLZW$(encodeLZW$)+{
}
Clipboard Doc$
Report Doc$
}
FastM2000- Output:
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Mathematica/Wolfram Language
compress[uncompressed_] :=
Module[{dictsize, dictionary, w, result, wc},
dictsize = 256;
dictionary = # -> # & /@ FromCharacterCode /@ Range@dictsize;
w = "";
result = {};
Do[wc = w <> c;
If[MemberQ[dictionary[[All, 1]], wc],
w = wc,
AppendTo[result, w /. dictionary];
AppendTo[dictionary, wc -> dictsize];
dictsize++;
w = c],
{c, Characters[uncompressed]}];
AppendTo[result, w /. dictionary];
result];
decompress::bc = "Bad compressed `1`";
decompress[compressed_] :=
Module[{dictsize, dictionary, w, result, entry},
dictsize = 256;
dictionary = # -> # & /@ FromCharacterCode /@ Range@dictsize;
w = result = compressed[[1]];
Do[Which[MemberQ[dictionary[[All, 1]], k],
entry = k /. dictionary,
k == dictsize,
entry = w <> StringTake[w, 1],
True,
Message[decompress::bc, k]];
result = result <> entry;
AppendTo[dictionary, dictsize -> w <> StringTake[entry, 1]];
dictsize++;
w = entry,
{k, compressed[[2 ;;]]}];
result];
(*How to use:*)
compress["TOBEORNOTTOBEORTOBEORNOT"]
decompress[%]
- Output:
{"T", "O", "B", "E", "O", "R", "N", "O", "T", 256, 258, 260, 265, 259, 261, 263}
"TOBEORNOTTOBEORTOBEORNOT"
(* stm, 6 June 2022: I did not edit the above code, but I believe that there is a small error. The Range@dictsize should be replaced by Range[0,dictsize-1] for LZW standard; Mathematica runs from 1 to 256 instead of 0 to 255. The code works as is in String format in Mathematica because Mathematica distinguishes between Strings and numbers. However, this example fails when the code is adapted to byte type input and output, as used in images. Adjusting Range[...] fixes the problem.*)
Nim
import tables
proc compress*(uncompressed: string): seq[int] =
# Build the dictionary.
var dictionary: Table[string, int]
for i in 0..255: dictionary[$chr(i)] = i
var w = ""
for c in uncompressed:
var wc = w & c
if wc in dictionary:
w = wc
else:
# Writes "w" to output.
result.add dictionary[w]
# "wc" is a new sequence: add it to the dictionary.
dictionary[wc] = dictionary.len
w = $c
# Write remaining output if necessary.
if w.len > 0: result.add dictionary[w]
proc decompress*(compressed: var seq[int]): string =
# Build the dictionary.
var dictionary: Table[int, string]
for i in 0..255: dictionary[i] = $chr(i)
var w = dictionary[compressed[0]]
compressed.delete(0)
result = w
for k in compressed:
var entry: string
if k in dictionary:
entry = dictionary[k]
elif k == dictionary.len:
entry = w & w[0]
else:
raise newException(ValueError, "Bad compressed k: " & $k)
result.add entry
# New sequence: add it to the dictionary.
dictionary[dictionary.len] = w & entry[0]
w = entry
when isMainModule:
var compressed = compress("TOBEORNOTTOBEORTOBEORNOT")
echo compressed
var decompressed = decompress(compressed)
echo decompressed
- Output:
@[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Objeck
use Collection;
class LZW {
function : Main(args : String[]) ~ Nil {
compressed := Compress("TOBEORNOTTOBEORTOBEORNOT");
Show(compressed);
decompressed := Decompress(compressed);
decompressed->PrintLine();
}
function : native : Compress(uncompressed : String) ~ IntVector {
# Build the dictionary.
dictSize := 256;
dictionary := StringMap->New();
for (i := 0; i < 256; i+=1;) {
key := "";
key->Append(i->As(Char));
dictionary->Insert(key, IntHolder->New(i));
};
w := "";
result := IntVector->New();
each (i : uncompressed) {
c := uncompressed->Get(i);
wc := String->New(w);
wc->Append(c);
if (dictionary->Has(wc)) {
w := wc;
}
else {
value := dictionary->Find(w)->As(IntHolder);
result->AddBack(value->Get());
# Add wc to the dictionary.
dictionary->Insert(wc, IntHolder->New(dictSize));
dictSize+=1;
w := "";
w->Append(c);
};
};
# Output the code for w.
if (w->Size() > 0) {
value := dictionary->Find(w)->As(IntHolder);
result->AddBack(value->Get());
};
return result;
}
function : Decompress(compressed : IntVector) ~ String {
# Build the dictionary.
dictSize := 256;
dictionary := IntMap->New();
for (i := 0; i < 256; i+=1;) {
value := "";
value->Append(i->As(Char));
dictionary->Insert(i, value);
};
w := "";
found := compressed->Remove(0);
w->Append(found->As(Char));
result := String->New(w);
each (i : compressed) {
k := compressed->Get(i);
entry : String;
if (dictionary->Has(k)) {
entry := dictionary->Find(k);
}
else if (k = dictSize) {
entry := String->New(w);
entry->Append(w->Get(0));
}
else {
return "";
};
result->Append(entry);
# Add w+entry[0] to the dictionary.
value := String->New(w);
value->Append(entry->Get(0));
dictionary->Insert(dictSize, value);
dictSize+=1;
w := entry;
};
return result;
}
function : Show(results : IntVector) ~ Nil {
"["->Print();
each(i : results) {
results->Get(i)->Print();
if(i + 1 < results->Size()) {
", "->Print();
};
};
"]"->PrintLine();
}
}[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Objective-C
The class for the LZW compression algorithm:
#import <Foundation/Foundation.h>
#import <stdio.h>
@interface LZWCompressor : NSObject
{
@private
NSMutableArray *iostream;
NSMutableDictionary *dict;
NSUInteger codemark;
}
-(instancetype) init;
-(instancetype) initWithArray: (NSMutableArray *) stream;
-(BOOL) compressData: (NSData *) string;
-(void) setArray: (NSMutableArray *) stream;
-(NSArray *) getArray;
@end
@implementation LZWCompressor : NSObject
-(instancetype) init
{
self = [super init];
if ( self )
{
iostream = nil;
codemark = 256;
dict = [[NSMutableDictionary alloc] initWithCapacity: 512];
}
return self;
}
-(instancetype) initWithArray: (NSMutableArray *) stream
{
self = [self init];
if ( self )
{
[self setArray: stream];
}
return self;
}
-(void) setArray: (NSMutableArray *) stream
{
iostream = stream;
}
-(BOOL) compressData: (NSData *) string;
{
// prepare dict
for(NSUInteger i=0; i < 256; i++)
{
unsigned char j = i;
NSData *s = [NSData dataWithBytes: &j length: 1];
dict[s] = @(i);
}
NSData *w = [NSData data];
for(NSUInteger i=0; i < [string length]; i++)
{
NSMutableData *wc = [NSMutableData dataWithData: w];
[wc appendData: [string subdataWithRange: NSMakeRange(i, 1)]];
if ( dict[wc] != nil )
{
w = wc;
} else {
[iostream addObject: dict[w]];
dict[wc] = @(codemark);
codemark++;
w = [string subdataWithRange: NSMakeRange(i, 1)];
}
}
if ( [w length] != 0 )
{
[iostream addObject: dict[w]];
}
return YES;
}
-(NSArray *) getArray
{
return iostream;
}
@end
Usage example:
NSString *text = @"TOBEORNOTTOBEORTOBEORNOT";
int main()
{
@autoreleasepool {
NSMutableArray *array = [[NSMutableArray alloc] init];
LZWCompressor *lzw = [[LZWCompressor alloc]
initWithArray: array ];
if ( lzw )
{
[lzw compressData: [text dataUsingEncoding: NSUTF8StringEncoding]];
for ( id obj in array )
{
printf("%u\n", [obj unsignedIntValue]);
}
}
}
return EXIT_SUCCESS;
}
- Output:
(reformatted by hand)
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
OCaml
#directory "+extlib" (* or maybe "+site-lib/extlib/" *)
#load "extLib.cma"
open ExtString
(** compress a string to a list of output symbols *)
let compress ~uncompressed =
(* build the dictionary *)
let dict_size = 256 in
let dictionary = Hashtbl.create 397 in
for i=0 to 255 do
let str = String.make 1 (char_of_int i) in
Hashtbl.add dictionary str i
done;
let f = (fun (w, dict_size, result) c ->
let c = String.make 1 c in
let wc = w ^ c in
if Hashtbl.mem dictionary wc then
(wc, dict_size, result)
else
begin
(* add wc to the dictionary *)
Hashtbl.add dictionary wc dict_size;
let this = Hashtbl.find dictionary w in
(c, dict_size + 1, this::result)
end
) in
let w, _, result =
String.fold_left f ("", dict_size, []) uncompressed
in
(* output the code for w *)
let result =
if w = ""
then result
else (Hashtbl.find dictionary w) :: result
in
(List.rev result)
;;
exception ValueError of string
(** decompress a list of output symbols to a string *)
let decompress ~compressed =
(* build the dictionary *)
let dict_size = 256 in
let dictionary = Hashtbl.create 397 in
for i=0 to pred dict_size do
let str = String.make 1 (char_of_int i) in
Hashtbl.add dictionary i str
done;
let w, compressed =
match compressed with
| hd::tl -> (String.make 1 (char_of_int hd)), tl
| [] -> failwith "empty input"
in
let result = [w] in
let result, _, _ =
List.fold_left (fun (result, w, dict_size) k ->
let entry =
if Hashtbl.mem dictionary k then
Hashtbl.find dictionary k
else if k = Hashtbl.length dictionary then
w ^ (String.make 1 w.[0])
else
raise(ValueError(Printf.sprintf "Bad compressed k: %d" k))
in
let result = entry :: result in
(* add (w ^ entry.[0]) to the dictionary *)
Hashtbl.add dictionary dict_size (w ^ (String.make 1 entry.[0]));
(result, entry, dict_size + 1)
) (result, w, dict_size) compressed
in
(List.rev result)
;;
here is the interface:
val compress : uncompressed:string -> int list
val decompress : compressed:int list -> string list
How to use:
The compressed datas are a list of symbols (of type int) that will require more than 8 bits to be saved.
So to know how many bits are required, you need to know how many bits are required for the greatest symbol in the list.
let greatest = List.fold_left max 0 ;;
(** number of bits needed to encode the integer m *)
let n_bits m =
let m = float m in
let rec aux n =
let max = (2. ** n) -. 1. in
if max >= m then int_of_float n
else aux (n +. 1.0)
in
aux 1.0
;;
let write_compressed ~filename ~compressed =
let nbits = n_bits(greatest compressed) in
let oc = open_out filename in
output_byte oc nbits;
let ob = IO.output_bits(IO.output_channel oc) in
List.iter (IO.write_bits ob nbits) compressed;
IO.flush_bits ob;
close_out oc;
;;
let read_compressed ~filename =
let ic = open_in filename in
let nbits = input_byte ic in
let ib = IO.input_bits(IO.input_channel ic) in
let rec loop acc =
try
let code = IO.read_bits ib nbits in
loop (code::acc)
with _ -> List.rev acc
in
let compressed = loop [] in
let result = decompress ~compressed in
let buf = Buffer.create 2048 in
List.iter (Buffer.add_string buf) result;
(Buffer.contents buf)
;;
Ol
This version use lazy streams which is pair (symbol . function-to-get-next-symbol).
(define (compress str)
(let loop ((dc (fold (lambda (f x) ; dictionary (simplest, not optimized), with reversed codes
(cons (list x) (cons x f)))
'() (iota 256)))
(w '()) ; output sequence (reversed)
(s 256) ; maximal dictionary code value + 1
(x '()) ; current sequence
(r (str-iter str))); input stream
(cond
((null? r)
(reverse (cons (cadr (member x dc)) w)))
((pair? r)
(let ((xy (cons (car r) x)))
(if (member xy dc)
(loop dc w s xy (cdr r))
(loop (cons xy (cons s dc)) ; update dictionary with xy . s
(cons (cadr (member x dc)) w) ; add code to output stream
(+ s 1) ; increase code
(list (car r)) ; new current sequence
(cdr r))))) ; next input
(else
(loop dc w s x (r)))))
)
(print (compress "TOBEORNOTTOBEORTOBEORNOT")) ; => (84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)
And decoder (runes->string used to unify functions - both used string iterators):
(define (decompress str)
(let loop ((dc (fold (lambda (f x) ; dictionary (simplest, not optimized), with reversed codes
(cons x (cons (list x) f)))
'() (iota 256)))
(w '()) ; output sequence (reversed)
(s 256) ; maximal dictionary code value + 1
(x '()) ; current symbols sequence
(r (str-iter str))); input stream
(cond
((null? r)
(reverse w))
((pair? r)
(let*((y (cadr (member (car r) dc)))
(xy (append y x)))
(if (member xy dc)
(loop dc (append y w) s xy (cdr r)) ; вряд ли такое будет...
(loop (cons s (cons xy dc)) ; update dictionary with xy . s
(append y w) ; add phrase to output stream
(+ s 1)
y ; new initial code
(cdr r))))) ; next input
(else
(loop dc w s x (r))))))
(print (runes->string
(decompress (runes->string '(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)))))
; => TOBEORNOTTOBEORTOBEEORNOT
Perl
In this version the hashes contain mixed typed data:
# Compress a string to a list of output symbols.
sub compress {
my $uncompressed = shift;
# Build the dictionary.
my $dict_size = 256;
my %dictionary = map {chr $_ => chr $_} 0..$dict_size-1;
my $w = "";
my @result;
foreach my $c (split '', $uncompressed) {
my $wc = $w . $c;
if (exists $dictionary{$wc}) {
$w = $wc;
} else {
push @result, $dictionary{$w};
# Add wc to the dictionary.
$dictionary{$wc} = $dict_size;
$dict_size++;
$w = $c;
}
}
# Output the code for w.
if ($w) {
push @result, $dictionary{$w};
}
return @result;
}
# Decompress a list of output ks to a string.
sub decompress {
my @compressed = @_;
# Build the dictionary.
my $dict_size = 256;
my %dictionary = map {chr $_ => chr $_} 0..$dict_size-1;
my $w = shift @compressed;
my $result = $w;
foreach my $k (@compressed) {
my $entry;
if (exists $dictionary{$k}) {
$entry = $dictionary{$k};
} elsif ($k == $dict_size) {
$entry = $w . substr($w,0,1);
} else {
die "Bad compressed k: $k";
}
$result .= $entry;
# Add w+entry[0] to the dictionary.
$dictionary{$dict_size} = $w . substr($entry,0,1);
$dict_size++;
$w = $entry;
}
return $result;
}
# How to use:
my @compressed = compress('TOBEORNOTTOBEORTOBEORNOT');
print "@compressed\n";
my $decompressed = decompress(@compressed);
print "$decompressed\n";
- Output:
T O B E O R N O T 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Phix
with javascript_semantics function compress(string uncompressed) integer dict = new_dict() sequence result = {} integer dictSize = 255, c string word = "" for i=0 to 255 do setd(""&i,i,dict) end for for i=1 to length(uncompressed) do c = uncompressed[i] if getd_index(word&c,dict) then word &= c else result &= getd(word,dict) dictSize += 1 setd(word&c,dictSize,dict) word = ""&c end if end for if word!="" then result &= getd(word,dict) end if destroy_dict(dict) return result end function function decompress(sequence compressed) integer dict = new_dict() integer dictSize = 255, k, ki string dent = "", result = "", word = "" for i=0 to 255 do setd(i,""&i,dict) end for for i=1 to length(compressed) do k = compressed[i] ki = getd_index(k,dict) if ki then dent = getd_by_index(ki,dict) elsif k=dictSize then dent = word&word[1] else return {NULL,i} end if result &= dent setd(dictSize,word&dent[1],dict) dictSize += 1 word = dent end for destroy_dict(dict) return result end function constant example = "TOBEORNOTTOBEORTOBEORNOT" sequence com = compress(example) pp(com,{pp_IntCh,true,pp_Maxlen,90}) ?decompress(com)
- Output:
{84'T',79'O',66'B',69'E',79'O',82'R',78'N',79'O',84'T',256,258,260,265,259,261,263}
"TOBEORNOTTOBEORTOBEORNOT"
PHP
class LZW
{
function compress($unc) {
$i;$c;$wc;
$w = "";
$dictionary = array();
$result = array();
$dictSize = 256;
for ($i = 0; $i < 256; $i += 1) {
$dictionary[chr($i)] = $i;
}
for ($i = 0; $i < strlen($unc); $i++) {
$c = $unc[$i];
$wc = $w.$c;
if (array_key_exists($w.$c, $dictionary)) {
$w = $w.$c;
} else {
array_push($result,$dictionary[$w]);
$dictionary[$wc] = $dictSize++;
$w = (string)$c;
}
}
if ($w !== "") {
array_push($result,$dictionary[$w]);
}
return implode(",",$result);
}
function decompress($com) {
$com = explode(",",$com);
$i;$w;$k;$result;
$dictionary = array();
$entry = "";
$dictSize = 256;
for ($i = 0; $i < 256; $i++) {
$dictionary[$i] = chr($i);
}
$w = chr($com[0]);
$result = $w;
for ($i = 1; $i < count($com);$i++) {
$k = $com[$i];
if (isset($dictionary[$k])) {
$entry = $dictionary[$k];
} else {
if ($k === $dictSize) {
$entry = $w.$w[0];
} else {
return null;
}
}
$result .= $entry;
$dictionary[$dictSize++] = $w . $entry[0];
$w = $entry;
}
return $result;
}
}
//How to use
$str = 'TOBEORNOTTOBEORTOBEORNOT';
$lzw = new LZW();
$com = $lzw->compress($str);
$dec = $lzw->decompress($com);
echo $com . "<br>" . $dec;
- Output:
84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263 TOBEORNOTTOBEORTOBEORNOT
Picat
go =>
S = "TOBEORNOTTOBEORTOBEORNOT",
println(s=S),
println(len=S.length),
Compressed = compress(S),
println(compressed=Compressed),
println(len=Compressed.length),
Uncompressed = uncompress(Compressed),
println(uncompressed=Uncompressed),
printf("compressed to %3.3f%%\n", 100*(Compressed.length / S.length)),
if S = Uncompressed then
println("Same!")
else
println("Error: S != Uncompressed!"),
printf("S.length: %d Uncompressed.length: %d\n", S.length, Uncompressed.length),
edit(S,Uncompressed,Distance,Diffs),
println(distance=Distance),
println(diffs=Diffs)
end,
nl.
compress(Uncompressed) = Compressed =>
DictSize = 256,
Dict = new_map([C=C : I in 1..DictSize-1, C=chr(I).to_string()]),
W = "",
Result = [],
foreach(C in Uncompressed)
C := C.to_string(),
WC = W ++ C,
if Dict.has_key(WC) then
W := WC
else
Result := Result ++ [Dict.get(W)],
Dict.put(WC, DictSize),
DictSize := DictSize + 1,
W := C
end
end,
if W.length > 0 then
Result := Result ++ [Dict.get(W)]
end,
Compressed = Result.
uncompress(Compressed) = Uncompressed =>
DictSize = 256,
Dict = new_map([ C=C : I in 1..DictSize-1, C=chr(I).to_string()]),
W = Compressed.first(),
Compressed := Compressed.tail(),
Result = W,
Entry = "",
foreach(K in Compressed)
if Dict.has_key(K) then
Entry := Dict.get(K)
elseif K == DictSize then
Entry := W ++ W[1].to_string()
else
printf("Bad compressed K: %w\n", K)
end,
Result := Result ++ Entry,
Dict.put(DictSize,(W ++ Entry[1].to_string())),
DictSize := DictSize + 1,
W := Entry
end,
Uncompressed = Result.flatten().
%
% Computing the minimal editing distance of two given lists
%
table(+,+,min,-)
edit([],[],D,Diffs) => D=0, Diffs=[].
edit([X|Xs],[X|Ys],D,Diffs) => % copy
edit(Xs,Ys,D,Diffs).
edit(Xs,[Y|Ys],D,Diffs) ?=> % insert
edit(Xs,Ys,D1,Diffs1),
D=D1+1,
Diffs = [insert=Y,xPos=Xs.length,yPos=Ys.length|Diffs1].
edit([X|Xs],Ys,D,Diffs) => % delete
edit(Xs,Ys,D1,Diffs1),
D=D1+1,
Diffs = [[delete=X,xPos=Xs.length,yPos=Ys.length]|Diffs1].- Output:
s = TOBEORNOTTOBEORTOBEORNOT len = 24 compressed = [T,O,B,E,O,R,N,O,T,256,258,260,265,259,261,263] len = 16 uncompressed = TOBEORNOTTOBEORTOBEORNOT compressed to 66.667% Same!
PicoLisp
(de lzwCompress (Lst)
(let (Codes 255 Dict)
(balance 'Dict
(make
(for C Codes
(link (cons (char C) C)) ) ) )
(make
(let W (pop 'Lst)
(for C Lst
(let WC (pack W C)
(if (lup Dict WC)
(setq W WC)
(link (cdr (lup Dict W)))
(idx 'Dict (cons WC (inc 'Codes)) T)
(setq W C) ) ) )
(and W (link (cdr (lup Dict W)))) ) ) ) )
(de lzwDecompress (Lst)
(let (Codes 255 Dict)
(balance 'Dict
(make
(for C Codes
(link (list C (char C))) ) ) )
(make
(let W NIL
(for N Lst
(let WC (if (lup Dict N) (cdr @) (cons (last W) W))
(chain (reverse WC))
(when W
(idx 'Dict (cons (inc 'Codes) (cons (last WC) W)) T) )
(setq W WC) ) ) ) ) ) )Test:
: (lzwCompress (chop "TOBEORNOTTOBEORTOBEORNOT")) -> (84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263) : (pack (lzwDecompress @)) -> "TOBEORNOTTOBEORTOBEORNOT"
PL/I
The interesting point is the implementation of REXX's associative array (compound variable).
*process source xref attributes or(!);
lzwt: Proc Options(main);
Dcl (LEFT,LENGTH,SUBSTR,TRANSLATE,TRIM,UNSPEC) Builtin;
Dcl SYSPRINT Print;
Dcl str Char(50) Var Init('TOBEORNOTTOBEORTOBEORNOT');
Dcl compressed Char(80) Var;
Dcl decompressed Char(80) Var;
Dcl 1 dict(0:300),
2 key Char(5) Var,
2 inx Bin Fixed(16) Unsigned;
Dcl dict_size Bin Fixed(31) Init(256);
Dcl hi Bin Fixed(16) Unsigned Init(65535);
Put Edit('str=',str)(Skip,a,a);
compressed = compress(str);
Put Edit(compressed)(Skip,a);
decompressed = decompress(compressed);
Put Edit('dec=',decompressed)(Skip,a,a);
If decompressed=str Then
Put Edit('decompression ok')(Skip,a);
Else
Put Edit('decompression not ok')(Skip,a);
compress: Proc(s) Returns(Char(80) Var);
Dcl s Char(*) Var;
Dcl res Char(80) Var;
Dcl i Bin Fixed(31);
Dcl c Char(1);
Dcl w Char(5) Var;
Dcl wc Char(5) Var;
dict.key='';
Dcl ii Bin Fixed(8) Unsigned;
Do i=0 To 255;
ii=i;
Unspec(c)=unspec(ii);
dict.key(i)=c;
dict.inx(i)=i;
End;
res='[';
w='';
Do i=1 To length(s);
c=substr(s,i,1);
wc=w!!c;
If dicti(wc)^=hi Then Do;
w=wc;
End;
Else Do;
res=res!!trim(dicti(w))!!', ';
Call dict_add(wc,dict_size);
w=c;
End;
End;
If w^='' Then
res=res!!trim(dicti(w))!!', ';
substr(res,length(res)-1,1)=']';
Return(res);
dicti: Proc(needle) Returns(Bin Fixed(31));
Dcl needle Char(*) Var;
Dcl i Bin Fixed(31);
Do i=1 To dict_size;
If dict.key(i)=needle Then
Return(i);
End;
Return(hi);
End;
dict_add: Proc(needle,dict_size);
Dcl needle Char(*) Var;
Dcl dict_size Bin Fixed(31);
dict.key(dict_size)=needle;
dict.inx(dict_size)=dict_size;
dict_size+=1;
End;
End;
decompress: Proc(s) Returns(Char(80) Var);
Dcl s Char(80) Var;
Dcl ss Char(80) Var;
Dcl words(50) Char(5) Var;
Dcl wn Bin Fixed(31);
Dcl ww Bin Fixed(31);
Dcl c Char(1);
Dcl entry Char(5) Var;
Dcl w Char(5) Var;
Dcl res Char(80) Var;
ss=translate(s,' ','[],');
Call mk_words(ss,words,wn);
dict.key='';
dict.inx=hi;
Dcl i Bin Fixed(31);
Dcl ii Bin Fixed(8) Unsigned;
Dcl dict(0:300) Char(5) Var;
Dcl dict_size Bin Fixed(31);
Do i=0 To 255;
ii=i;
Unspec(c)=unspec(ii);
dict(i)=c;
End;
dict_size=256;
ww=words(1);
w=dict(ww);
res=w;
Do i=2 To wn;
ww=words(i);
Select;
When(dict(ww)^='')
entry=dict(ww);
When(ww=dict_size)
entry=w!!substr(w,1,1);
Otherwise
Put Edit('Bad compressed k: ',ww)(Skip,a,a);
End;
res=res!!entry;
dict(dict_size)=w!!substr(entry,1,1);
dict_size+=1;
w=entry;
End;
Return(res);
End;
mk_words: Proc(st,arr,arrn);
Dcl st Char(*) Var;
Dcl sv Char(80) Var;
Dcl arr(*) Char(5) Var;
Dcl arrn Bin fixed(31);
Dcl elem Char(5) Var;
arrn=0;
sv=st!!' ';
elem='';
Do While(length(sv)>0);
If left(sv,1)=' ' Then Do;
If elem>'' Then Do;
arrn+=1;
arr(arrn)=elem;
elem='';
End;
End;
Else
elem=elem!!left(sv,1);
sv=substr(sv,2);
End;
End;
Return;
End;- Output:
str=TOBEORNOTTOBEORTOBEORNOT [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] dec=TOBEORNOTTOBEORTOBEORNOT decompression ok
PureBasic
This version encodes character sequences as 16-bit values. Because this version only encodes an input string it won't handle Null values. This is because PureBasic uses these to terminate strings. Only slight modifications are necessary to handle Null values that would be present for a more generic routine that could be used with a buffer containing any data type.
Procedure compress(uncompressed.s, List result.u())
;Compress a string to a list of output symbols
;Build the dictionary.
Protected dict_size = 255, i
newmap dict.u()
For i = 0 To 254
dict(Chr(i + 1)) = i
Next
Protected w.s, wc.s, *c.Character = @uncompressed
w = ""
LastElement(result())
While *c\c <> #Null
wc = w + Chr(*c\c)
If FindMapElement(dict(), wc)
w = wc
Else
AddElement(result())
result() = dict(w)
;Add wc to the dictionary
dict(wc) = dict_size
dict_size + 1 ;no check is performed for overfilling the dictionary.
w = Chr(*c\c)
EndIf
*c + 1
Wend
;Output the code for w
If w
AddElement(result())
result() = dict(w)
EndIf
EndProcedure
Procedure.s decompress(List compressed.u())
;Decompress a list of encoded values to a string
If ListSize(compressed()) = 0: ProcedureReturn "": EndIf
;Build the dictionary.
Protected dict_size = 255, i
Dim dict.s(255)
For i = 1 To 255
dict(i - 1) = Chr(i)
Next
Protected w.s, entry.s, result.s
FirstElement(compressed())
w = dict(compressed())
result = w
i = 0
While NextElement(compressed())
i + 1
If compressed() < dict_size
entry = dict(compressed())
ElseIf i = dict_size
entry = w + Left(w, 1)
Else
MessageRequester("Error","Bad compression at [" + Str(i) + "]")
ProcedureReturn result;abort
EndIf
result + entry
;Add w + Left(entry, 1) to the dictionary
If ArraySize(dict()) <= dict_size
Redim dict(dict_size + 256)
EndIf
dict(dict_size) = w + Left(entry, 1)
dict_size + 1 ;no check is performed for overfilling the dictionary.
w = entry
Wend
ProcedureReturn result
EndProcedure
If OpenConsole()
;How to use:
Define initial.s, decompressed.s
Print("Type something: ")
initial = Input()
NewList compressed.u()
compress(initial, compressed())
ForEach compressed()
Print(Str(compressed()) + " ")
Next
PrintN("")
decompressed = decompress(compressed())
PrintN(decompressed)
Print(#CRLF$ + #CRLF$ + "Press ENTER to exit")
Input()
CloseConsole()
EndIfSample output:
Type something: TOBEORNOTTOBEORTOBEORNOT 83 78 65 68 78 81 77 78 83 255 257 259 264 258 260 262 TOBEORNOTTOBEORTOBEORNOT
Python
In this version the dicts contain mixed typed data:
def compress(uncompressed):
"""Compress a string to a list of output symbols."""
# Build the dictionary.
dict_size = 256
dictionary = dict((chr(i), i) for i in range(dict_size))
# in Python 3: dictionary = {chr(i): i for i in range(dict_size)}
w = ""
result = []
for c in uncompressed:
wc = w + c
if wc in dictionary:
w = wc
else:
result.append(dictionary[w])
# Add wc to the dictionary.
dictionary[wc] = dict_size
dict_size += 1
w = c
# Output the code for w.
if w:
result.append(dictionary[w])
return result
def decompress(compressed):
"""Decompress a list of output ks to a string."""
from io import StringIO
# Build the dictionary.
dict_size = 256
dictionary = dict((i, chr(i)) for i in range(dict_size))
# in Python 3: dictionary = {i: chr(i) for i in range(dict_size)}
# use StringIO, otherwise this becomes O(N^2)
# due to string concatenation in a loop
result = StringIO()
w = chr(compressed.pop(0))
result.write(w)
for k in compressed:
if k in dictionary:
entry = dictionary[k]
elif k == dict_size:
entry = w + w[0]
else:
raise ValueError('Bad compressed k: %s' % k)
result.write(entry)
# Add w+entry[0] to the dictionary.
dictionary[dict_size] = w + entry[0]
dict_size += 1
w = entry
return result.getvalue()
# How to use:
compressed = compress('TOBEORNOTTOBEORTOBEORNOT')
print (compressed)
decompressed = decompress(compressed)
print (decompressed)
Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Racket
#lang racket
; utilities
(define-syntax def (make-rename-transformer #'define))
(define (dict-ref d w) (hash-ref d w #f))
(define (append-char w c) (string-append w (string c)))
(define (append-first w s) (append-char w (string-ref s 0)))
;; Compress a string with LZW
(define (compress uncompressed)
(def d (make-hash))
(def (dict-add d w) (hash-set! d w (hash-count d)))
; build initial dictionary
(for ([i (in-range 256)])
(def s (string (integer->char i)))
(hash-set! d s s))
; compress the string
(def result '())
(def (emit! i) (set! result (cons i result)))
(def w "")
(for ([c uncompressed])
(define wc (append-char w c))
(cond
[(dict-ref d wc) (set! w wc)]
[else (emit! (dict-ref d w))
(dict-add d wc)
(set! w (string c))]))
(emit! (dict-ref d w))
(reverse result))
;; Decompress a LZW compressed string
(define (decompress compressed)
(def d (make-hash))
(def (dict-add! w) (hash-set! d (hash-count d) w))
; build initial dictionary
(for ([i (in-range 256)])
(def s (string (integer->char i)))
(hash-set! d s s))
; decompress the list
(def w (first compressed))
(apply string-append
w
(for/list ([k (rest compressed)])
(def entry
(or (dict-ref d k)
(if (eqv? k (hash-count d))
(append-first w w)
(error 'lzq-decompress "faulty input"))))
(dict-add! (append-first w entry))
(set! w entry)
entry)))
(def uncompressed "TOBEORNOTTOBEORTOBEORNOT")
(displayln uncompressed)
(def compressed (compress uncompressed))
(displayln compressed)
(def decompressed (decompress compressed))
(displayln decompressed)
Output:
TOBEORNOTTOBEORTOBEORNOT (T O B E O R N O T 256 258 260 265 259 261 263) TOBEORNOTTOBEORTOBEORNOT
Raku
(formerly Perl 6) I just came across this SO question by chance hence the update. Notably the ancestor Perl entry simply works without any further tweak.
# 20200421 Updated Raku programming solution ; add unicode support
sub compress(Str $uncompressed --> Seq) {
my $dict-size = 256;
my %dictionary = (.chr => .chr for ^$dict-size);
my $w = "";
gather {
for $uncompressed.encode('utf8').list.chrs.comb -> $c {
my $wc = $w ~ $c;
if %dictionary{$wc}:exists { $w = $wc }
else {
take %dictionary{$w};
%dictionary{$wc} = +%dictionary;
$w = $c;
}
}
take %dictionary{$w} if $w.chars;
}
}
sub decompress(@compressed --> Str) {
my $dict-size = 256;
my %dictionary = (.chr => .chr for ^$dict-size);
my $w = shift @compressed;
( Blob.new: flat ( gather {
take $w;
for @compressed -> $k {
my $entry;
if %dictionary{$k}:exists { take $entry = %dictionary{$k} }
elsif $k == $dict-size { take $entry = $w ~ $w.substr(0,1) }
else { die "Bad compressed k: $k" }
%dictionary{$dict-size++} = $w ~ $entry.substr(0,1);
$w = $entry;
}
} )».ords ).decode('utf-8')
}
say my @compressed = compress('TOBEORNOTTOBEORTOBEORNOT');
say decompress(@compressed);
@compressed = compress('こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵');
say decompress(@compressed);
- Output:
[T O B E O R N O T 256 258 260 265 259 261 263] TOBEORNOTTOBEORTOBEORNOT こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵
REXX
version 1
/* REXX ---------------------------------------------------------------
* 20.07.2014 Walter Pachl translated from Java
* 21.07.2014 WP allow for blanks in the string
*--------------------------------------------------------------------*/
Parse Arg str
default="TOBEORNOTTOBEORTOBEORNOT"
If str='' Then
str=default
/* str=space(str,0) */
Say 'str='str
compressed = compress(str)
Say compressed
If str=default Then Do
cx='[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]'
If cx==compressed Then Say 'compression ok'
End
decompressed = decompress(compressed)
Say 'dec='decompressed
If decompressed=str Then Say 'decompression ok'
Exit
compress: Procedure
Parse Arg uncompressed
dict.=''
Do i=0 To 255
z=d2c(i)
d.i=z
dict.z=i
End
dict_size=256
res='['
w=''
Do i=1 To length(uncompressed)
c=substr(uncompressed,i,1)
wc=w||c
If dict.wc<>'' Then
w=wc
Else Do
res=res||dict.w', '
dict.wc=dict_size
dict_size=dict_size+1
w=c
End
End
If w<>'' Then
res=res||dict.w', '
Return left(res,length(res)-2)']'
decompress: Procedure
Parse Arg compressed
compressed=space(translate(compressed,'','[],'))
d.=''
Do i=0 To 255
z=d2c(i)
d.i=z
End
dict_size=256
Parse Var compressed w compressed
res=d.w
w=d.w
Do i=1 To words(compressed)
k=word(compressed,i)
Select
When d.k<>'' | d.k==' ' then /* allow for blank */
entry=d.k
When k=dict_size Then
entry=w||substr(w,1,1)
Otherwise
Say "Bad compressed k: " k
End
res=res||entry
d.dict_size=w||substr(entry,1,1)
dict_size=dict_size+1
w=entry
End
Return res
Output:
str=TOBEORNOTTOBEORTOBEORNOT [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] compression ok dec=TOBEORNOTTOBEORTOBEORNOT decompression ok str=To be or not to be that is the question! [84, 111, 32, 98, 101, 32, 111, 114, 32, 110, 111, 116, 32, 116, 257, 259, 268, 104, 97, 267, 105, 115, 272, 260, 113, 117, 101, 115, 116, 105, 111, 110, 33] dec=To be or not to be that is the question! decompression ok
version 2
(Based on the Java program.)
This REXX version can execute on ASCII or EBCDIC systems.
/*REXX program compresses text using the LZW (Lempel─Ziv─Welch), and reconstitutes it.*/
$$$= '"There is nothing permanent except change." ─── Heraclitus [540 ── 475 BCE]'
parse arg text; if text='' then text= $$$ /*get an optional argument from the CL.*/
say 'original text=' text /* [↑] Not specified? Then use default*/
cypher= LZWc(text) /*compress text using the LZW algorithm*/
say 'reconstituted=' LZWd(cypher) /*display the reconstituted string. */
say; say ' LZW integers=' cypher /* " " LZW integers used. */
exit 0 /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
LZWi: arg i,@.; #=256; do j=0 for #; _=d2c(j); if i then @.j=_; else @._=j; end; return
/*──────────────────────────────────────────────────────────────────────────────────────*/
LZWc: procedure; parse arg y,,$; call LZWi 0; w= /*LZW compress.*/
do k=1 for length(y)+1; z= w || substr(y, k, 1)
if @.z=='' then do; $= $ @.w; @.z= #; #= # + 1; w= substr(y, k, 1); end
else w= z /*#: the dictionary size.*/
end /*k*/; return substr($, 2) /*elide a leading blank. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
LZWd: procedure; parse arg x y; call LZWi 1; $= @.x; w= $ /*LZW decompress.*/
do k=1 for words(y); z= word(y, k)
if @.z\=='' | @.k==" " then ?= @.z
else if z==# then ?= w || left(w, 1)
$= $ || ?
@.#= w || left(?, 1); w= ?; #= # + 1 /*bump dict. size*/
end /*k*/; return $
- output when using the default input:
original text= "There is nothing permanent except change." ─── Heraclitus [540 ── 475 BCE] reconstituted= "There is nothing permanent except change." ─── Heraclitus [540 ── 475 BCE] LZW integers= 34 84 104 101 114 101 32 105 115 32 110 111 116 104 105 110 103 32 112 259 109 97 110 101 110 116 32 101 120 99 101 112 281 99 104 277 103 101 46 34 32 296 196 298 296 32 72 259 97 99 108 105 116 117 264 32 91 53 52 48 32 299 52 55 53 32 66 67 69 93
Ring
# Project : LZW compression
plaintext = "TOBEORNOTTOBEORTOBEORNOT"
result = []
encode = encodelzw(plaintext)
for i = 1 to len(encode) step 2
add(result,ascii(substr(encode,i,1)) + 256*ascii(substr(encode,i+1,1)))
next
showarray(result)
see decodelzw(encode)
func encodelzw(text)
o = ""
dict = list(4096)
for i = 1 to 255
dict[i] = char(i)
next
l = i
i = 1
w = left(text,1)
while i < len(text)
d = 0
while d < l
c = d
if i > len(text)
exit
ok
for d = 1 to l
if w = dict[d]
exit
ok
next
if d < l
i = i + 1
w = w + substr(text,i,1)
ok
end
dict[l] = w
l = l + 1
w = right(w,1)
o = o + char(c % 256) + char(floor(c / 256))
end
return o
func decodelzw(text)
o = ""
dict = list(4096)
for i = 1 to 255
dict[i] = char(i)
next
l = i
c = ascii(left(text,1)) + 256*ascii(substr(text,2,1))
w = dict[c]
o = w
if len(text) < 4
return o
ok
for i = 3 to len(text) step 2
c = ascii(substr(text,i,1)) + 256*ascii(substr(text,i+1,1))
if c < l
t = dict[c]
else
t = w + left(w,1)
ok
o = o + t
dict[l] = w + left(t,1)
l = l + 1
w = t
next
return o
func showarray(vect)
svect = ""
for n = 1 to len(vect)
svect = svect + vect[n] + " "
next
svect = left(svect, len(svect) - 1)
see svect + nlOutput:
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Ruby
In this version the hashes contain mixed typed data:
# Compress a string to a list of output symbols.
def compress(uncompressed)
# Build the dictionary.
dict_size = 256
dictionary = Hash[ Array.new(dict_size) {|i| [i.chr, i.chr]} ]
w = ""
result = []
for c in uncompressed.split('')
wc = w + c
if dictionary.has_key?(wc)
w = wc
else
result << dictionary[w]
# Add wc to the dictionary.
dictionary[wc] = dict_size
dict_size += 1
w = c
end
end
# Output the code for w.
result << dictionary[w] unless w.empty?
result
end
# Decompress a list of output ks to a string.
def decompress(compressed)
# Build the dictionary.
dict_size = 256
dictionary = Hash[ Array.new(dict_size) {|i| [i.chr, i.chr]} ]
w = result = compressed.shift
for k in compressed
if dictionary.has_key?(k)
entry = dictionary[k]
elsif k == dict_size
entry = w + w[0,1]
else
raise 'Bad compressed k: %s' % k
end
result += entry
# Add w+entry[0] to the dictionary.
dictionary[dict_size] = w + entry[0,1]
dict_size += 1
w = entry
end
result
end
# How to use:
compressed = compress('TOBEORNOTTOBEORTOBEORNOT')
p compressed
decompressed = decompress(compressed)
puts decompressed
Output:
["T", "O", "B", "E", "O", "R", "N", "O", "T", 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Rust
Handles arbitrary byte sequences.
use std::collections::HashMap;
fn compress(data: &[u8]) -> Vec<u32> {
// Build initial dictionary.
let mut dictionary: HashMap<Vec<u8>, u32> = (0u32..=255)
.map(|i| (vec![i as u8], i))
.collect();
let mut w = Vec::new();
let mut compressed = Vec::new();
for &b in data {
let mut wc = w.clone();
wc.push(b);
if dictionary.contains_key(&wc) {
w = wc;
} else {
// Write w to output.
compressed.push(dictionary[&w]);
// wc is a new sequence; add it to the dictionary.
dictionary.insert(wc, dictionary.len() as u32);
w.clear();
w.push(b);
}
}
// Write remaining output if necessary.
if !w.is_empty() {
compressed.push(dictionary[&w]);
}
compressed
}
fn decompress(mut data: &[u32]) -> Vec<u8> {
// Build the dictionary.
let mut dictionary: HashMap::<u32, Vec<u8>> = (0u32..=255)
.map(|i| (i, vec![i as u8]))
.collect();
let mut w = dictionary[&data[0]].clone();
data = &data[1..];
let mut decompressed = w.clone();
for &k in data {
let entry = if dictionary.contains_key(&k) {
dictionary[&k].clone()
} else if k == dictionary.len() as u32 {
let mut entry = w.clone();
entry.push(w[0]);
entry
} else {
panic!("Invalid dictionary!");
};
decompressed.extend_from_slice(&entry);
// New sequence; add it to the dictionary.
w.push(entry[0]);
dictionary.insert(dictionary.len() as u32, w);
w = entry;
}
decompressed
}
fn main() {
let compressed = compress("TOBEORNOTTOBEORTOBEORNOT".as_bytes());
println!("{:?}", compressed);
let decompressed = decompress(&compressed);
let decompressed = String::from_utf8(decompressed).unwrap();
println!("{}", decompressed);
}
Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Scala
def compress(tc:String) = {
//initial dictionary
val startDict = (1 to 255).map(a=>(""+a.toChar,a)).toMap
val (fullDict, result, remain) = tc.foldLeft ((startDict, List[Int](), "")) {
case ((dict,res,leftOver),nextChar) =>
if (dict.contains(leftOver + nextChar)) // current substring already in dict
(dict, res, leftOver+nextChar)
else if (dict.size < 4096) // add to dictionary
(dict + ((leftOver+nextChar, dict.size+1)), dict(leftOver) :: res, ""+nextChar)
else // dictionary is full
(dict, dict(leftOver) :: res, ""+nextChar)
}
if (remain.isEmpty) result.reverse else (fullDict(remain) :: result).reverse
}
def decompress(ns: List[Int]): String = {
val startDict = (1 to 255).map(a=>(a,""+a.toChar)).toMap
val (_, result, _) =
ns.foldLeft[(Map[Int, String], List[String], Option[(Int, String)])]((startDict, Nil, None)) {
case ((dict, result, conjecture), n) => {
dict.get(n) match {
case Some(output) => {
val (newDict, newCode) = conjecture match {
case Some((code, prefix)) => ((dict + (code -> (prefix + output.head))), code + 1)
case None => (dict, dict.size + 1)
}
(newDict, output :: result, Some(newCode -> output))
}
case None => {
// conjecture being None would be an encoding error
val (code, prefix) = conjecture.get
val output = prefix + prefix.head
(dict + (code -> output), output :: result, Some(code + 1 -> output))
}
}
}
}
result.reverse.mkString("")
}
// test
val text = "TOBEORNOTTOBEORTOBEORNOT"
val compressed = compress(text)
println(compressed)
val result = decompress(compressed)
println(result)
Scheme
; Get the list reference number for a member or #f if not found
(define (member-string-ref m l)
(define r #f)
(let loop ((i 0))
(if (< i (length l))
(if (not (string=? (list-ref l i) m))
(loop (+ i 1))
(set! r i))))
r)
;; Compress a string with LZW
(define (lzw-compress uncompressed)
(define dictionary '())
(define n 0)
(define result '())
(set! uncompressed (string->list uncompressed))
;; Setup Dictionary
(let dict-setup ((c 0))
(if (> 256 c)
(begin
(set! dictionary (append dictionary
(list (string (integer->char c)))))
(set! n (+ n 1))
(dict-setup (+ c 1)))))
;; Compress the string
(let compress ((w "") (ci 0))
(define c (string (list-ref uncompressed ci)))
(define wc "")
(set! wc (string-append w c))
(if (member-string-ref wc dictionary)
(set! w wc)
(begin
(set! result (append result
(list (member-string-ref w dictionary))))
(set! dictionary (append dictionary (list wc)))
(set! n (+ n 1))
(set! w c)))
(if (eqv? ci (- (length uncompressed) 1))
(set! result (append result
(list (member-string-ref w dictionary))))
(compress w (+ ci 1))))
result)
;; Decompress a LZW compressed string (input should be a list of integers)
(define (lzw-decompress compressed)
(define dictionary '())
(define n 0)
(define result "")
;; Setup Dictionary
(let dict-setup ((c 0))
(if (> 256 c)
(begin
(set! dictionary (append dictionary
(list (string (integer->char c)))))
(set! n (+ n 1))
(dict-setup (+ c 1)))))
;; Decompress the list
(let decompress ((k (list-ref compressed 0)) (ci 0))
(define kn #f)
;; Add to dictionary
(if (> (length compressed) (+ ci 1))
(begin
(set! kn (list-ref compressed (+ ci 1)))
(if (< kn (length dictionary))
(set! dictionary
(append dictionary
(list (string-append
(list-ref dictionary k)
(string (string-ref (list-ref dictionary kn) 0)))))))))
;; Build the resulting string
(set! result (string-append result (list-ref dictionary k)))
(if (not (eqv? ci (- (length compressed) 1)))
(decompress kn (+ ci 1))))
result)
(define compressed (lzw-compress "TOBEORNOTTOBEORTOBEORNOT"))
(display compressed) (newline)
(define decompressed (lzw-decompress compressed))
(display decompressed) (newline)
Output:
(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263) TOBEORNOTTOBEORTOBEORNOT
Seed7
$ include "seed7_05.s7i";
const func string: lzwCompress (in string: uncompressed) is func
result
var string: compressed is "";
local
var char: ch is ' ';
var hash [string] char: mydict is (hash [string] char).value;
var string: buffer is "";
var string: xstr is "";
begin
for ch range chr(0) to chr(255) do
mydict @:= [str(ch)] ch;
end for;
for ch range uncompressed do
xstr := buffer & str(ch);
if xstr in mydict then
buffer &:= str(ch)
else
compressed &:= str(mydict[buffer]);
mydict @:= [xstr] chr(length(mydict));
buffer := str(ch);
end if;
end for;
if buffer <> "" then
compressed &:= str(mydict[buffer]);
end if;
end func;
const func string: lzwDecompress (in string: compressed) is func
result
var string: uncompressed is "";
local
var char: ch is ' ';
var hash [char] string: mydict is (hash [char] string).value;
var string: buffer is "";
var string: current is "";
var string: chain is "";
begin
for ch range chr(0) to chr(255) do
mydict @:= [ch] str(ch);
end for;
for ch range compressed do
if buffer = "" then
buffer := mydict[ch];
uncompressed &:= buffer;
elsif ch <= chr(255) then
current := mydict[ch];
uncompressed &:= current;
chain := buffer & current;
mydict @:= [chr(length(mydict))] chain;
buffer := current;
else
if ch in mydict then
chain := mydict[ch];
else
chain := buffer & str(buffer[1]);
end if;
uncompressed &:= chain;
mydict @:= [chr(length(mydict))] buffer & str(chain[1]);
buffer := chain;
end if;
end for;
end func;
const proc: main is func
local
var string: compressed is "";
var string: uncompressed is "";
begin
compressed := lzwCompress("TOBEORNOTTOBEORTOBEORNOT");
writeln(literal(compressed));
uncompressed := lzwDecompress(compressed);
writeln(uncompressed);
end func;Output:
"TOBEORNOT\256;\258;\260;\265;\259;\261;\263;" TOBEORNOTTOBEORTOBEORNOT
Sidef
# Compress a string to a list of output symbols.
func compress(String uncompressed) -> Array {
# Build the dictionary.
var dict_size = 256
var dictionary = Hash()
for i in range(dict_size) {
dictionary{i.chr} = i.chr
}
var w = ''
var result = []
uncompressed.each { |c|
var wc = w+c
if (dictionary.exists(wc)) {
w = wc
} else {
result << dictionary{w}
# Add wc to the dictionary.
dictionary{wc} = dict_size
dict_size++
w = c
}
}
# Output the code for w.
if (w != '') {
result << dictionary{w}
}
return result
}
# Decompress a list of output ks to a string.
func decompress(Array compressed) -> String {
# Build the dictionary.
var dict_size = 256
var dictionary = Hash()
for i in range(dict_size) {
dictionary{i.chr} = i.chr
}
var w = compressed.shift
var result = w
compressed.each { |k|
var entry = nil
if (dictionary.exists(k)) {
entry = dictionary{k}
} elsif (k == dict_size) {
entry = w+w.first
} else {
die "Bad compressed k: #{k}"
}
result += entry
# Add w+entry[0] to the dictionary.
dictionary{dict_size} = w+entry.first
dict_size++
w = entry
}
return result
}
# How to use:
var compressed = compress('TOBEORNOTTOBEORTOBEORNOT')
say compressed.join(' ')
var decompressed = decompress(compressed)
say decompressed
- Output:
T O B E O R N O T 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Swift
class LZW {
class func compress(_ uncompressed:String) -> [Int] {
var dict = [String : Int]()
for i in 0 ..< 256 {
let s = String(Unicode.Scalar(UInt8(i)))
dict[s] = i
}
var dictSize = 256
var w = ""
var result = [Int]()
for c in uncompressed {
let wc = w + String(c)
if dict[wc] != nil {
w = wc
} else {
result.append(dict[w]!)
dict[wc] = dictSize
dictSize += 1
w = String(c)
}
}
if w != "" {
result.append(dict[w]!)
}
return result
}
class func decompress(_ compressed:[Int]) -> String? {
var dict = [Int : String]()
for i in 0 ..< 256 {
dict[i] = String(Unicode.Scalar(UInt8(i)))
}
var dictSize = 256
var w = String(Unicode.Scalar(UInt8(compressed[0])))
var result = w
for k in compressed[1 ..< compressed.count] {
let entry : String
if let x = dict[k] {
entry = x
} else if k == dictSize {
entry = w + String(w[w.startIndex])
} else {
return nil
}
result += entry
dict[dictSize] = w + String(entry[entry.startIndex])
dictSize += 1
w = entry
}
return result
}
}
let comp = LZW.compress("TOBEORNOTTOBEORTOBEORNOT")
print(comp)
if let decomp = LZW.decompress(comp) {
print(decomp)
}
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Tcl
namespace eval LZW {
variable char2int
variable chars
for {set i 0} {$i < 256} {incr i} {
set char [binary format c $i]
set char2int($char) $i
lappend chars $char
}
}
proc LZW::encode {data} {
variable char2int
array set dict [array get char2int]
set w ""
set result [list]
foreach c [split $data ""] {
set wc $w$c
if {[info exists dict($wc)]} {
set w $wc
} else {
lappend result $dict($w)
set dict($wc) [array size dict]
set w $c
}
}
lappend result $dict($w)
}
proc LZW::decode {cdata} {
variable chars
set dict $chars
set k [lindex $cdata 0]
set w [lindex $dict $k]
set result $w
foreach k [lrange $cdata 1 end] {
set currSizeDict [llength $dict]
if {$k < $currSizeDict} {
set entry [lindex $dict $k]
} elseif {$k == $currSizeDict} {
set entry $w[string index $w 0]
} else {
error "invalid code ($k) in ($cdata)"
}
append result $entry
lappend dict $w[string index $entry 0]
set w $entry
}
return $result
}
set s TOBEORNOTTOBEORTOBEORNOT#
set e [LZW::encode $s] ;# ==> 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 35
set d [LZW::decode $e] ;# ==> TOBEORNOTTOBEORTOBEORNOT#
# or
if {$s eq [LZW::decode [LZW::encode $s]]} then {puts success} else {puts fail} ;# ==> success
VBScript
Option Explicit
Const numchars=127 'plain ASCII
Function LZWCompress(si)
Dim oDict, intMaxCode, i,z,ii,ss,strCurrent,strNext,j
Set oDict = CreateObject("Scripting.Dictionary")
ReDim a(Len(si))
intMaxCode = numchars
For i = 0 To numchars
oDict.Add Chr(i), i
Next
'strCurrent = ofread.ReadText(1)
strCurrent = Left(si,1)
j=0
For ii=2 To Len(si)
strNext = Mid(si,ii,1)
ss=strCurrent & strNext
If oDict.Exists(ss) Then
strCurrent = ss
Else
a(j)=oDict.Item(strCurrent) :j=j+1
intMaxCode = intMaxCode + 1
oDict.Add ss, intMaxCode
strCurrent = strNext
End If
Next
a(j)=oDict.Item(strCurrent)
ReDim preserve a(j)
LZWCompress=a
Set oDict = Nothing
End Function
Function lzwUncompress(sc)
Dim intNext, intCurrent, intMaxCode, i,ss,istr,s,j
s=""
reDim dict(1000)
intMaxCode = numchars
For i = 0 To numchars : dict(i)= Chr(i) : Next
intCurrent=sc(0)
For j=1 To UBound(sc)
ss=dict(intCurrent)
s= s & ss
intMaxCode = intMaxCode + 1
intnext=sc(j)
If intNext<intMaxCode Then
dict(intMaxCode)=ss & Left(dict(intNext), 1)
Else
dict(intMaxCode)=ss & Left(ss, 1)
End If
intCurrent = intNext
Next
s= s & dict(intCurrent)
lzwUncompress=s
End function
Sub printvec(a)
Dim s,i,x
s="("
For i=0 To UBound (a)
s=s & x & a(i)
x=", "
Next
WScript.echo s &")"
End sub
Dim a,b
b="TOBEORNOTTOBEORTOBEORNOT"
WScript.Echo b
a=LZWCompress (b)
printvec(a)
WScript.echo lzwUncompress (a )
wscript.quit 1- Output:
TOBEORNOTTOBEORTOBEORNOT (84, 79, 66, 69, 79, 82, 78, 79, 84, 128, 130, 132, 137, 131, 133, 135) TOBEORNOTTOBEORTOBEORNOT
Wren
class LZW {
/* Compress a string to a list of output symbols. */
static compress(uncompressed) {
// Build the dictionary.
var dictSize = 256
var dictionary = {}
for (i in 0...dictSize) dictionary[String.fromByte(i)] = i
var w = ""
var result = []
for (c in uncompressed.bytes) {
var cs = String.fromByte(c)
var wc = w + cs
if (dictionary.containsKey(wc)) {
w = wc
} else {
result.add(dictionary[w])
// Add wc to the dictionary.
dictionary[wc] = dictSize
dictSize = dictSize + 1
w = cs
}
}
// Output the code for w
if (w != "") result.add(dictionary[w])
return result
}
/* Decompress a list of output symbols to a string. */
static decompress(compressed) {
// Build the dictionary.
var dictSize = 256
var dictionary = {}
for (i in 0...dictSize) dictionary[i] = String.fromByte(i)
var w = String.fromByte(compressed[0])
var result = w
for (k in compressed.skip(1)) {
var entry
if (dictionary.containsKey(k)) {
entry = dictionary[k]
} else if (k == dictSize) {
entry = w + String.fromByte(w.bytes[0])
} else {
Fiber.abort("Bad compressed k: %(k)")
}
result = result + entry
// Add w + entry[0] to the dictionary.
dictionary[dictSize] = w + String.fromByte(entry.bytes[0])
dictSize = dictSize + 1
w = entry
}
return result
}
}
var compressed = LZW.compress("TOBEORNOTTOBEORTOBEORNOT")
System.print(compressed)
var decompressed = LZW.decompress(compressed)
System.print(decompressed)
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Xojo
Function compress(str as String) As String
Dim i as integer
Dim w as String = ""
Dim c as String
Dim strLen as Integer
Dim wc as string
Dim dic() as string
Dim result() as string
Dim lookup as integer
Dim startPos as integer = 0
Dim combined as String
strLen = len(str)
dic = populateDictionary
for i = 1 to strLen
c = str.mid(i, 1)
wc = w + c
startPos = getStartPos(wc)
lookup = findArrayPos(dic, wc, startPos)
if (lookup <> -1) then
w = wc
Else
startPos = getStartPos(w)
lookup = findArrayPos(dic, w, startPos)
if lookup <> -1 then
result.Append(lookup.ToText)
end if
dic.append(wc)
w = c
end if
next i
if (w <> "") then
startPos = getStartPos(w)
lookup = findArrayPos(dic, w, startPos)
result.Append(lookup.ToText)
end if
return join(result, ",")
End Function
Function decompress(str as string) As string
dim comStr() as string
dim w as string
dim result as string
dim comStrLen as integer
dim entry as string
dim dic() as string
dim i as integer
comStr = str.Split(",")
comStrLen = comStr.Ubound
dic = populateDictionary
w = chr(val(comStr(0)))
result = w
for i = 1 to comStrLen
entry = dic(val(comStr(i)))
result = result + entry
dic.append(w + entry.mid(1,1))
w = entry
next i
return result
End Function
Private Function findArrayPos(arr() as String, search as String, start as integer) As Integer
dim arraySize as Integer
dim arrayPosition as Integer = -1
dim i as Integer
arraySize = UBound(arr)
for i = start to arraySize
if (strcomp(arr(i), search, 0) = 0) then
arrayPosition = i
exit
end if
next i
return arrayPosition
End Function
Private Function getStartPos(str as String) As integer
if (len(str) = 1) then
return 0
else
return 255
end if
End Function
Private Function populateDictionary() As string()
dim dic() as string
dim i as integer
for i = 0 to 255
dic.append(Chr(i))
next i
return dic
End FunctionTest:
Dim str as String = "TOBEORNOTTOBEORTOBEORNOT" Dim com as string = LZW.compress(str) print com print LZW.decompress(com)
Output:
84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263 TOBEORNOTTOBEORTOBEORNOT
zkl
fcn lzwCompress(uncompressed){ // text-->list of 12 bit ints
dictionary:=(256).pump(Dictionary(),fcn(n){ return(n.toChar(),n) });
w,compressed:="",List();
foreach c in (uncompressed){
wc:=w+c;
if(dictionary.holds(wc)) w=wc;
else{
compressed.append(dictionary[w]); // 12 bits
dictionary[wc]=dictionary.len();
w=c;
}
}
if(w) compressed.append(dictionary[w]);
compressed
}
fcn lzwUncompress(compressed){ // compressed data-->text
dictionary:=(256).pump(Dictionary(),fcn(n){ return(n,n.toChar()) });
w,decommpressed:=dictionary[compressed[0]],Data(Void,w);
foreach k in (compressed[1,*]){
if(dictionary.holds(k)) entry:=dictionary[k];
else if(k==dictionary.len()) entry:=w+w[0];
else throw(Exception.ValueError("Invalid compressed data"));
decommpressed.append(entry);
dictionary.add(dictionary.len(),w+entry[0]);
w=entry;
}
decommpressed.text
}compressed:=lzwCompress("TOBEORNOTTOBEORTOBEORNOT");
compressed.toString(*).println();
lzwUncompress(compressed).println();- Output:
L(84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263) TOBEORNOTTOBEORTOBEORNOT
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