LZW compression: Difference between revisions
(fixed compression bug in Python & D; dunno how to fix for Forth) |
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// output the code for w |
// output the code for w |
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if (w.length) |
if (w.length) |
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result ~= dictionary[w]; |
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result ~= c; |
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return result; |
return result; |
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} |
} |
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The output: |
The output: |
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<d> |
<d> |
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[84,79,66,69,79,82,78,79,84,256,258,260,265,259,261, |
[84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263] |
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TOBEORNOTTOBEORTOBEORNOT |
TOBEORNOTTOBEORTOBEORNOT |
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</d> |
</d> |
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=={{header|Forth}}== |
=={{header|Forth}}== |
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{{works with|GNU Forth|0.6.2}} |
{{works with|GNU Forth|0.6.2}} |
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{{incorrect}} |
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256 value next-symbol |
256 value next-symbol |
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# Output the code for w. |
# Output the code for w. |
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if w: |
if w: |
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result. |
result.append(dictionary[w]) |
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return result |
return result |
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Line 383: | Line 383: | ||
Output: |
Output: |
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<python> |
<python> |
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['T', 'O', 'B', 'E', 'O', 'R', 'N', 'O', 'T', 256, 258, 260, 265, 259, 261, |
['T', 'O', 'B', 'E', 'O', 'R', 'N', 'O', 'T', 256, 258, 260, 265, 259, 261, 263] |
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TOBEORNOTTOBEORTOBEORNOT |
TOBEORNOTTOBEORTOBEORNOT |
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</python> |
</python> |
Revision as of 02:44, 25 November 2008
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 lossless data compression. You can read a complete description of it in the Wikipedia article on the subject. It was patented, but it fell in the public domain in 2004.
D
D 1, with Phobos, from the Python version (the final writefln works only on 7-bit ASCII strings): <d> import std.stdio: writefln;
/// Compress a string to a list of output symbols. int[] compress(string uncompressed) {
// build the dictionary int dict_size = 256; int[string] dictionary; for (int i; i < dict_size; i++) dictionary["" ~ cast(char)i] = i;
string w; int[] result; foreach (c; uncompressed) { string wc = w ~ c; if (wc in dictionary) 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.length) result ~= dictionary[w]; return result;
}
/// Decompress a list of output ks to a string.
string decompress(int[] compressed) {
// build the dictionary int dict_size = 256; string[int] dictionary; for (int i; i < dict_size; i++) dictionary[i] = "" ~ cast(char)i;
string w = "" ~ cast(char)compressed[0]; string result = w; foreach (k; compressed[1 .. $]) { string entry; if (k in dictionary) entry = dictionary[k]; else if (k == dictionary.length) entry = w ~ w[0]; else throw new Exception("Bad compressed k"); result ~= entry;
// add w+entry[0] to the dictionary dictionary[dict_size] = w ~ entry[0]; dict_size++;
w = entry; } return result;
}
void main() {
auto compressed = compress("TOBEORNOTTOBEORTOBEORNOT"); writefln(compressed);
auto decompressed = decompress(compressed); writefln(decompressed);
} </d> The output: <d> [84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263] TOBEORNOTTOBEORTOBEORNOT </d>
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 bounds do i c@ output loop 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 O T out out-size @ decompress cr \ TOBEORNOTTOBEORTOBEORNOT
OCaml
<ocaml>#directory "+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)
- </ocaml>
here is the interface: <ocaml>val compress : uncompressed:string -> int list val decompress : compressed:int list -> string list</ocaml>
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.
<ocaml>let greatest = List.fold_left (fun m x -> max m x) 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 (fun code -> IO.write_bits ob nbits code ) 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)
- </ocaml>
Python
In this version the dicts contain mixed typed data: <python> def compress(uncompressed):
"""Compress a string to a list of output symbols."""
# Build the dictionary. dict_size = 256 dictionary = dict((chr(i), chr(i)) for i in xrange(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."""
# Build the dictionary. dict_size = 256 dictionary = dict((chr(i), chr(i)) for i in xrange(dict_size))
w = result = compressed.pop(0) for k in compressed: if k in dictionary: entry = dictionary[k] elif k == len(dictionary): entry = w + w[0] else: raise ValueError('Bad compressed k: %s' % k) result += entry
# Add w+entry[0] to the dictionary. dictionary[dict_size] = w + entry[0] dict_size += 1
w = entry return result
- How to use:
compressed = compress('TOBEORNOTTOBEORTOBEORNOT') print compressed decompressed = decompress(compressed) print decompressed </python>
Output: <python> ['T', 'O', 'B', 'E', 'O', 'R', 'N', 'O', 'T', 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT </python>