Move-to-front algorithm

From Rosetta Code
Task
Move-to-front algorithm
You are encouraged to solve this task according to the task description, using any language you may know.

Given a symbol table of a zero-indexed array of all possible input symbols this algorithm reversibly transforms a sequence of input symbols into an array of output numbers (indices).
The transform in many cases acts to give frequently repeated input symbols lower indices which is useful in some compression algorithms.

Encoding algorithm
    for each symbol of the input sequence:
        output the index of the symbol in the symbol table
        move that symbol to the front of the symbol table
Decoding algorithm
    # Using the same starting symbol table
    for each index of the input sequence:
        output the symbol at that index of the symbol table
        move that symbol to the front of the symbol table
Example

Encoding the string of character symbols 'broood' using a symbol table of the characters 'a'-to-'z'

Input Output SymbolTable
broood 1 'abcdefghijklmnopqrstuvwxyz'
broood 1 17 'bacdefghijklmnopqrstuvwxyz'
broood 1 17 15 'rbacdefghijklmnopqstuvwxyz'
broood 1 17 15 0 'orbacdefghijklmnpqstuvwxyz'
broood 1 17 15 0 0 'orbacdefghijklmnpqstuvwxyz'
broood 1 17 15 0 0 5 'orbacdefghijklmnpqstuvwxyz'

Decoding the indices back to the original symbol order:

Input Output SymbolTable
1 17 15 0 0 5 b 'abcdefghijklmnopqrstuvwxyz'
1 17 15 0 0 5 br 'bacdefghijklmnopqrstuvwxyz'
1 17 15 0 0 5 bro 'rbacdefghijklmnopqstuvwxyz'
1 17 15 0 0 5 broo 'orbacdefghijklmnpqstuvwxyz'
1 17 15 0 0 5 brooo 'orbacdefghijklmnpqstuvwxyz'
1 17 15 0 0 5 broood 'orbacdefghijklmnpqstuvwxyz'
Task
  • Encode and decode the following three strings of characters using the symbol table of the characters 'a'-to-'z' as above.
  • Show the strings and their encoding here.
  • Add a check to ensure that the decoded string is the same as the original.


The strings are:

   broood          
   bananaaa     
   hiphophiphop  

(Note the spellings.)

Ada[edit]

with Ada.Text_IO;
 
procedure Move_To_Front is
 
subtype Lower_Case is Character range 'a' .. 'z';
subtype Index is Integer range 0 .. 25;
type Table is array (Index) of Lower_Case;
Alphabet: constant Table := "abcdefghijklmnopqrstuvwxyz";
type Number_String is array(Positive range <>) of Natural;
 
function Encode(S: String) return Number_String is
Key: Table := Alphabet;
 
function Encode(S: String; Tab: in out Table) return Number_String is
 
procedure Look_Up(A: in out Table; Ch: Lower_Case; Pos: out Index) is
begin
for I in A'Range loop
if A(I) = Ch then
Pos := I;
A := A(Pos) & A(A'First .. Pos-1) & A(Pos+1 .. A'Last);
return;
end if;
end loop;
raise Program_Error with "unknown character";
end Look_Up;
 
Empty: Number_String(1 .. 0);
Result: Natural;
begin
if S'Length = 0 then
return Empty;
else
Look_Up(Tab, S(S'First), Result);
return Result & Encode(S(S'First+1 .. S'Last), Tab);
end if;
end Encode;
 
begin
return Encode(S, Key);
end Encode;
 
function Decode(N: Number_String) return String is
Key: Table := Alphabet;
 
function Decode(N: Number_String; Tab: in out Table) return String is
 
procedure Look_Up(A: in out Table; Pos: Index; Ch: out Lower_Case) is
begin
Ch := A(Pos);
A := A(Pos) & A(A'First .. Pos-1) & A(Pos+1 .. A'Last);
end Look_Up;
 
Result: String(N'Range);
begin
for I in N'Range loop
Look_Up(Tab, N(I), Result(I));
end loop;
return Result;
end Decode;
 
begin
return Decode(N, Key);
end Decode;
 
procedure Encode_Write_Check(S: String) is
N: Number_String := Encode(S);
T: String := Decode(N);
Check: String := (if S=T then "Correct!" else "*WRONG*!");
begin
Ada.Text_IO.Put("'" & S & "' encodes to");
for Num of N loop
Ada.Text_IO.Put(Integer'Image(Num));
end loop;
Ada.Text_IO.Put_Line(". This decodes to '" & T & "'. " & Check);
end Encode_Write_Check;
 
begin
Encode_Write_Check("broood");
Encode_Write_Check("bananaaa");
Encode_Write_Check("hiphophiphop");
end Move_To_Front;
Output:
'broood' encodes to 1 17 15 0 0 5. This decodes to 'broood'. Correct!
'bananaaa' encodes to 1 1 13 1 1 1 0 0. This decodes to 'bananaaa'. Correct!
'hiphophiphop' encodes to 7 8 15 2 15 2 2 3 2 2 3 2. This decodes to 'hiphophiphop'. Correct!

ALGOL 68[edit]

Works with: ALGOL 68G version Any - tested with release 2.8.win32
# move the character at text pos to the front of text                         #
# note text pos is based from 0 #
PROC move to front = ( STRING text, INT text pos )STRING:
IF text pos < 1
THEN
# the character is already at the front (or not in the string) #
text
ELSE
# the character isn't already at the front - construct the new string #
 
INT char pos = LWB text + text pos;
 
( text[ char pos : char pos ]
+ text[ : char pos - 1 ]
+ text[ char pos + 1 : ]
)
 
FI;
 
 
# encode the string "text", using "initial table" as the starting symbol table#
PROC encode = ( STRING text, STRING initial table )[]INT:
BEGIN
 
[ 1 : ( UPB text - LWB text ) + 1 ]INT result;
 
STRING symbol table := initial table;
 
FOR text pos FROM LWB text TO UPB text
DO
INT symbol pos := 0;
 
result[ text pos ]
:= IF char in string( text[ text pos ], symbol pos, symbol table )
THEN
# the character is in the symbol table at symbol pos #
# (indexed from LWB text) - we store the positions #
# indexed from 0 #
symbol pos - LWB text
ELSE
# the character isn't in the symbol table #
-1
FI;
 
# modify the symbol table so the latest character is at the front #
symbol table := move to front( symbol table, result[ text pos ] )
 
OD;
 
result
END; # encode #
 
 
 
# decode "encoded", using "initial table" as the starting symbol table #
PROC decode = ( []INT encoded, STRING initial table )STRING:
BEGIN
 
STRING result := "";
STRING symbol table := initial table;
 
FOR text pos FROM LWB encoded TO UPB encoded
DO
result
+:= IF encoded[ text pos ] < 0
THEN
# the encoded character wasn't in the string #
"?"
ELSE
# the character is in the symbol table #
symbol table[ encoded[ text pos ] + LWB symbol table ]
FI;
 
# modify the symbol table so the latest character is at the front #
symbol table := move to front( symbol table, encoded[ text pos ] )
 
OD;
 
result
END; # decode #
 
 
 
# routine to test the encode and decode routines #
PROC test encode and decode = ( STRING text )VOID:
BEGIN
 
# initial value for the symbol table #
[]CHAR initial table = "abcdefghijklmnopqrstuvwxyz";
 
# procedure to format the encoded value #
PROC format encoded value = ( []INT values )STRING:
BEGIN
STRING result := "";
FOR value pos FROM LWB values TO UPB values
DO
result +:= ( " " + whole( values[ value pos ], 0 ) )
OD;
 
result
END; # format encoded value #
 
[]INT encoded = encode( text, initial table );
STRING decoded = decode( encoded, initial table );
 
print( ( ( text
+ " encodes to ["
+ format encoded value( encoded )
+ " ] which "
+ IF text = decoded
THEN
"correctly"
ELSE
"INCORRECTLY"
FI
+ " decodes to """
+ decoded
+ """"
)
, newline
)
)
 
END; # test encode and decode #
 
 
 
main: (
 
test encode and decode( "broood" )
; test encode and decode( "bananaaa" )
; test encode and decode( "hiphophiphop" )
 
# ; test encode and decode( "abcdefghijklmnopqrstuvwxyz" ) #
# ; test encode and decode( "zyxwvutsrqponmlkjihgfedcba" ) #
 
)
Output:
broood encodes to [ 1 17 15 0 0 5 ] which correctly decodes to "broood"
bananaaa encodes to [ 1 1 13 1 1 1 0 0 ] which correctly decodes to "bananaaa"
hiphophiphop encodes to [ 7 8 15 2 15 2 2 3 2 2 3 2 ] which correctly decodes to "hiphophiphop"

AutoHotkey[edit]

MTF_Encode(string){
str := "abcdefghijklmnopqrstuvwxyz"
loop, parse, string
code .= (A_Index>1 ? ",":"") . InStr(str, A_LoopField) - 1 , str := A_LoopField . StrReplace(str, A_LoopField)
return code
}
 
MTF_Decode(code){
str := "abcdefghijklmnopqrstuvwxyz"
loop, parse, code, `,
string .= (letter := SubStr(str, A_LoopField+1, 1)) , str := letter . StrReplace(str, letter)
return string
}
 
Examples:
testStrings = broood,bananaaa,hiphophiphop
loop, parse, testStrings, `,
Output .= A_LoopField "`t" MTF_Encode(A_LoopField) "`t" MTF_Decode(MTF_Encode(A_LoopField)) "`n"
MsgBox  % Output
return
Outputs:
broood		1,17,15,0,0,5			broood
bananaaa	1,1,13,1,1,1,0,0		bananaaa
hiphophiphop	7,8,15,2,15,2,2,3,2,2,3,2	hiphophiphop

C[edit]

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
 
#define MAX_SIZE 100
 
int move_to_front(char *str,char c)
{
char *q,*p;
int shift=0;
p=(char *)malloc(strlen(str)+1);
strcpy(p,str);
q=strchr(p,c); //returns pointer to location of char c in string str
shift=q-p; // no of characters from 0 to position of c in str
strncpy(str+1,p,shift);
str[0]=c;
free(p);
// printf("\n%s\n",str);
return shift;
}
 
void decode(int* pass,int size,char *sym)
{
int i,index;
char c;
char table[]="abcdefghijklmnopqrstuvwxyz";
for(i=0;i<size;i++)
{
c=table[pass[i]];
index=move_to_front(table,c);
if(pass[i]!=index) printf("there is an error");
sym[i]=c;
}
sym[size]='\0';
}
 
void encode(char *sym,int size,int *pass)
{
int i=0;
char c;
char table[]="abcdefghijklmnopqrstuvwxyz";
for(i=0;i<size;i++)
{
c=sym[i];
pass[i]=move_to_front(table,c);
}
}
 
int check(char *sym,int size,int *pass)
{
int *pass2=malloc(sizeof(int)*size);
char *sym2=malloc(sizeof(char)*size);
int i,val=1;
 
encode(sym,size,pass2);
i=0;
while(i<size && pass[i]==pass2[i])i++;
if(i!=size)val=0;
 
decode(pass,size,sym2);
if(strcmp(sym,sym2)!=0)val=0;
 
free(sym2);
free(pass2);
 
return val;
}
 
int main()
{
char sym[3][MAX_SIZE]={"broood","bananaaa","hiphophiphop"};
int pass[MAX_SIZE]={0};
int i,len,j;
for(i=0;i<3;i++)
{
len=strlen(sym[i]);
encode(sym[i],len,pass);
printf("%s : [",sym[i]);
for(j=0;j<len;j++)
printf("%d ",pass[j]);
printf("]\n");
if(check(sym[i],len,pass))
printf("Correct :)\n");
else
printf("Incorrect :(\n");
}
return 0;
}
Output:
broood : [1 17 15 0 0 5 ]
Correct :)
bananaaa : [1 1 13 1 1 1 0 0 ]
Correct :)
hiphophiphop : [7 8 15 2 15 2 2 3 2 2 3 2 ]
Correct :)

C++[edit]

 
#include <iostream>
#include <iterator>
#include <sstream>
#include <vector>
 
using namespace std;
 
class MTF
{
public:
string encode( string str )
{
fillSymbolTable();
vector<int> output;
for( string::iterator it = str.begin(); it != str.end(); it++ )
{
for( int i = 0; i < 26; i++ )
{
if( *it == symbolTable[i] )
{
output.push_back( i );
moveToFront( i );
break;
}
}
}
string r;
for( vector<int>::iterator it = output.begin(); it != output.end(); it++ )
{
ostringstream ss;
ss << *it;
r += ss.str() + " ";
}
return r;
}
 
string decode( string str )
{
fillSymbolTable();
istringstream iss( str ); vector<int> output;
copy( istream_iterator<int>( iss ), istream_iterator<int>(), back_inserter<vector<int> >( output ) );
string r;
for( vector<int>::iterator it = output.begin(); it != output.end(); it++ )
{
r.append( 1, symbolTable[*it] );
moveToFront( *it );
}
return r;
}
 
private:
void moveToFront( int i )
{
char t = symbolTable[i];
for( int z = i - 1; z >= 0; z-- )
symbolTable[z + 1] = symbolTable[z];
 
symbolTable[0] = t;
}
 
void fillSymbolTable()
{
for( int x = 0; x < 26; x++ )
symbolTable[x] = x + 'a';
}
 
char symbolTable[26];
};
 
int main()
{
MTF mtf;
string a, str[] = { "broood", "bananaaa", "hiphophiphop" };
for( int x = 0; x < 3; x++ )
{
a = str[x];
cout << a << " -> encoded = ";
a = mtf.encode( a );
cout << a << "; decoded = " << mtf.decode( a ) << endl;
}
return 0;
}
 
Output:
broood -> encoded = 1 17 15 0 0 5 ; decoded = broood
bananaaa -> encoded = 1 1 13 1 1 1 0 0 ; decoded = bananaaa
hiphophiphop -> encoded = 7 8 15 2 15 2 2 3 2 2 3 2 ; decoded = hiphophiphop

C#[edit]

 
using System;
using System.Collections.Generic;
using System.Text;
 
namespace MoveToFront
{
class Program
{
private static char[] symbolTable;
private static void setSymbolTable()
{
symbolTable = "abcdefghijklmnopqrstuvwxyz".ToCharArray();
}
 
private static void moveToFront(int charIndex)
{
char toFront = symbolTable[charIndex];
for (int j = charIndex; j > 0; j--)
{
symbolTable[j] = symbolTable[j - 1];
}
symbolTable[0] = toFront;
}
 
public static int[] Encode(string input)
{
setSymbolTable();
var output = new List<int>();
foreach (char c in input)
{
for (int i = 0; i < 26; i++)
{
if (symbolTable[i] == c)
{
output.Add(i);
moveToFront(i);
break;
}
}
}
return output.ToArray();
}
 
public static string Decode(int[] input)
{
setSymbolTable();
var output = new StringBuilder(input.Length);
foreach (int n in input)
{
output.Append(symbolTable[n]);
moveToFront(n);
}
return output.ToString();
}
 
static void Main(string[] args)
{
string[] testInputs = new string[] { "broood", "bananaaa", "hiphophiphop" };
int[] encoding;
foreach (string s in testInputs)
{
Console.WriteLine($"Encoding for '{s}':");
encoding = Encode(s);
foreach (int i in encoding)
{
Console.Write($"{i} ");
}
Console.WriteLine($"\nDecoding for '{s}':");
Console.WriteLine($"{Decode(encoding)}\n");
}
}
}
}
 

Common Lisp[edit]

(defconstant +lower+ (coerce "abcdefghijklmnopqrstuvwxyz" 'list))
 
(defun move-to-front (x xs)
(cons x (remove x xs)))
 
(defun enc (text table)
(map 'list
(lambda (c)
(let ((idx (position c table)))
(setf table (move-to-front c table))
idx))
text))
 
(defun dec (indices table)
(coerce (mapcar (lambda (idx)
(let ((c (nth idx table)))
(setf table (move-to-front c table))
c))
indices)
'string))
 
(loop for word in '("broood" "bananaaa" "hiphophiphop")
do (let* ((encoded (enc word +lower+))
(decoded (dec encoded +lower+)))
(assert (string= word decoded))
(format T "~s encodes to ~a which decodes back to ~s.~%"
word encoded decoded)))
Output:
"broood" encodes to (1 17 15 0 0 5) which decodes back to "broood".
"bananaaa" encodes to (1 1 13 1 1 1 0 0) which decodes back to "bananaaa".
"hiphophiphop" encodes to (7 8 15 2 15 2 2 3 2 2 3 2) which decodes back to "hiphophiphop".

D[edit]

import std.stdio, std.string, std.ascii, std.algorithm;
 
ptrdiff_t[] mtfEncoder(in string data) pure nothrow @safe
in {
assert(data.countchars("a-z") == data.length);
} out(result) {
assert(result.length == data.length);
assert(result.all!(e => e >= 0 && e < lowercase.length));
} body {
ubyte[lowercase.length] order = lowercase.representation;
auto encoded = new typeof(return)(data.length);
 
size_t i = 0;
foreach (immutable b; data) {
immutable j = encoded[i++] = order[].countUntil(b);
bringToFront(order[0 .. j], order[j .. j + 1]);
}
 
return encoded;
}
 
string mtfDecoder(in ptrdiff_t[] encoded) pure nothrow @safe
in {
assert(encoded.all!(e => e >= 0 && e < lowercase.length));
} out(result) {
assert(result.length == encoded.length);
assert(result.countchars("a-z") == result.length);
} body {
ubyte[lowercase.length] order = lowercase.representation;
auto decoded = new char[encoded.length];
 
size_t i = 0;
foreach (immutable code; encoded) {
decoded[i++] = order[code];
bringToFront(order[0 .. code], order[code .. code + 1]);
}
 
return decoded;
}
 
void main() {
foreach (immutable word; ["broood", "bananaaa", "hiphophiphop"]) {
immutable encoded = word.mtfEncoder;
immutable decoded = encoded.mtfDecoder;
writefln("'%s' encodes to %s, which decodes back to '%s'",
word, encoded, decoded);
assert(word == decoded);
}
}
Output:
'broood' encodes to [1, 17, 15, 0, 0, 5], which decodes back to 'broood'
'bananaaa' encodes to [1, 1, 13, 1, 1, 1, 0, 0], which decodes back to 'bananaaa'
'hiphophiphop' encodes to [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2], which decodes back to 'hiphophiphop'

Elixir[edit]

defmodule MoveToFront do
@table Enum.to_list(?a..?z)
 
def encode(text), do: encode(to_char_list(text), @table, [])
 
defp encode([], _, output), do: Enum.reverse(output)
defp encode([h|t], table, output) do
i = Enum.find_index(table, &(&1 == h))
encode(t, move2front(table, i), [i | output])
end
 
def decode(indices), do: decode(indices, @table, [])
 
defp decode([], _, output), do: Enum.reverse(output) |> to_string
defp decode([h|t], table, output) do
decode(t, move2front(table, h), [Enum.at(table, h) | output])
end
 
def move2front(table, i), do: [Enum.at(table,i) | List.delete_at(table, i)]
end
 
Enum.each(["broood", "bananaaa", "hiphophiphop"], fn word ->
IO.inspect word
IO.inspect enc = MoveToFront.encode(word)
IO.puts "#{word == MoveToFront.decode(enc)}\n"
end)
Output:
"broood"
[1, 17, 15, 0, 0, 5]
true

"bananaaa"
[1, 1, 13, 1, 1, 1, 0, 0]
true

"hiphophiphop"
[7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2]
true

Gambas[edit]

Public Sub Main()
Dim sToCode As String[] = ["broood", "bananaaa", "hiphophiphop"] 'Samples to process
Dim sHold As New String[] 'To store results
Dim siCount, siCounter, siPos As Short 'Various variables
Dim sOutput, sCode, sWork, sEach As String 'Various variables
 
For siCounter = 0 To sToCode.Max 'To loop through each 'Sample'
sCode = "abcdefghijklmnopqrstuvwxyz" 'Set sCode to default setting
For siCount = 1 To Len(sToCode[siCounter]) 'Loop through each letter in 'Sample'
sWork = Mid(sToCode[siCounter], siCount, 1) 'sWork to store the Letter
siPos = InStr(scode, sWork) - 1 'Find the position of the letter in sCode, -1 for '0' based array
sOutput &= Str(siPos) & " " 'Add the postion to sOutput
sCode = Mid(sCode, siPos + 1, 1) & Replace(sCode, sWork, "") 'sCode = the letter + the rest of sCode except the letter
Next
Print sToCode[siCounter] & " = " & sOutput 'Print the 'Sample' and the coded version
sHold.Add(Trim(sOutput)) 'Add the code to the sHold array
sOutput = "" 'Clear sOutput
Next
 
Print 'Print a blank line
 
For siCounter = 0 To sHold.Max 'To loop through each coded 'Sample'
sCode = "abcdefghijklmnopqrstuvwxyz" 'Set sCode to default setting
For Each sEach In Split(sHold[siCounter], " ") 'For each 'code' in coded 'Sample'
sWork = Mid(sCode, Val(sEach) + 1, 1) 'sWork = the decoded letter
sOutput &= sWork 'Add the decoded letter to sOutput
sCode = sWork & Replace(sCode, sWork, "") 'sCode = the decoded letter + the rest of sCode except the letter
Next
Print sHold[siCounter] & " = " & sOutput 'Print the coded and decoded result
sOutput = "" 'Clear sOutput
Next

Output:

broood = 1 17 15 0 0 5 
bananaaa = 1 1 13 1 1 1 0 0 
hiphophiphop = 7 8 15 2 15 2 2 3 2 2 3 2 

1 17 15 0 0 5 = broood
1 1 13 1 1 1 0 0 = bananaaa
7 8 15 2 15 2 2 3 2 2 3 2 = hiphophiphop

Go[edit]

Translation of: Python
package main
 
import (
"bytes"
"fmt"
)
 
type symbolTable string
 
func (symbols symbolTable) encode(s string) []byte {
seq := make([]byte, len(s))
pad := []byte(symbols)
c1 := []byte{0}
for i := 0; i < len(s); i++ {
c := s[i]
c1[0] = c
x := byte(bytes.Index(pad, c1))
seq[i] = x
copy(pad[1:], pad[:x])
pad[0] = c
}
return seq
}
 
func (symbols symbolTable) decode(seq []byte) string {
chars := make([]byte, len(seq))
pad := []byte(symbols)
for i, x := range seq {
c := pad[x]
chars[i] = c
copy(pad[1:], pad[:x])
pad[0] = c
}
return string(chars)
}
 
func main() {
m := symbolTable("abcdefghijklmnopqrstuvwxyz")
for _, s := range []string{"broood", "bananaaa", "hiphophiphop"} {
enc := m.encode(s)
dec := m.decode(enc)
fmt.Println(s, enc, dec)
if dec != s {
panic("Whoops!")
}
}
}
Output:
broood [1 17 15 0 0 5] broood
bananaaa [1 1 13 1 1 1 0 0] bananaaa
hiphophiphop [7 8 15 2 15 2 2 3 2 2 3 2] hiphophiphop

Haskell[edit]

import Data.List (delete, elemIndex, mapAccumL)
 
import Data.Maybe (fromJust)
 
table :: String
table = ['a' .. 'z']
 
encode :: String -> [Int]
encode =
let f t s = (s : delete s t, fromJust (elemIndex s t))
in snd . mapAccumL f table
 
decode :: [Int] -> String
decode = snd . mapAccumL f table
where
f t i =
let s = (t !! i)
in (s : delete s t, s)
 
main :: IO ()
main =
mapM_ print $
(,) <*> uncurry ((==) . fst) <$> -- Test that ((fst . fst) x) == snd x)
((,) <*> (decode . snd) <$>
((,) <*> encode <$> ["broood", "bananaaa", "hiphophiphop"]))
Output:
((("broood",[1,17,15,0,0,5]),"broood"),True)
((("bananaaa",[1,1,13,1,1,1,0,0]),"bananaaa"),True)
((("hiphophiphop",[7,8,15,2,15,2,2,3,2,2,3,2]),"hiphophiphop"),True)

Icon and Unicon[edit]

Works in both languages:

procedure main(A)
every writes(s := !A, " -> [") do {
every writes(!(enc := encode(&lcase,s))," ")
writes("] -> ",s2 := decode(&lcase,enc))
write((s == s2, " (Correct)") | " (Incorrect)")
}
end
 
procedure encode(m,s)
enc := []
every c := !s do {
m ?:= reorder(tab(i := upto(c)),move(1),tab(0))
put(enc,i-1) # Strings are 1-based
}
return enc
end
 
procedure decode(m,enc)
dec := ""
every i := 1 + !enc do { # Lists are 1-based
dec ||:= m[i]
m ?:= reorder(tab(i),move(1),tab(0))
}
return dec
end
 
procedure reorder(s1,s2,s3)
return s2||s1||s3
end

Sample run:

->mtfa broood bananaaa hiphophiphop
broood -> [1 17 15 0 0 5 ] -> broood (Correct)
bananaaa -> [1 1 13 1 1 1 0 0 ] -> bananaaa (Correct)
hiphophiphop -> [7 8 15 2 15 2 2 3 2 2 3 2 ] -> hiphophiphop (Correct)
->

J[edit]

spindizzy=:3 :0
'seq table'=. y
ndx=.$0
orig=. table
for_sym. seq do.
ndx=.ndx,table i.sym
table=. sym,table-.sym
end.
ndx
assert. seq-:yzzidnips ndx;orig
)
 
yzzidnips=:3 :0
'ndx table'=. y
seq=.''
for_n. ndx do.
seq=.seq,sym=. n{table
table=. sym,table-.sym
end.
seq
)

Required examples:

   spindizzy 'broood';'abcdefghijklmnopqrstuvwxyz'
1 17 15 0 0 5
spindizzy 'bananaaa';'abcdefghijklmnopqrstuvwxyz'
1 1 13 1 1 1 0 0
spindizzy 'hiphophiphop';'abcdefghijklmnopqrstuvwxyz'
7 8 15 2 15 2 2 3 2 2 3 2

It's not clear, though, why anyone would think that this is any better than lookups against an unmodified symbol table.

Java[edit]

Works with: Java version 1.5+
import java.util.LinkedList;
import java.util.List;
 
public class MTF{
public static List<Integer> encode(String msg, String symTable){
List<Integer> output = new LinkedList<Integer>();
StringBuilder s = new StringBuilder(symTable);
for(char c : msg.toCharArray()){
int idx = s.indexOf("" + c);
output.add(idx);
s = s.deleteCharAt(idx).insert(0, c);
}
return output;
}
 
public static String decode(List<Integer> idxs, String symTable){
StringBuilder output = new StringBuilder();
StringBuilder s = new StringBuilder(symTable);
for(int idx : idxs){
char c = s.charAt(idx);
output = output.append(c);
s = s.deleteCharAt(idx).insert(0, c);
}
return output.toString();
}
 
private static void test(String toEncode, String symTable){
List<Integer> encoded = encode(toEncode, symTable);
System.out.println(toEncode + ": " + encoded);
String decoded = decode(encoded, symTable);
System.out.println((toEncode.equals(decoded) ? "" : "in") + "correctly decoded to " + decoded);
}
 
public static void main(String[] args){
String symTable = "abcdefghijklmnopqrstuvwxyz";
test("broood", symTable);
test("bananaaa", symTable);
test("hiphophiphop", symTable);
}
}
Output:
broood: [1, 17, 15, 0, 0, 5]
correctly decoded to broood
bananaaa: [1, 1, 13, 1, 1, 1, 0, 0]
correctly decoded to bananaaa
hiphophiphop: [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2]
correctly decoded to hiphophiphop

JavaScript[edit]

var encodeMTF = function (word) {
var init = {wordAsNumbers: [], charList: 'abcdefghijklmnopqrstuvwxyz'.split('')};
 
return word.split('').reduce(function (acc, char) {
var charNum = acc.charList.indexOf(char); //get index of char
acc.wordAsNumbers.push(charNum); //add original index to acc
acc.charList.unshift(acc.charList.splice(charNum, 1)[0]); //put at beginning of list
return acc;
}, init).wordAsNumbers; //return number list
};
 
var decodeMTF = function (numList) {
var init = {word: '', charList: 'abcdefghijklmnopqrstuvwxyz'.split('')};
 
return numList.reduce(function (acc, num) {
acc.word += acc.charList[num];
acc.charList.unshift(acc.charList.splice(num, 1)[0]); //put at beginning of list
return acc;
}, init).word;
};
 
//test our algorithms
var words = ['broood', 'bananaaa', 'hiphophiphop'];
var encoded = words.map(encodeMTF);
var decoded = encoded.map(decodeMTF);
 
//print results
console.log("from encoded:");
console.log(encoded);
console.log("from decoded:");
console.log(decoded);
Output:
from encoded:
[
  [1, 17, 15, 0, 0, 5],
  [1, 1, 13, 1, 1, 1, 0, 0],
  [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2]
]
from decoded:
['broood', 'bananaaa', 'hiphophiphop']

jq[edit]

Works with: jq version q.4
# Input is the string to be encoded, st is the initial symbol table (an array)
# Output: the encoded string (an array)
def m2f_encode(st):
reduce explode[] as $ch
( [ [], st]; # state: [ans, st]
(.[1]|index($ch)) as $ix
| .[1] as $st
| [ (.[0] + [ $ix ]), [$st[$ix]] + $st[0:$ix] + $st[$ix+1:] ] )
| .[0];
 
# Input should be the encoded string (an array)
# and st should be the initial symbol table (an array)
def m2f_decode(st):
reduce .[] as $ix
( [ [], st]; # state: [ans, st]
.[1] as $st
| [ (.[0] + [ $st[$ix] ]), [$st[$ix]] + $st[0:$ix] + $st[$ix+1:] ] )
| .[0]
| implode;

Example:

("abcdefghijklmnopqrstuvwxyz" | explode) as $ST
| ("broood", "bananaaa", "hiphophiphop")
| . as $string
| m2f_encode($ST)
| . as $encoded
| m2f_decode($ST) as $decoded
| if $string == $decoded then "\($string) => \($encoded) => \($decoded)"
else "INTERNAL ERROR: encoding of \($string) => \($encoded) => \($decoded)"
end
Output:
$ jq -r -n -f move_to_front.jq
broood => [1,17,15,0,0,5] => broood
bananaaa => [1,1,13,1,1,1,0,0] => bananaaa
hiphophiphop => [7,8,15,2,15,2,2,3,2,2,3,2] => hiphophiphop

Kotlin[edit]

// version 1.1.2
 
fun encode(s: String): IntArray {
if (s.isEmpty()) return intArrayOf()
val symbols = "abcdefghijklmnopqrstuvwxyz".toCharArray()
val result = IntArray(s.length)
for ((i, c) in s.withIndex()) {
val index = symbols.indexOf(c)
if (index == -1)
throw IllegalArgumentException("$s contains a non-alphabetic character")
result[i] = index
if (index == 0) continue
for (j in index - 1 downTo 0) symbols[j + 1] = symbols[j]
symbols[0] = c
}
return result
}
 
fun decode(a: IntArray): String {
if (a.isEmpty()) return ""
val symbols = "abcdefghijklmnopqrstuvwxyz".toCharArray()
val result = CharArray(a.size)
for ((i, n) in a.withIndex()) {
if (n !in 0..25)
throw IllegalArgumentException("${a.contentToString()} contains an invalid number")
result[i] = symbols[n]
if (n == 0) continue
for (j in n - 1 downTo 0) symbols[j + 1] = symbols[j]
symbols[0] = result[i]
}
return result.joinToString("")
}
 
fun main(args: Array<String>) {
val strings = arrayOf("broood", "bananaaa", "hiphophiphop")
val encoded = Array<IntArray?>(strings.size) { null }
for ((i, s) in strings.withIndex()) {
encoded[i] = encode(s)
println("${s.padEnd(12)} -> ${encoded[i]!!.contentToString()}")
}
println()
val decoded = Array<String?>(encoded.size) { null }
for ((i, a) in encoded.withIndex()) {
decoded[i] = decode(a!!)
print("${a.contentToString().padEnd(38)} -> ${decoded[i]!!.padEnd(12)}")
println(" -> ${if (decoded[i] == strings[i]) "correct" else "incorrect"}")
}
}
Output:
broood       -> [1, 17, 15, 0, 0, 5]
bananaaa     -> [1, 1, 13, 1, 1, 1, 0, 0]
hiphophiphop -> [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2]

[1, 17, 15, 0, 0, 5]                   -> broood       -> correct
[1, 1, 13, 1, 1, 1, 0, 0]              -> bananaaa     -> correct
[7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] -> hiphophiphop -> correct

Lua[edit]

-- Return table of the alphabet in lower case
function getAlphabet ()
local letters = {}
for ascii = 97, 122 do table.insert(letters, string.char(ascii)) end
return letters
end
 
-- Move the table value at ind to the front of tab
function moveToFront (tab, ind)
local toMove = tab[ind]
for i = ind - 1, 1, -1 do tab[i + 1] = tab[i] end
tab[1] = toMove
end
 
-- Perform move-to-front encoding on input
function encode (input)
local symbolTable, output, index = getAlphabet(), {}
for pos = 1, #input do
for k, v in pairs(symbolTable) do
if v == input:sub(pos, pos) then index = k end
end
moveToFront(symbolTable, index)
table.insert(output, index - 1)
end
return table.concat(output, " ")
end
 
-- Perform move-to-front decoding on input
function decode (input)
local symbolTable, output = getAlphabet(), ""
for num in input:gmatch("%d+") do
output = output .. symbolTable[num + 1]
moveToFront(symbolTable, num + 1)
end
return output
end
 
-- Main procedure
local testCases, output = {"broood", "bananaaa", "hiphophiphop"}
for _, case in pairs(testCases) do
output = encode(case)
print("Original string: " .. case)
print("Encoded: " .. output)
print("Decoded: " .. decode(output))
print()
end
Output:
Original string: broood
Encoded: 1 17 15 0 0 5
Decoded: broood

Original string: bananaaa
Encoded: 1 1 13 1 1 1 0 0
Decoded: bananaaa

Original string: hiphophiphop
Encoded: 7 8 15 2 15 2 2 3 2 2 3 2
Decoded: hiphophiphop

Mathematica[edit]

mtf[word_]:=Module[{f,f2,p,q},
f[{output_,symList_},next_]:=Module[{index},index=Position[symList,next][[1,1]]-1;
{output~Append~index,Prepend[Delete[symList,index+1],next]}];
p=Fold[f,{{},CharacterRange["a","z"]},Characters[ToString[word]]][[1]];
f2[{output_,symList_},next_]:=Module[{index},index=symList[[next+1]];
{output~Append~index,Prepend[DeleteCases[symList,ToString[index]],index]}];
q=Fold[f2,{{},CharacterRange["a","z"]},p][[1]];
Print["'", word,"' encodes to: ",p, " - " ,p," decodes to: '",[email protected],"' - Input equals Output: " ,[email protected]];]

Testing out the function:

mtf /@ {"broood", "bananaaa", "hiphophiphop"}
Output:
'broood' encodes to: {1,17,15,0,0,5} - {1,17,15,0,0,5} decodes to: 'broood' - Input equals Output: True
'bananaaa' encodes to: {1,1,13,1,1,1,0,0} - {1,1,13,1,1,1,0,0} decodes to: 'bananaaa' - Input equals Output: True
'hiphophiphop' encodes to: {7,8,15,2,15,2,2,3,2,2,3,2} - {7,8,15,2,15,2,2,3,2,2,3,2} decodes to: 'hiphophiphop' - Input equals Output: True

MATLAB[edit]

function testMTF
symTable = 'abcdefghijklmnopqrstuvwxyz';
inStr = {'broood' 'bananaaa' 'hiphophiphop'};
for k = 1:length(inStr)
outArr = encodeMTF(inStr{k}, symTable);
outStr = decodeMTF(outArr, symTable);
fprintf('%s: [ %s]\n', inStr{k}, sprintf('%d ', outArr))
fprintf('%scorrectly decoded to %s\n', char('in'.*~strcmp(outStr, inStr{k})), outStr)
end
end
 
function arr = encodeMTF(str, symTable)
n = length(str);
arr = zeros(1, n);
for k = 1:n
arr(k) = find(str(k) == symTable, 1);
symTable = [symTable(arr(k)) symTable(1:arr(k)-1) symTable(arr(k)+1:end)];
end
arr = arr-1; % Change to zero-indexed array
end
 
function str = decodeMTF(arr, symTable)
arr = arr+1; % Change to one-indexed array
n = length(arr);
str = char(zeros(1, n));
for k = 1:n
str(k) = symTable(arr(k));
symTable = [symTable(arr(k)) symTable(1:arr(k)-1) symTable(arr(k)+1:end)];
end
end
Output:
broood: [ 1 17 15 0 0 5 ]
correctly decoded to broood
bananaaa: [ 1 1 13 1 1 1 0 0 ]
correctly decoded to bananaaa
hiphophiphop: [ 7 8 15 2 15 2 2 3 2 2 3 2 ]
correctly decoded to hiphophiphop

Perl[edit]

use strict;
use warnings;
sub encode {
my ($str) = @_;
my $table = join '', 'a' .. 'z';
map {
$table =~ s/(.*?)$_/$_$1/ or die;
length($1);
} split //, $str;
}
 
sub decode {
my $table = join '', 'a' .. 'z';
join "", map {
$table =~ s/(.{$_})(.)/$2$1/ or die;
$2;
} @_;
}
 
for my $test ( qw(broood bananaaa hiphophiphop) ) {
my @encoded = encode($test);
print "$test: @encoded\n";
my $decoded = decode(@encoded);
print "in" x ( $decoded ne $test );
print "correctly decoded to $decoded\n";
}
 
Output:
broood: 1 17 15 0 0 5
correctly decoded to broood
bananaaa: 1 1 13 1 1 1 0 0
correctly decoded to bananaaa
hiphophiphop: 7 8 15 2 15 2 2 3 2 2 3 2
correctly decoded to hiphophiphop

Perl 6[edit]

Works with: rakudo version 2015-09-24
sub encode ( Str $word ) {
my @sym = 'a' .. 'z';
gather for $word.comb -> $c {
die "Symbol '$c' not found in @sym" if $c eq @sym.none;
@sym[0 .. take (@sym ... $c).end] .= rotate(-1);
}
}
 
sub decode ( @enc ) {
my @sym = 'a' .. 'z';
[~] gather for @enc -> $pos {
take @sym[$pos];
@sym[0..$pos] .= rotate(-1);
}
}
 
use Test;
plan 3;
for <broood bananaaa hiphophiphop> -> $word {
my $enc = encode($word);
my $dec = decode($enc);
is $word, $dec, "$word.fmt('%-12s') ($enc[])";
}
Output:
1..3
ok 1 - broood       (1 17 15 0 0 5)
ok 2 - bananaaa     (1 1 13 1 1 1 0 0)
ok 3 - hiphophiphop (7 8 15 2 15 2 2 3 2 2 3 2)

Phix[edit]

function encode(string s)
string symtab = "abcdefghijklmnopqrstuvwxyz"
sequence res = {}
for i=1 to length(s) do
integer ch = s[i]
integer k = find(ch,symtab)
res &= k-1
for j=k to 2 by -1 do
symtab[j] = symtab[j-1]
end for
symtab[1] = ch
end for
return res
end function
 
function decode(sequence s)
string symtab = "abcdefghijklmnopqrstuvwxyz"
string res = ""
for i=1 to length(s) do
integer k = s[i]+1
integer ch = symtab[k]
res &= ch
for j=k to 2 by -1 do
symtab[j] = symtab[j-1]
end for
symtab[1] = ch
end for
return res
end function
 
procedure test(string s)
sequence e = encode(s)
string d = decode(e)
 ?{s,e,d,{"**ERROR**","ok"}[(s=d)+1]}
end procedure
test("broood")
test("bananaaa")
test("hiphophiphop")
Output:
{"broood",{1,17,15,0,0,5},"broood","ok"}
{"bananaaa",{1,1,13,1,1,1,0,0},"bananaaa","ok"}
{"hiphophiphop",{7,8,15,2,15,2,2,3,2,2,3,2},"hiphophiphop","ok"}

PicoLisp[edit]

(de encode (Str)
(let Table (chop "abcdefghijklmnopqrstuvwxyz")
(mapcar
'((C)
(dec
(prog1
(index C Table)
(rot Table @) ) ) )
(chop Str) ) ) )
 
(de decode (Lst)
(let Table (chop "abcdefghijklmnopqrstuvwxyz")
(pack
(mapcar
'((N)
(prog1
(get Table (inc 'N))
(rot Table N) ) )
Lst ) ) ) )

Test:

(test (1 17 15 0 0 5)
(encode "broood") )
(test "broood"
(decode (1 17 15 0 0 5)) )
 
(test (1 1 13 1 1 1 0 0)
(encode "bananaaa") )
(test "bananaaa"
(decode (1 1 13 1 1 1 0 0)) )
 
(test (7 8 15 2 15 2 2 3 2 2 3 2)
(encode "hiphophiphop") )
(test "hiphophiphop"
(decode (7 8 15 2 15 2 2 3 2 2 3 2)) )

PL/I[edit]

*process source attributes xref or(!);
/*********************************************************************
* 25.5.2014 Walter Pachl translated from REXX
*********************************************************************/

ed: Proc Options(main);
Call enc_dec('broood');
Call enc_dec('bananaaa');
Call enc_dec('hiphophiphop');
 
enc_dec: Proc(in);
Dcl in Char(*);
Dcl out Char(20) Var Init('');
Dcl st Char(26) Init('abcdefghijklmnopqrstuvwxyz');
Dcl sta Char(26) Init((st));
Dcl enc(20) Bin Fixed(31);
Dcl encn Bin Fixed(31) Init(0);
Dcl (i,p.k) Bin Fixed(31);
Dcl c Char(1);
Do i=1 To length(in);
c=substr(in,i,1);
p=index(st,c);
encn+=1;
enc(encn)=p-1;
st=c!!left(st,p-1)!!substr(st,p+1);
End;
Put Skip List(' in='!!in);
Put Skip List('sta='!!sta!!' original symbol table');
Put Skip Edit('enc=',(enc(i) do i=1 To encn))(a,20(F(3)));
Put Skip List(' st='!!st!!' symbol table after encoding');
Do i=1 To encn;
k=enc(i)+1;
out=out!!substr(sta,k,1);
sta=substr(sta,k,1)!!left(sta,k-1)!!substr(sta,k+1);
End;
Put Skip List('out='!!out);
Put Skip List(' ');
If out=in Then;
Else
Put Skip List('all wrong!!');
End;
Output:
 in=broood
sta=abcdefghijklmnopqrstuvwxyz original symbol table
enc=  1 17 15  0  0  5
 st=dorbacefghijklmnpqstuvwxyz symbol table after encoding
out=broood

 in=bananaaa
sta=abcdefghijklmnopqrstuvwxyz original symbol table
enc=  1  1 13  1  1  1  0  0
 st=anbcdefghijklmopqrstuvwxyz symbol table after encoding
out=bananaaa

 in=hiphophiphop
sta=abcdefghijklmnopqrstuvwxyz original symbol table
enc=  7  8 15  2 15  2  2  3  2  2  3  2
 st=pohiabcdefgjklmnqrstuvwxyz symbol table after encoding
out=hiphophiphop

PowerShell[edit]

Function Test-MTF
{
[CmdletBinding()]
Param
(
[Parameter(Mandatory=$true,Position=0)]
[string]$word,
 
[Parameter(Mandatory=$false)]
[string]$SymbolTable = 'abcdefghijklmnopqrstuvwxyz'
)
Begin
{
Function Encode
{
Param
(
[Parameter(Mandatory=$true,Position=0)]
[string]$word,
 
[Parameter(Mandatory=$false)]
[string]$SymbolTable = 'abcdefghijklmnopqrstuvwxyz'
)
foreach ($letter in $word.ToCharArray())
{
$index = $SymbolTable.IndexOf($letter)
 
$prop = [ordered]@{
Input = $letter
Output = [int]$index
SymbolTable = $SymbolTable
}
New-Object PSobject -Property $prop
$SymbolTable = $SymbolTable.Remove($index,1).Insert(0,$letter)
}
}
Function Decode
{
Param
(
[Parameter(Mandatory=$true,Position=0)]
[int[]]$index,
 
[Parameter(Mandatory=$false)]
[string]$SymbolTable = 'abcdefghijklmnopqrstuvwxyz'
)
foreach ($i in $index)
{
#Write-host $i -ForegroundColor Red
$letter = $SymbolTable.Chars($i)
 
$prop = [ordered]@{
Input = $i
Output = $letter
SymbolTable = $SymbolTable
}
New-Object PSObject -Property $prop
$SymbolTable = $SymbolTable.Remove($i,1).Insert(0,$letter)
}
}
}
Process
{
#Encoding
Write-Host "Encoding $word" -NoNewline
$Encoded = (Encode -word $word).output
Write-Host -NoNewline ": $($Encoded -join ',')"
 
#Decoding
Write-Host "`nDecoding $($Encoded -join ',')" -NoNewline
$Decoded = (Decode -index $Encoded).output -join ''
Write-Host -NoNewline ": $Decoded`n"
}
End{}
}
Output:
Test-MTF -word broood
Test-MTF -word bananaaa
Test-MTF -word hiphophiphop
Encoding broood: 1,17,15,0,0,5
Decoding 1,17,15,0,0,5: broood
Encoding bananaaa: 1,1,13,1,1,1,0,0
Decoding 1,1,13,1,1,1,0,0: bananaaa
Encoding hiphophiphop: 7,8,15,2,15,2,2,3,2,2,3,2
Decoding 7,8,15,2,15,2,2,3,2,2,3,2: hiphophiphop

Python[edit]

Python: Procedural[edit]

from __future__ import print_function
from string import ascii_lowercase
 
SYMBOLTABLE = list(ascii_lowercase)
 
def move2front_encode(strng, symboltable):
sequence, pad = [], symboltable[::]
for char in strng:
indx = pad.index(char)
sequence.append(indx)
pad = [pad.pop(indx)] + pad
return sequence
 
def move2front_decode(sequence, symboltable):
chars, pad = [], symboltable[::]
for indx in sequence:
char = pad[indx]
chars.append(char)
pad = [pad.pop(indx)] + pad
return ''.join(chars)
 
if __name__ == '__main__':
for s in ['broood', 'bananaaa', 'hiphophiphop']:
encode = move2front_encode(s, SYMBOLTABLE)
print('%14r encodes to %r' % (s, encode), end=', ')
decode = move2front_decode(encode, SYMBOLTABLE)
print('which decodes back to %r' % decode)
assert s == decode, 'Whoops!'
Output:
      'broood' encodes to [1, 17, 15, 0, 0, 5], which decodes back to 'broood'
    'bananaaa' encodes to [1, 1, 13, 1, 1, 1, 0, 0], which decodes back to 'bananaaa'
'hiphophiphop' encodes to [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2], which decodes back to 'hiphophiphop'

Python: Functional[edit]

From the procedural version note that through the encoding or decoding loops both the final output is accumulated as well as the symbol table being rearranged.
For the functional forms a 2-item list of output to be accumulated and symboltable manipulation is calculated then only the former accumulated as the later works to transform the symbol table in-place.

def m2f_e(s, st):
return [[st.index(ch), st.insert(0, st.pop(st.index(ch)))][0] for ch in s]
 
def m2f_d(sq, st):
return ''.join([st[i], st.insert(0, st.pop(i))][0] for i in sq)
 
ST = list('abcdefghijklmnopqrstuvwxyz')
for s in ['broood', 'bananaaa', 'hiphophiphop']:
encode = m2f_e(s, ST[::])
print('%14r encodes to %r' % (s, encode), end=', ')
decode = m2f_d(encode, ST[::])
print('decodes back to %r' % decode)
assert s == decode, 'Whoops!'
Output:

Similar to that of the procedural version above.

Racket[edit]

#lang racket
(define default-symtab "abcdefghijklmnopqrstuvwxyz")
 
(define (move-to-front:encode in (symtab default-symtab))
(define inner-encode
(match-lambda**
[((? string? (app string->list in)) st acc) ; make input all listy
(inner-encode in st acc)]
[(in (? string? (app string->list st)) acc) ; make symtab all listy
(inner-encode in st acc)]
[((list) _ (app reverse rv)) ; nothing more to encode
rv]
[((list a tail ...) (list left ... a right ...) acc) ; encode and recur
(inner-encode tail `(,a ,@left ,@right) (cons (length left) acc))]))
(inner-encode in symtab null))
 
(define (move-to-front:decode in (symtab default-symtab))
(define inner-decode
(match-lambda**
[(in (? string? (app string->list st)) acc) ; make symtab all listy
(inner-decode in st acc)]
[((list) _ (app (compose list->string reverse) rv)) ; nothing more to encode
rv]
[((list a tail ...) symbols acc) ; decode and recur
(match/values
(split-at symbols a)
[(l (cons ra rd))
(inner-decode tail (cons ra (append l rd)) (cons ra acc))])]))
(inner-decode in symtab null))
 
(module+ test
 ;; Test against the example in the task
(require rackunit)
(check-equal? (move-to-front:encode "broood") '(1 17 15 0 0 5))
(check-equal? (move-to-front:decode '(1 17 15 0 0 5)) "broood")
(check-equal? (move-to-front:decode (move-to-front:encode "broood")) "broood"))
 
(module+ main
(define (encode+decode-string str)
(define enc (move-to-front:encode str))
(define dec (move-to-front:decode enc))
(define crt (if (equal? dec str) "correctly" "incorrectly"))
(printf "~s encodes to ~s, which decodes ~s to ~s.~%" str enc crt dec))
 
(for-each encode+decode-string '("broood" "bananaaa" "hiphophiphop")))
Output:
"broood" encodes to (1 17 15 0 0 5), which decodes "correctly" to "broood".
"bananaaa" encodes to (1 1 13 1 1 1 0 0), which decodes "correctly" to "bananaaa".
"hiphophiphop" encodes to (7 8 15 2 15 2 2 3 2 2 3 2), which decodes "correctly" to "hiphophiphop".

REXX[edit]

version 1[edit]

/* REXX ***************************************************************
* 25.05.2014 Walter Pachl
* REXX strings start with position 1
**********************************************************************/

Call enc_dec 'broood'
Call enc_dec 'bananaaa'
Call enc_dec 'hiphophiphop'
Exit
enc_dec: Procedure
Parse Arg in
st='abcdefghijklmnopqrstuvwxyz'
sta=st /* remember this for decoding */
enc=''
Do i=1 To length(in)
c=substr(in,i,1)
p=pos(c,st)
enc=enc (p-1)
st=c||left(st,p-1)substr(st,p+1)
End
Say ' in='in
Say 'sta='sta 'original symbol table'
Say 'enc='enc
Say ' st='st 'symbol table after encoding'
out=''
Do i=1 To words(enc)
k=word(enc,i)+1
out=out||substr(sta,k,1)
sta=substr(sta,k,1)left(sta,k-1)substr(sta,k+1)
End
Say 'out='out
Say ' '
If out==in Then Nop
Else
Say 'all wrong!!'
Return
 
Output:
 in=broood
sta=abcdefghijklmnopqrstuvwxyz original symbol table
enc= 1 17 15 0 0 5
 st=dorbacefghijklmnpqstuvwxyz symbol table after encoding
out=broood

 in=bananaaa
sta=abcdefghijklmnopqrstuvwxyz original symbol table
enc= 1 1 13 1 1 1 0 0
 st=anbcdefghijklmopqrstuvwxyz symbol table after encoding
out=bananaaa

 in=hiphophiphop
sta=abcdefghijklmnopqrstuvwxyz original symbol table
enc= 7 8 15 2 15 2 2 3 2 2 3 2
 st=pohiabcdefgjklmnqrstuvwxyz symbol table after encoding
out=hiphophiphop

version 2[edit]

Programming note:   the two REXX statements that add/subtract   one   deal with the task's requirement that the symbol table be   zero-indexed   (the REXX language uses   unity-based   strings).

/*REXX program demonstrates the  move─to─front  algorithm  encode/decode  symbol table. */
parse arg xxx; if xxx='' then xxx= 'broood bananaaa hiphophiphop' /*use the default?*/
one=1 /*(offset) for this task's requirement.*/
do j=1 for words(xxx); x=word(xxx, j) /*process a single word at a time. */
@= 'abcdefghijklmnopqrstuvwxyz'; @@=@ /*symbol table: the lowercase alphabet */
$= /*set the decode string to a null. */
do k=1 for length(x); z=substr(x, k, 1) /*encrypt a symbol in the word. */
_=pos(z, @); if _==0 then iterate /*the symbol position in symbol table. */
$=$ _ - one; @=z || delstr(@, _, 1) /*(re-)adjust the symbol table string. */
end /*k*/ /* [↑] the move─to─front encoding. */
 != /*set the encode string to a null. */
do m=1 for words($); n=word($, m) +one /*decode the sequence table string. */
y=substr(@@, n, 1);  !=! || y /*the decode symbol for the N word.*/
@@=y || delstr(@@, n, 1) /*rebuild the symbol table string. */
end /*m*/ /* [↑] the move─to─front decoding. */
say ' word: ' left(x, 20) "encoding:" left($, 35) word('wrong OK', 1+(!==x) )
end /*j*/ /*stick a fork in it, we're all done. */
output   when using the default input:
  word:  broood               encoding:  1 17 15 0 0 5                      OK
  word:  bananaaa             encoding:  1 1 13 1 1 1 0 0                   OK
  word:  hiphophiphop         encoding:  7 8 15 2 15 2 2 3 2 2 3 2          OK

Ruby[edit]

Use a module as namespace:

module MoveToFront
 
ABC = ("a".."z").to_a.freeze
 
def self.encode(str)
ar = ABC.dup
str.chars.each_with_object([]) do |char, memo|
memo << (i = ar.index(char))
ar = m2f(ar,i)
end
end
 
def self.decode(indices)
ar = ABC.dup
indices.each_with_object("") do |i, str|
str << ar[i]
ar = m2f(ar,i)
end
end
 
private
def self.m2f(ar,i)
[ar.delete_at(i)] + ar
end
 
end
 
['broood', 'bananaaa', 'hiphophiphop'].each do |word|
p word == MoveToFront.decode(p MoveToFront.encode(p word))
end
Output:
"broood"
[1, 17, 15, 0, 0, 5]
true
"bananaaa"
[1, 1, 13, 1, 1, 1, 0, 0]
true
"hiphophiphop"
[7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2]
true

Scala[edit]

Works with: Scala version 2.11.8+
package rosetta
 
import scala.annotation.tailrec
 
object MoveToFront {
/**
* Default radix
*/

private val R = 256
 
/**
* Default symbol table
*/

private def symbolTable = (0 until R).map(_.toChar).mkString
 
/**
* Apply move-to-front encoding using default symbol table.
*/

def encode(s: String): List[Int] = {
encode(s, symbolTable)
}
 
/**
* Apply move-to-front encoding using symbol table <tt>symTable</tt>.
*/

def encode(s: String, symTable: String): List[Int] = {
val table = symTable.toCharArray
 
@inline @tailrec def moveToFront(ch: Char, index: Int, tmpout: Char): Int = {
val tmpin = table(index)
table(index) = tmpout
if (ch != tmpin)
moveToFront(ch, index + 1, tmpin)
else {
table(0) = ch
index
}
}
 
@tailrec def encodeString(output: List[Int], s: List[Char]): List[Int] = s match {
case Nil => output
case x :: xs => {
encodeString(moveToFront(x, 0, table(0)) :: output, s.tail)
}
}
encodeString(Nil, s.toList).reverse
}
 
/**
* Apply move-to-front decoding using default symbol table.
*/

def decode(ints: List[Int]): String = {
decode(ints, symbolTable)
}
 
/**
* Apply move-to-front decoding using symbol table <tt>symTable</tt>.
*/

def decode(lst: List[Int], symTable: String): String = {
val table = symTable.toCharArray
 
@inline def moveToFront(c: Char, index: Int) {
for (i <- index-1 to 0 by -1)
table(i+1) = table(i)
table(0) = c
}
 
@tailrec def decodeList(output: List[Char], lst: List[Int]): List[Char] = lst match {
case Nil => output
case x :: xs => {
val c = table(x)
moveToFront(c, x)
decodeList(c :: output, xs)
}
}
decodeList(Nil, lst).reverse.mkString
}
 
def test(toEncode: String, symTable: String) {
val encoded = encode(toEncode, symTable)
println(toEncode + ": " + encoded)
val decoded = decode(encoded, symTable)
if (toEncode != decoded)
print("in")
println("correctly decoded to " + decoded)
}
}
 
/**
* Unit tests the <tt>MoveToFront</tt> data type.
*/

object RosettaCodeMTF extends App {
val symTable = "abcdefghijklmnopqrstuvwxyz"
MoveToFront.test("broood", symTable)
MoveToFront.test("bananaaa", symTable)
MoveToFront.test("hiphophiphop", symTable)
}
Output:
broood: List(1, 17, 15, 0, 0, 5)
correctly decoded to broood
bananaaa: List(1, 1, 13, 1, 1, 1, 0, 0)
correctly decoded to bananaaa
hiphophiphop: List(7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2)
correctly decoded to hiphophiphop

Sidef[edit]

Translation of: Perl

Implemented using regular expressions:

func encode(str) {
var table = ('a'..'z' -> join);
str.chars.map { |c|
var s = '';
table.sub!(Regex('(.*?)' + c), {|s1| s=s1; c + s1});
s.len;
}
}
 
func decode(nums) {
var table = ('a'..'z' -> join);
nums.map { |n|
var s = '';
table.sub!(Regex('(.{' + n + '})(.)'), {|s1, s2| s=s2; s2 + s1});
s;
}.join;
}
 
%w(broood bananaaa hiphophiphop).each { |test|
var encoded = encode(test);
say "#{test}: #{encoded}";
var decoded = decode(encoded);
print "in" if (decoded != test);
say "correctly decoded to #{decoded}";
}

Alternatively, implemented as a module, using arrays:

module MoveToFront {
 
define ABC = @("a".."z")
 
func m2f(ar,i) {
[ar.delete_index(i)] + ar
}
 
func encode(str) {
var ar = ABC+[]
gather {
str.each_char { |char|
take(var i = ar.index(char))
ar = m2f(ar, i);
}
}
}
 
func decode(indices) {
var ar = ABC+[]
gather {
indices.each { |i|
take ar[i];
ar = m2f(ar, i)
}
}.join
}
}
 
%w(broood bananaaa hiphophiphop).each { |test|
var encoded = MoveToFront::encode(test);
say "#{test}: #{encoded}";
var decoded = MoveToFront::decode(encoded);
print "in" if (decoded != test);
say "correctly decoded to #{decoded}";
}
Output:
broood: 1 17 15 0 0 5
correctly decoded to broood
bananaaa: 1 1 13 1 1 1 0 0
correctly decoded to bananaaa
hiphophiphop: 7 8 15 2 15 2 2 3 2 2 3 2
correctly decoded to hiphophiphop

Swift[edit]

 
 
var str="broood"
var number:[Int]=[1,17,15,0,0,5]
 
//function to encode the string
func encode(st:String)->[Int]
{
 
var array:[Character]=["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
 
var num:[Int]=[]
var temp:Character="a"
var i1:Int=0
for i in st.characters
{
for j in 0...25
{
if i==array[j]
{
num.append(j)
temp=array[j]
i1=j
while(i1>0)
{
array[i1]=array[i1-1]
i1=i1-1
 
}
array[0]=temp
}
 
}
 
 
}
 
return num
 
}
 
func decode(s:[Int])->[Character]
{
 
var st1:[Character]=[]
var alph:[Character]=["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
var temp1:Character="a"
var i2:Int=0
for i in 0...s.character.count-1
{
i2=s[i]
st1.append(alph[i2])
temp1=alph[i2]
 
while(i2>0)
{
alph[i2]=alph[i2-1]
i2=i2-1
 
}
alph[0]=temp1
 
}
return st1
 
}
 
var encarr:[Int]=encode(st:str)
var decarr:[Character]=decode(s:number)
print(encarr)
print(decarr)
 

Tcl[edit]

Works with: Tcl version 8.6
package require Tcl 8.6
 
oo::class create MoveToFront {
variable symbolTable
constructor {symbols} {
set symbolTable [split $symbols ""]
}
 
method MoveToFront {table index} {
list [lindex $table $index] {*}[lreplace $table $index $index]
}
method encode {text} {
set t $symbolTable
set r {}
foreach c [split $text ""] {
set i [lsearch -exact $t $c]
lappend r $i
set t [my MoveToFront $t $i]
}
return $r
}
method decode {numbers} {
set t $symbolTable
set r ""
foreach n $numbers {
append r [lindex $t $n]
set t [my MoveToFront $t $n]
}
return $r
}
}
 
MoveToFront create mtf "abcdefghijklmnopqrstuvwxyz"
foreach tester {"broood" "bananaaa" "hiphophiphop"} {
set enc [mtf encode $tester]
set dec [mtf decode $enc]
puts [format "'%s' encodes to %s. This decodes to '%s'. %s" \
$tester $enc $dec [expr {$tester eq $dec ? "Correct!" : "WRONG!"}]]
}
Output:
'broood' encodes to 1 17 15 0 0 5. This decodes to 'broood'. Correct!
'bananaaa' encodes to 1 1 13 1 1 1 0 0. This decodes to 'bananaaa'. Correct!
'hiphophiphop' encodes to 7 8 15 2 15 2 2 3 2 2 3 2. This decodes to 'hiphophiphop'. Correct!

VBScript[edit]

Function mtf_encode(s)
'create the array list and populate it with the initial symbol position
Set symbol_table = CreateObject("System.Collections.ArrayList")
For j = 97 To 122 'a to z in decimal.
symbol_table.Add Chr(j)
Next
output = ""
For i = 1 To Len(s)
char = Mid(s,i,1)
If i = Len(s) Then
output = output & symbol_table.IndexOf(char,0)
symbol_table.RemoveAt(symbol_table.LastIndexOf(char))
symbol_table.Insert 0,char
Else
output = output & symbol_table.IndexOf(char,0) & " "
symbol_table.RemoveAt(symbol_table.LastIndexOf(char))
symbol_table.Insert 0,char
End If
Next
mtf_encode = output
End Function
 
Function mtf_decode(s)
'break the function argument into an array
code = Split(s," ")
'create the array list and populate it with the initial symbol position
Set symbol_table = CreateObject("System.Collections.ArrayList")
For j = 97 To 122 'a to z in decimal.
symbol_table.Add Chr(j)
Next
output = ""
For i = 0 To UBound(code)
char = symbol_table(code(i))
output = output & char
If code(i) <> 0 Then
symbol_table.RemoveAt(symbol_table.LastIndexOf(char))
symbol_table.Insert 0,char
End If
Next
mtf_decode = output
End Function
 
'Testing the functions
wordlist = Array("broood","bananaaa","hiphophiphop")
For Each word In wordlist
WScript.StdOut.Write word & " encodes as " & mtf_encode(word) & " and decodes as " &_
mtf_decode(mtf_encode(word)) & "."
WScript.StdOut.WriteBlankLines(1)
Next
Output:
broood encodes as 1 17 15 0 0 5 and decodes as broood.
bananaaa encodes as 1 1 13 1 1 1 0 0 and decodes as bananaaa.
hiphophiphop encodes as 7 8 15 2 15 2 2 3 2 2 3 2 and decodes as hiphophiphop.

zkl[edit]

fcn encode(text){ //-->List
st:=["a".."z"].pump(Data); //"abcd..z" as byte array
text.reduce(fcn(st,c,sink){
n:=st.index(c); sink.write(n); st.del(n).insert(0,c); },st,sink:=L());
sink;
}

Strings are immutable so we create a bit bucket (which is mutable) to hold the symbol table which can then be modified in place.

fcn decode(list){ //-->String
st:=["a".."z"].pump(String); //"abcd..z"
sink:=Sink(String);
list.reduce('wrap(st,n){ c:=st[n]; sink.write(c); c+st.del(n); },st);
sink.close();
}

Here, we create a new symbol table each round as we would have to convert the byte we got from the bit bucket to string (so it is a wash garbage wise).

texts:=T("broood","bananaaa","hiphophiphop");
out:=texts.apply(encode);
texts.zipWith(fcn(t,e){ println(t,"-->",e) },out);
 
out.apply(decode).println();
texts.zipWith('==,out.apply(decode)).println();
Output:
broood-->L(1,17,15,0,0,5)
bananaaa-->L(1,1,13,1,1,1,0,0)
hiphophiphop-->L(7,8,15,2,15,2,2,3,2,2,3,2)
L("broood","bananaaa","hiphophiphop")
L(True,True,True)