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', and 'hiphophiphop'. (Note the spellings).
Ada
<lang Ada>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;</lang>
- 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
<lang algol68># 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" ) #
)</lang>
- 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"
Common Lisp
<lang lisp>(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)))</lang>
- 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
<lang d>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);
}
}</lang>
- 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'
Go
<lang go>package main
import (
"bytes" "fmt"
)
type moveToFront string
func (symbols moveToFront) encode(s string) []int {
seq := make([]int, len(s))
pad := []byte(symbols)
c1 := []byte{0}
for i := 0; i < len(s); i++ {
c := s[i]
c1[0] = c
x := bytes.Index(pad, c1)
seq[i] = x
copy(pad[1:], pad[:x])
pad[0] = c
}
return seq
} func (symbols moveToFront) decode(seq []int) 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 := moveToFront("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!")
}
}
}</lang>
- 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
Icon and Unicon
Works in both languages: <lang unicon>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</lang>
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
<lang J>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
)</lang>
Required examples:
<lang J> 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</lang>
It's not clear, though, why anyone would think that this is any better than lookups against an unmodified symbol table.
Java
<lang java5>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); } }</lang>
- 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
Mathematica
<lang Mathematica>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=symListnext+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: '",StringJoin@q,"' - Input equals Output: " ,word===StringJoin@q];]</lang>
Testing out the function: <lang Mathematica>mtf /@ {"broood", "bananaaa", "hiphophiphop"}</lang>
- 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
<lang MATLAB>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</lang>
- 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
<lang perl>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"; } </lang>
- 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
<lang perl6>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 (Nil, @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[])";
} </lang>
- 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)
PL/I
<lang pli>*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;</lang>
- 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
Python
Python: Procedural
<lang python>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!'</lang>
- 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
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.
<lang python>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!'</lang>
- Output:
Similar to that of the procedural version above.
Racket
<lang racket>#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")))</lang>
- 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
version 1
<lang rexx>/* 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 </lang>
- 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
<lang rexx>/*REXX pgm demonstrates move-to-front algorithm encode/decode sym table.*/ parse arg stuff; if stuff= then stuff='broood bananaaa hiphophiphop'
do j=1 for words(stuff); x=word(stuff,j) /*process 1 word at a time*/
abc='abcdefghijklmnopqrstuvwxyz'; $= /*define the symbol table.*/
do k=1 for length(x); z=substr(x,k,1) /*encrypt a letter in word*/
_=pos(z,abc); if _==0 then iterate /*pos of letter in sym tab*/
$=$ _-1; abc=z || delstr(abc,_,1) /*adjust the symbol table.*/
end /*k*/ /* [↑] move-to-front alg.*/
abc='abcdefghijklmnopqrstuvwxyz'; != /*define the symbol table.*/
do m=1 for words($); n=word($,m)+1 /*decode the sequence tab.*/
y=substr(abc,n,1); !=!||y /*decode a letter of word.*/
abc=y || delstr(abc,n,1) /*re-build the symbol tab.*/
end /*m*/ /* [↑] show word, enc, OK*/
say 'word: ' left(x,20) 'encoding:' left($,35) word('wrong OK',1+(!==x))
end /*j*/ /*done encoding/decoding words. */
/*stick a fork in it, we're done.*/</lang>
output 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
Use a module as namespace: <lang ruby>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</lang>
- 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
Tcl
<lang tcl>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!"}]] }</lang>
- 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!
zkl
<lang zkl>fcn encode(text){ //-->List
st:=["a".."z"].aggregate(String).walk().toData().copy(); //"abcd..z"
text.reduce(fcn(st,c,sink){
n:=st.index(c); sink.write(n); st.del(n).insert(0,c); },st,sink:=L());
sink;
}</lang> String are immutable so we create a bit bucket (which is mutable) to hold the symbol table which can then be modified in place. <lang zkl>fcn decode(list){ //-->String
st:=["a".."z"].aggregate(String).walk(); //"abcd..z"
sink:=Sink(String);
list.reduce('wrap(st,n){ c:=st[n]; sink.write(c); c+st.del(n); },st);
sink.close();
}></lang> 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). <lang zkl>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();</lang>
- 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)