LZW compression: Difference between revisions
Line 2,958:
c = d
IF i > LEN(i$) THEN EXIT
FOR d = 1 TO l-1
IF w$ = dict$(d) THEN EXIT
IF d < l Then i += 1 : w$ += MID$(i$, i, 1)
} UNTIL d >= l
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Revision as of 08:33, 17 May 2019
![Task](http://static.miraheze.org/rosettacodewiki/thumb/b/ba/Rcode-button-task-crushed.png/64px-Rcode-button-task-crushed.png)
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
<lang 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))</lang>
Ada
lzw.ads: <lang Ada>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;</lang>
lzw.adb: <lang Ada>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;</lang>
test.adb: <lang Ada>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;</lang>
BaCon
<lang 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</lang>
- 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. <lang bbcbasic> 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$</lang>
- 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.)
<lang c>#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;
}</lang>
CoffeeScript
This only does the encoding step for now.
<lang coffeescript> 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" </lang>
- 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).
<lang lisp>(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)</lang>
And the format used:
<lang lisp>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"</lang>
C++
<lang cpp>#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;
}</lang>
C#
<lang C sharp>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(); } }
}</lang>
- Output:
84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263 TOBEORNOTTOBEORTOBEORNOT
Clojure
<lang lisp>(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")</lang>
- Output:
<lang lisp>(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)</lang>
D
Simpler Version
<lang d>import std.stdio, std.array;
auto compress(in string original) pure nothrow {
int[string] dict; foreach (immutable char c; char.min .. char.max + 1) dictc = 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);
}</lang>
- 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. <lang d>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) dictb = 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;
}</lang>
- 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. <lang d>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;
}</lang>
- Output:
Input size: 206403 Encoded size: 97633 Decoded size: 206403 Decoded OK.
Dylan
<lang 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"))</lang>
Eiffel
<lang 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 </lang>
- Output:
84 79 66 69 79 82 78 79 84 257 259 261 266 260 262 264 TOBEORNOTTOBEORTOBEORNOT
Elixir
<lang 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([dh | out]) defp encode([h|t], d, free, out) do val = dh 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</lang>
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] "TOBEORNOTTOBEORTOBEORNOT" true
Erlang
<lang 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).</lang>
Forth
<lang 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</lang>
FreeBASIC
<lang 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</lang>
- 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). <lang go>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) }</lang>
- Output:
[84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263] TOBEORNOTTOBEORTOBEORNOT
Groovy
<lang 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${entry[0]}" w = entry }
result.toString()
}</lang> Testing: <lang groovy>def plaintext = 'TOBEORNOTTOBEORTOBEORNOT' def compressed = compress(plaintext) def result = decompress(compressed)
println """\
Plaintext: '$plaintext' Compressed: $compressed Uncompressed: '$result'""".stripIndent()</lang>
- Output:
Plaintext: 'TOBEORNOTTOBEORTOBEORNOT' Compressed: [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] Uncompressed: 'TOBEORNOTTOBEORTOBEORNOT'
Haskell
<lang Haskell>import Data.List (elemIndex, tails) import Control.Applicative (liftA2) import Data.Maybe (fromJust)
take2 :: [a] -> a take2 xs = filter ((2 ==) . length) (take 2 <$> tails xs)
doLZW :: String -> String -> [Int] doLZW _ [] = [] doLZW as (x:xs) = lzw (return <$> 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 :: String -> [Int] -> String undoLZW _ [] = [] undoLZW a cs =
cs >>= (!!) (foldl (liftA2 (.) (++) (\x xs -> return (liftA2 (++) head (take 1 . last) ((x !!) <$> xs)))) (return <$> a) (take2 cs))</lang>
Testing: <lang Haskell>*Main> doLZW ['\0'..'\255'] "TOBEORNOTTOBEORTOBEORNOT" [84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263]
- Main> undoLZW ['\0'..'\255'] [84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263]
"TOBEORNOTTOBEORTOBEORNOT"</lang> Encode --> decode --> compare with original text. <lang Haskell>*Main> (ap (==) . liftM2 (.) undoLZW doLZW) ['\0'..'\255'] "TOBEORNOTTOBEORTOBEORNOT" True</lang>
Other (elegant) code can be found at Haskell wiki Toy compression
J
Straightforward implementations of encoding and decoding: <lang J>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
)</lang> Test:
a. encodeLZW 'TOBEORNOTTOBEORTOBEORNOT' 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
Decoding: <lang J>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
)</lang> 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
<lang java5>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); }
}</lang>
- Output:
(Command Line direct output)
<lang java5>[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT</lang>
JavaScript
<lang javascript>//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 + '
' + decomp);</lang>
ES6 Version
This is the the same thing, but for ES6. The code has been refactored and cleaned up a bit to look neater.
<lang javascript>'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}`);</lang>
- Output:
84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263 TOBEORNOTTOBEORTOBEORNOT
jq
<lang 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]
- </lang>
Example: <lang jq>"TOBEORNOTTOBEORTOBEORNOT" | lzw_compress| lzw_decompress</lang>
- Output:
$ jq -n -f LZW.jq "TOBEORNOTTOBEORTOBEORNOT"
Julia
<lang julia>function compressLZW(decompressed::String)
dictsize = 256 dict = Dict{String,Int}(string(Char(i)) => i for i in range(0, dictsize)) result = Vector{Int}(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 range(0, dictsize)) result = IOBuffer() w = string(Char(shift!(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(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, 2))%)\n-> Decompressed: $decomp")
end</lang>
- 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
<lang scala>// 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: $k") result.append(entry)
// Add w + entry[0] to the dictionary. dictionary.put(dictSize++, w + entry[0]) w = entry } return result.toString() }
}
fun main(args: Array<String>) {
val compressed = Lzw.compress("TOBEORNOTTOBEORTOBEORNOT") println(compressed) val decompressed = Lzw.decompress(compressed) println(decompressed)
}</lang>
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Liberty BASIC
The encoder features variable-bit output, a 12 to 21 bit rotating dictionary (that can also be set to "Static"), and an unbalanced binary search tree that assures a worst-case-scenario maximum of 256 searches to find any given index, regardless of the dictionary's size. It uses both read and write buffers so is capable of handling files of any size, and it adds a settings-byte to the beginning of the encoded file to retain the maximum bit-width and rotating status of the dictionary. It also has the option to write the encoding/decoding dictionaries to file so the encoder can be checked for accuracy. This code directly follows the methodology described in an excellent web article by Juha Nieminen entitled "An efficient LZW implementation". <lang> DIM LZW(1, 1)
DIM JDlzw(1) DIM JDch$(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$="" FOR dNext = 0 TO 255 ' initialize dictionary for LZW ' LZW(dNext, PREFIX) = EMPTY ' prefix index of '<index>' ' LZW(dNext, BYTE) = dNext ' byte value of <index> '' LZW(dNext, FIRST) = EMPTY ' first index to use <index> as prefix ' LZW(dNext, LESS) = EMPTY ' lesser index of binary search tree for ' LZW(dNext, MORE) = EMPTY ' greater index of binary search tree for NEXT dNext OPEN inputName$ 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 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> is found IF newByte < LZW(dSearch, BYTE) THEN ' else if new is less than <index> 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> was not found in the dictionary GOSUB [WriteIndex] ' write <index> to the output IF dNext < dSize THEN ' save <index> 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> = ELSE ' if <index> was found in the dictionary, orgIndex = dSearch ' then set <index> = <index> 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$(tagCount) + "_" RETURN
'''''''''''''''''''''''''''''''''''</lang>
Lua
<lang lua>local function compress(uncompressed) -- string
local dictionary, result, dictSize, w, c = {}, {}, 255, "" for i = 0, 255 do dictionary[string.char(i)] = i end for i = 1, #uncompressed do c = string.sub(uncompressed, i, i) if dictionary[w .. c] then w = w .. c else table.insert(result, dictionary[w]) dictSize = dictSize + 1 dictionary[w .. c] = dictSize w = c end end if w ~= "" then table.insert(result, dictionary[w]) end return result
end
local function decompress(compressed) -- table
local dictionary, dictSize, entry, result, w, k = {}, 255, "", "", "" for i = 0, 255 do dictionary[i] = string.char(i) end for i = 1, #compressed do k = compressed[i] if dictionary[k] then entry = dictionary[k] elseif k == dictSize then entry = w .. string.sub(w, 1, 1) else return nil, i end result = result .. entry dictionary[dictSize] = w .. string.sub(entry, 1, 1) dictSize = dictSize + 1 w = entry end return result
end
local example = "TOBEORNOTTOBEORTOBEORNOT" local com = compress(example) local dec = decompress(com) print(table.concat(com, ", ")) print(dec)</lang>
- Output:
84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263 TOBEORNOTTOBEORTOBEORNOT
M2000 Interpreter
<lang M2000 Interpreter> Module BBCtrans {
\\ LZW compression plaintext$="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$(encodeLZW$)
} BBCtrans </lang>
And here a change for using Inventories, where we have hash function, and we find entry in O(1). <lang M2000 Interpreter> 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 </lang>
- Output:
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Mathematica
<lang>compress[uncompressed_] :=
Module[{dictsize, dictionary, w, result, wc}, dictsize = 256; dictionary = # -> # & /@ FromCharacterCode /@ Range@dictsize; w = ""; result = {}; Do[wc = w <> c; If[MemberQ[dictionaryAll, 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 = compressed1; Do[Which[MemberQ[dictionaryAll, 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, compressed2 ;;}]; result];
(*How to use:*) compress["TOBEORNOTTOBEORTOBEORNOT"] decompress[%]</lang>
- Output:
{"T", "O", "B", "E", "O", "R", "N", "O", "T", 256, 258, 260, 265, 259, 261, 263} "TOBEORNOTTOBEORTOBEORNOT"
Nim
<lang> import tables
proc compress*(uncompressed: string): seq[int] =
## build the dictionary var dictionary = initTable[string, int]() for i in 0..255: dictionary.add($char(i), i)
var w: string = newString(0) var compressed = newSeq[int]()
for c in uncompressed: var wc = w & c if(dictionary.hasKey(wc)): w = wc else: # writes w to output compressed.add(dictionary[w]) # wc is a new sequence; add it to the dictionary dictionary.add(wc, dictionary.len) w = $c
# write remaining output if necessary if(w != nil): compressed.add(dictionary[w])
result = compressed
proc decompress*(compressed: var seq[int]): string =
# build the dictionary var dictionary = initTable[int, string]() for i in 0..255: dictionary.add(i, $char(i))
var w: string = dictionary[compressed[0]]
compressed.delete(0)
var decompressed = w
for k in compressed: var entry: string = newString(0) if(dictionary.hasKey(k)): entry = dictionary[k] elif(k == dictionary.len): entry = w & w[0] else: raise newException(ValueError, "Bad compressed k: " & $k)
decompressed &= entry
# new sequence; add it to the dictionary dictionary.add(dictionary.len, w & entry[0])
w = entry
result = decompressed
when isMainModule:
var compressed = compress("TOBEORNOTTOBEORTOBEORNOT") echo compressed var decompressed = decompress(compressed) echo decompressed
</lang>
- Output:
@[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Objeck
<lang 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(); }
}</lang>
[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:
<lang objc>#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</lang>
Usage example:
<lang objc>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;
}</lang>
- Output:
(reformatted by hand)
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
OCaml
<lang 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)
- </lang>
here is the interface: <lang ocaml>val compress : uncompressed:string -> int list val decompress : compressed:int list -> string list</lang>
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.
<lang ocaml>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)
- </lang>
Ol
This version use lazy streams which is pair (symbol . function-to-get-next-symbol). <lang scheme> (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) </lang>
And decoder (runes->string used to unify functions - both used string iterators): <lang scheme> (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
</lang>
Perl
In this version the hashes contain mixed typed data: <lang perl># 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";</lang>
- Output:
T O B E O R N O T 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Perl 6
<lang perl6>sub compress(Str $uncompressed --> Seq) {
my $dict-size = 256; my %dictionary = (.chr => .chr for ^$dict-size); my $w = ""; gather { for $uncompressed.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; join , 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; } }
}
my @compressed = compress('TOBEORNOTTOBEORTOBEORNOT'); say @compressed; my $decompressed = decompress(@compressed); say $decompressed;</lang>
- Output:
T O B E O R N O T 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Phix
<lang Phix>function compress(string uncompressed) integer dict = new_dict() sequence result = {} integer dictSize = 255, c string word = ""
for i=0 to 255 do setd(""&i,i,dict) end for for i=1 to length(uncompressed) do c = uncompressed[i] if getd_index(word&c,dict) then word &= c else result &= getd(word,dict) dictSize += 1 setd(word&c,dictSize,dict) word = ""&c end if end for if word!="" then result &= getd(word,dict) end if destroy_dict(dict) return result
end function
function decompress(sequence compressed) integer dict = new_dict() integer dictSize = 255, k, ki string dent = "", result = "", word = ""
for i=0 to 255 do setd(i,""&i,dict) end for for i=1 to length(compressed) do k = compressed[i] ki = getd_index(k,dict) if ki then dent = getd_by_index(ki,dict) elsif k=dictSize then dent = word&word[1] else return {NULL,i} end if result &= dent setd(dictSize,word&dent[1],dict) dictSize += 1 word = dent end for destroy_dict(dict) return result
end function
constant example = "TOBEORNOTTOBEORTOBEORNOT" sequence com = compress(example) --?com pp(com) ?decompress(com) </lang>
- Output:
{84'T',79'O',66'B',69'E',79'O',82'R',78'N',79'O',84'T',256,258,260,265,259,261,263} "TOBEORNOTTOBEORTOBEORNOT"
PHP
<lang PHP>class LZW {
function compress($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 ($dictionary[$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 . "
" . $dec;
</lang>
- Output:
84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263 TOBEORNOTTOBEORTOBEORNOT
PicoLisp
<lang 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) ) ) ) ) ) )</lang>
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). <lang pli>*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;</lang>
- 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. <lang PureBasic>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()
EndIf</lang> Sample 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: <lang python>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 xrange(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 cStringIO import StringIO
# Build the dictionary. dict_size = 256 dictionary = dict((i, chr(i)) for i in xrange(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)</lang>
Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Racket
<lang 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) </lang>
Output:
TOBEORNOTTOBEORTOBEORNOT (T O B E O R N O T 256 258 260 265 259 261 263) TOBEORNOTTOBEORTOBEORNOT
REXX
version 1
<lang rexx>/* 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</lang> 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. <lang rexx>/*REXX program compresses text using the LZW (Lempel─Ziv─Welch), and reconstitutes it.*/ parse arg x; if x== then /*get an optional argument from the CL.*/
x= '"There is nothing permanent except change." ─── Heraclitus [540-475 BC]' say 'original text=' x /* [↑] Not specified? Then use default*/
cypher= LZWc(x) /*compress text using the LZW algorithm*/
say 'reconstituted=' LZWd(cypher) /*display the reconstituted string. */ say ' LZW integers=' cypher /* " " LZW integers used. */
exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ LZWc: procedure; parse arg y,,w $ @.; #= 256 /*LZW compress algorithm.*/
do j=0 for #; _= d2c(j); @._= j; end /*j*/ 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 end /*k*/; return substr($, 2) /*del leading blank.*/
/*──────────────────────────────────────────────────────────────────────────────────────*/ LZWd: procedure; parse arg x y,,@.; #= 256 /*LZW decompress algorithm.*/
do j=0 for #; @.j= d2c(j); end /*j*/ $= @.x; w= $ /*#: is the dictionay size*/ 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); #= # + 1; w= ? end /*k*/; return $</lang>
- output when using the default input:
original text= "There is nothing permanent except change." ─── Heraclitus [540-475 BC] reconstituted= "There is nothing permanent except change." ─── Heraclitus [540-475 BC] 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 72 259 97 99 108 105 116 117 264 32 91 53 52 48 45 52 55 53 32 66 67 93
Ring
<lang 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 + nl
</lang> Output:
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: <lang ruby># 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</lang>
Output:
["T", "O", "B", "E", "O", "R", "N", "O", "T", 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Scala
<lang 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) </lang>
Scheme
<lang 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)</lang>
Output:
(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263) TOBEORNOTTOBEORTOBEORNOT
Seed7
<lang 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;</lang>
Output:
"TOBEORNOT\256;\258;\260;\265;\259;\261;\263;" TOBEORNOTTOBEORTOBEORNOT
Sidef
<lang ruby># 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</lang>
- Output:
T O B E O R N O T 256 258 260 265 259 261 263 TOBEORNOTTOBEORTOBEORNOT
Swift
<lang swift>class LZW {
class func compress(uncompressed:String) -> [Int] { var dict = [String : Int]() for i in 0 ..< 256 { let s = String(UnicodeScalar(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++ 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(UnicodeScalar(i)) } var dictSize = 256 var w = String(UnicodeScalar(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(first(w)!) } else { return nil } result += entry dict[dictSize++] = w + String(first(entry)!) w = entry } return result }
}
let comp = LZW.compress("TOBEORNOTTOBEORTOBEORNOT") println(comp) if let decomp = LZW.decompress(comp) {
println(decomp)
}</lang>
- Output:
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263] TOBEORNOTTOBEORTOBEORNOT
Tcl
<lang 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</lang>
Xojo
<lang vb>
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 Function
</lang>
Test:
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
<lang 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
}</lang> <lang zkl>compressed:=lzwCompress("TOBEORNOTTOBEORTOBEORNOT"); compressed.toString(*).println();
lzwUncompress(compressed).println();</lang>
- Output:
L(84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263) TOBEORNOTTOBEORTOBEORNOT
- Programming Tasks
- Compression
- 11l
- Ada
- BaCon
- BBC BASIC
- C
- CoffeeScript
- Common Lisp
- Babel (library)
- C++
- C sharp
- Clojure
- D
- Dylan
- Eiffel
- Elixir
- Erlang
- Forth
- FreeBASIC
- Go
- Groovy
- Haskell
- J
- Java
- JavaScript
- Jq
- Julia
- Kotlin
- Liberty BASIC
- Lua
- M2000 Interpreter
- Mathematica
- Nim
- Objeck
- Objective-C
- OCaml
- Ol
- Perl
- Perl 6
- Phix
- PHP
- PicoLisp
- PL/I
- PureBasic
- Python
- Racket
- REXX
- Ring
- Ruby
- Scala
- Scheme
- Seed7
- Sidef
- Swift
- Tcl
- Xojo
- Zkl
- Lilypond/Omit