⚠️ Warning: This is a draft ⚠️

This means it might contain formatting issues, incorrect code, conceptual problems, or other severe issues.

If you want to help to improve and eventually enable this page, please fork RosettaGit's repository and open a merge request on GitHub.

The Lempel-Ziv-Welch (LZW) algorithm provides loss-less data compression.

You can read a complete description of it in the [[wp:Lempel-Ziv-Welch|Wikipedia article]] on the subject. It was patented, but it entered the public domain in 2004.

## 11l

{{trans|Python}}

```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
result ‘’= entry
dictionary[dict_size] = w‘’entry[0]
dict_size++
w = entry

R result

V compressed = compress(‘TOBEORNOTTOBEORTOBEORNOT’)
print(compressed)
print(decompress(&compressed))
```

```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;
```

```with Ada.Containers.Ordered_Maps;

package body LZW is
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;
```

```with LZW;

procedure Test is
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;
```

## 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
```

{{out}}

```LZWData: TOBEORNOTTOBEORTOBEORNOT
Encoded: 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
Decoded: TOBEORNOTTOBEORTOBEORNOT

```

## BBC BASIC

{{works with|BBC BASIC for Windows}} Uses fixed bit-width (16 bits) and initial dictionary size = 256.

```      plaintext\$ = "TOBEORNOTTOBEORTOBEORNOT"
encodeLZW\$ = FNencodeLZW(plaintext\$)
FOR i% = 1 TO LEN(encodeLZW\$) STEP 2
PRINT ; ASCMID\$(encodeLZW\$,i%) + 256*ASCMID\$(encodeLZW\$,i%+1) " " ;
NEXT
PRINT ' FNdecodeLZW(encodeLZW\$)
END

DEF FNencodeLZW(i\$)
LOCAL c%, d%, i%, l%, o\$, w\$, dict\$()
DIM dict\$(4095)
FOR i% = 0 TO 255 : dict\$(i%) = CHR\$(i%) : NEXT
l% = i%
i% = 1
w\$ = LEFT\$(i\$,1)
REPEAT
d% = 0
REPEAT
c% = d%
IF i% > LEN(i\$) EXIT REPEAT
FOR d% = 1 TO l%-1
IF w\$ = dict\$(d%) EXIT FOR
NEXT d%
IF d% < l% i% += 1 : w\$ += MID\$(i\$, i%, 1)
UNTIL d% >= l%
dict\$(l%) = w\$ : l% += 1 : w\$ = RIGHT\$(w\$)
o\$ += CHR\$(c% MOD 256) + CHR\$(c% DIV 256)
UNTIL i% >= LEN(i\$)
= o\$

DEF FNdecodeLZW(i\$)
LOCAL c%, i%, l%, o\$, t\$, w\$, dict\$()
DIM dict\$(4095)
FOR i% = 0 TO 255 : dict\$(i%) = CHR\$(i%) : NEXT
l% = i%
c% = ASC(i\$) + 256*ASCMID\$(i\$,2)
w\$ = dict\$(c%)
o\$ = w\$
IF LEN(i\$) < 4 THEN = o\$
FOR i% = 3 TO LEN(i\$) STEP 2
c% = ASCMID\$(i\$,i%) + 256*ASCMID\$(i\$,i%+1)
IF c% < l% t\$ = dict\$(c%) ELSE t\$ = w\$ + LEFT\$(w\$,1)
o\$ += t\$
dict\$(l%) = w\$ + LEFT\$(t\$,1)
l% += 1
w\$ = t\$
NEXT
= o\$
```

{{out}}

```
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT

```

## C

LZW encoder/decoder. Using variable bit length from 9 to up to 15. Encoder needs to know max allow bits, decoder doesn't. Code 256 for clear table, 257 for end of data, everything else are either byte values (<256) or code values.

'''WARNING: This code appears to have come from a GIF codec that has been modified to meet the requirements of this page, provided that the decoder works with the encoder to produce correct output. For writing GIF files the write_bits subroutine is wrong for Little Endian systems (it may be wrong for Big Endian as well.) The encoder also increases the number of bits in the variable length GIF-LZW after the N-2 code, whereas this must be done after N-1 to produce a working GIF file (just looking at the encoder, it's easy to see how this mistake could be made.)'''

```#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

/* -------- aux stuff ---------- */
void* mem_alloc(size_t item_size, size_t n_item)
{
size_t *x = calloc(1, sizeof(size_t)*2 + n_item * item_size);
x[0] = item_size;
x[1] = n_item;
return x + 2;
}

void* mem_extend(void *m, size_t new_n)
{
size_t *x = (size_t*)m - 2;
x = realloc(x, sizeof(size_t) * 2 + *x * new_n);
if (new_n > x[1])
memset((char*)(x + 2) + x[0] * x[1], 0, x[0] * (new_n - x[1]));
x[1] = new_n;
return x + 2;
}

inline void _clear(void *m)
{
size_t *x = (size_t*)m - 2;
memset(m, 0, x[0] * x[1]);
}

#define _new(type, n) mem_alloc(sizeof(type), n)
#define _del(m)   { free((size_t*)(m) - 2); m = 0; }
#define _len(m)   *((size_t*)m - 1)
#define _setsize(m, n)  m = mem_extend(m, n)
#define _extend(m)  m = mem_extend(m, _len(m) * 2)

/* ----------- LZW stuff -------------- */
typedef uint8_t byte;
typedef uint16_t ushort;

#define M_CLR 256 /* clear table marker */
#define M_EOD 257 /* end-of-data marker */
#define M_NEW 258 /* new code index */

/* encode and decode dictionary structures.
for encoding, entry at code index is a list of indices that follow current one,
i.e. if code 97 is 'a', code 387 is 'ab', and code 1022 is 'abc',
then dict[97].next['b'] = 387, dict[387].next['c'] = 1022, etc. */
typedef struct {
ushort next[256];
} lzw_enc_t;

/* for decoding, dictionary contains index of whatever prefix index plus trailing
byte.  i.e. like previous example,
dict[1022] = { c: 'c', prev: 387 },
dict[387]  = { c: 'b', prev: 97 },
dict[97]   = { c: 'a', prev: 0 }
the "back" element is used for temporarily chaining indices when resolving
a code to bytes
*/
typedef struct {
ushort prev, back;
byte c;
} lzw_dec_t;

byte* lzw_encode(byte *in, int max_bits)
{
int len = _len(in), bits = 9, next_shift = 512;
ushort code, c, nc, next_code = M_NEW;
lzw_enc_t *d = _new(lzw_enc_t, 512);

if (max_bits > 15) max_bits = 15;
if (max_bits < 9 ) max_bits = 12;

byte *out = _new(ushort, 4);
int out_len = 0, o_bits = 0;
uint32_t tmp = 0;

inline void write_bits(ushort x) {
tmp = (tmp << bits) | x;
o_bits += bits;
if (_len(out) <= out_len) _extend(out);
while (o_bits >= 8) {
o_bits -= 8;
out[out_len++] = tmp >> o_bits;
tmp &= (1 << o_bits) - 1;
}
}

//write_bits(M_CLR);
for (code = *(in++); --len; ) {
c = *(in++);
if ((nc = d[code].next[c]))
code = nc;
else {
write_bits(code);
nc = d[code].next[c] = next_code++;
code = c;
}

/* next new code would be too long for current table */
if (next_code == next_shift) {
/* either reset table back to 9 bits */
if (++bits > max_bits) {
/* table clear marker must occur before bit reset */
write_bits(M_CLR);

bits = 9;
next_shift = 512;
next_code = M_NEW;
_clear(d);
} else  /* or extend table */
_setsize(d, next_shift *= 2);
}
}

write_bits(code);
write_bits(M_EOD);
if (tmp) write_bits(tmp);

_del(d);

_setsize(out, out_len);
return out;
}

byte* lzw_decode(byte *in)
{
byte *out = _new(byte, 4);
int out_len = 0;

inline void write_out(byte c)
{
while (out_len >= _len(out)) _extend(out);
out[out_len++] = c;
}

lzw_dec_t *d = _new(lzw_dec_t, 512);
int len, j, next_shift = 512, bits = 9, n_bits = 0;
ushort code, c, t, next_code = M_NEW;

uint32_t tmp = 0;
inline void get_code() {
while(n_bits < bits) {
if (len > 0) {
len --;
tmp = (tmp << 8) | *(in++);
n_bits += 8;
} else {
tmp = tmp << (bits - n_bits);
n_bits = bits;
}
}
n_bits -= bits;
code = tmp >> n_bits;
tmp &= (1 << n_bits) - 1;
}

inline void clear_table() {
_clear(d);
for (j = 0; j < 256; j++) d[j].c = j;
next_code = M_NEW;
next_shift = 512;
bits = 9;
};

for (len = _len(in); len;) {
get_code();
if (code == M_EOD) break;
if (code == M_CLR) {
clear_table();
continue;
}

if (code >= next_code) {
_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) {
return 1;
};

struct stat st;
fstat(fd, &st);

byte *in = _new(char, 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]) {
break;
}

if (i == _len(dec)) printf("Decoded ok\n");

_del(in);
_del(enc);
_del(dec);

return 0;
}
```

## CoffeeScript

This only does the encoding step for now.

```
lzw = (s) ->
dct = {} # map substrings to codes between 256 and 4096
stream = [] # array of compression results

# initialize basic ASCII characters
for code_num in [0..255]
c = String.fromCharCode(code_num)
dct[c] = code_num
code_num = 256

i = 0
while i < s.length
# Find word and new_word
#   word = longest substr already encountered, or next character
#   new_word = word plus next character, a new substr to encode
word = ''
j = i
while j < s.length
new_word = word + s[j]
break if !dct[new_word]
word = new_word
j += 1

# stream out the code for the substring
stream.push dct[word]

# build up our encoding dictionary
if code_num < 4096
dct[new_word] = code_num
code_num += 1

i += word.length
stream

console.log lzw "TOBEORNOTTOBEORTOBEORNOT"

```

{{out}}

```
> coffee lzw.coffee
[ 84,
79,
66,
69,
79,
82,
78,
79,
84,
256,
258,
260,
265,
259,
261,
263 ]

```

## Common Lisp

'''Library:''' [[SMW::off]][[:Category:Babel (library)|Babel]][[Category:Babel (library)]][[SMW::on]]{{#set:Uses library=Babel (library)}}
{{trans|Perl}} This version is based upon the Perl one. It doesn't contain mixed type data at the cost of being more consy. It includes vector operation routines, since using `VECTOR-PUSH-APPEND` reallocates the whole vector with each call.

The Babel library is required to convert octet vectors to strings. Lisp strings can contain characters out of the ASCII/latin1 character set, including the whole Unicode range in them. The exact encoding used is dependent upon the user's locale (`LC_CTYPE` on Unix).

```(declaim (ftype (function (vector vector &optional fixnum fixnum) vector)
vector-append))
(defun vector-append (old new &optional (start2 0) end2)
(declare (optimize (speed 3) (safety 0) (debug 0)))
(prog1 old
(let* ((old-fill (fill-pointer old))
(new-fill (+ old-fill (length new))))
(when (> new-fill (array-dimension old 0))
(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))
(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
:initial-element elt))

(declaim (ftype (function (vector t) vector) vector-append1-new))
(defun vector-append1-new (old new)
(vector-append1 (vector-append (make-empty-vector 'octet) old)
new))

(declaim (ftype (function (vector vector) vector) vector-append-new))
(defun vector-append-new (old new)
(vector-append (vector-append (make-empty-vector 'octet) old)
new))

(deftype octet () '(unsigned-byte 8))

(declaim (ftype (function () hash-table) build-dictionary))
(defun build-dictionary ()
(let ((dictionary (make-hash-table :test #'equalp)))
(loop for i below 256
do (let ((vec (make-vector-with-elt i 'octet)))
(setf (gethash vec dictionary) vec)))
dictionary))

(declaim (ftype (function ((vector octet)) (vector octet))
lzw-compress-octets))
(defun lzw-compress-octets (octets)
(declare (optimize (speed 3) (safety 0) (debug 0)))
(loop with dictionary-size of-type fixnum = 256
with w = (make-empty-vector 'octet)
with result = (make-empty-vector 't)
with dictionary = (build-dictionary)
for c across octets
for wc = (vector-append1-new w c)
if (gethash wc dictionary) do (setq w wc)
else do
(vector-append result (gethash w dictionary))
(setf (gethash wc dictionary)
(make-vector-with-elt dictionary-size))
(incf dictionary-size)
(setq w (make-vector-with-elt c 'octet))
finally (unless (zerop (length (the (vector octet) w)))
(vector-append result (gethash w dictionary)))
(return result)))

(declaim (ftype (function (vector) (vector octet)) lzw-decompress))
(defun #1=lzw-decompress (octets)
(declare (optimize (speed 3) (safety 0) (debug 0)))
(when (zerop (length octets))
(return-from #1# (make-empty-vector 'octet)))
(loop with dictionary-size = 256
with dictionary = (build-dictionary)
with result = (make-vector-with-elt (aref octets 0) 'octet)
with w = (copy-seq result)
for i from 1 below (length octets)
for k = (make-vector-with-elt (aref octets i) 't)
for entry = (or (gethash k dictionary)
(if (equalp k dictionary-size)
(vector-append1-new w (aref w 0))
(error "bad compresed entry at pos ~S" i)))
do (vector-append result entry)
(setf (gethash (make-vector-with-elt dictionary-size) dictionary)
(vector-append1-new w (aref entry 0)))
(incf dictionary-size)
(setq w entry)
finally (return result)))

(defgeneric lzw-compress (datum)
(:method ((string string))
(lzw-compress (babel:string-to-octets string)))
(:method ((octets vector))
(lzw-compress-octets octets)))

(defun lzw-decompress-to-string (octets)
(babel:octets-to-string (lzw-decompress octets)))

(defun test (string)
(assert (equal #2=(lzw-decompress-to-string (lzw-compress string)) string) ()
"Can't compress ~S properly, got ~S instead" string #2#)
t)
```

And the format used:

```CL-USER> (test "TOBEORNOTTOBEORTOBEORNOT")
T
CL-USER> (lzw-compress "TOBEORNOTTOBEORTOBEORNOT")
#(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)
CL-USER> (lzw-decompress-to-string *)
"TOBEORNOTTOBEORTOBEORNOT"
```

## C++

{{trans|D}}

```#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

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;
}
```

## C#

{{trans|Java}}

```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++)

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
// wc is a new sequence; add it to the dictionary
w = c.ToString();
}
}

// write remaining output if necessary
if (!string.IsNullOrEmpty(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++)

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

w = entry;
}

return decompressed.ToString();
}
}
}
```

{{out}}

```84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263
TOBEORNOTTOBEORTOBEORNOT
```

## Clojure

```(defn make-dict []
(let [vals (range 0 256)]
(zipmap (map (comp #'list #'char) vals) vals)))

(defn compress [#^String text]
(loop [t (seq text)
r '()
w '()
dict (make-dict)
s 256]
(let [c (first t)]
(if c
(let [wc (cons c w)]
(if (get dict wc)
(recur (rest t) r wc dict s)
(recur (rest t) (cons (get dict w) r) (list c) (assoc dict wc s) (inc s))))
(reverse (if w (cons (get dict w) r) r))))))

(compress "TOBEORNOTTOBEORTOBEORNOT")
```

{{out}}

```(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)
```

## D

### Simpler Version

```import std.stdio, std.array;

auto compress(in string original) pure nothrow {
int[string] dict;
foreach (immutable char c; char.min .. char.max + 1)
dict[[c]] = c;

string w;
int[] result;
foreach (immutable ch; original)
if (w ~ ch in dict)
w = w ~ ch;
else {
result ~= dict[w];
dict[w ~ ch] = dict.length;
w = [ch];
}
return w.empty ? result : (result ~ dict[w]);
}

auto decompress(in int[] compressed) pure nothrow {
auto dict = new string[char.max - char.min + 1];
foreach (immutable char c; char.min .. char.max + 1)
dict[c] = [c];

auto w = dict[compressed[0]];
auto result = w;
foreach (immutable k; compressed[1 .. \$]) {
auto entry = (k < dict.length) ? dict[k] : w ~ w[0];
result ~= entry;
dict ~= w ~ entry[0];
w = entry;
}
return result;
}

void main() {
auto comp = "TOBEORNOTTOBEORTOBEORNOT".compress;
writeln(comp, "\n", comp.decompress);
}
```

{{out}}

```[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT
```

### More Refined Version

This longer version is a little more efficient and it uses stronger static typing.

```struct LZW {
import std.array: empty;

// T is ubyte instead of char because D strings are UTF-8.
alias T = ubyte;
alias Tcomp = ushort;
static assert(Tcomp.sizeof > 1);
alias Ta = immutable(T)[];

enum int initDictSize = 256;
static immutable ubyte[initDictSize] bytes;
static this() {
foreach (immutable T i; 0 .. initDictSize)
bytes[i] = i;
}

static Tcomp[] compress(immutable scope T[] original) pure nothrow @safe
out(result) {
if (!original.empty)
assert(result[0] < initDictSize);
} body {
if (original.empty)
return [];
Tcomp[Ta] dict;
foreach (immutable b; bytes)
dict[[b]] = b;

// Here built-in slices give lower efficiency.
struct Slice {
size_t start, end;
@property opSlice() const pure nothrow @safe @nogc {
return original[start .. end];
}
alias opSlice this;
}

Slice w;
Tcomp[] result;
foreach (immutable i; 0 .. original.length) {
auto wc = Slice(w.start, w.end + 1); // Extend slice.
if (wc in dict) {
w = wc;
} else {
result ~= dict[w];
assert(dict.length < Tcomp.max); // Overflow guard.
dict[wc] = cast(Tcomp)dict.length;
w = Slice(i, i + 1);
}
}

if (!w.empty)
result ~= dict[w];
return result;
}

static Ta decompress(in Tcomp[] compressed) pure @safe
in {
if (!compressed.empty)
} 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
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;
}
```

{{out}}

```[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT
```

### More Efficient Version

{{trans|C}} This code retains part of the style of the original C code.

```enum Marker: ushort {
CLR = 256, // Clear table marker.
EOD = 257, // End-of-data marker.
NEW = 258  // New code index.
}

ubyte[] lzwEncode(scope const(ubyte)[] inp, in uint maxBits) pure nothrow
in {
assert(maxBits >= 9 && maxBits <= 16);
} body {
// Encode dictionary array. For encoding, entry at
// code index is a list of indices that follow current one,
// i.e. if code 97 is 'a', code 387 is 'ab', and code 1022 is 'abc',
// then dict[97].next['b'] = 387, dict[387].next['c'] = 1022, etc.
alias LZWenc = ushort[256];

auto len = inp.length;
uint bits = 9;
auto d = new LZWenc[512];

auto result = new ubyte[16];
size_t outLen = 0;
size_t oBits = 0;
uint tmp = 0;

void writeBits(in ushort x) nothrow {
tmp = (tmp << bits) | x;
oBits += bits;
if (result.length / 2 <= outLen)
result.length *= 2;
while (oBits >= 8) {
oBits -= 8;
assert(tmp >> oBits <= ubyte.max);
result[outLen] = cast(ubyte)(tmp >> oBits);
outLen++;
tmp &= (1 << oBits) - 1;
}
}

// writeBits(Marker.CLR);
ushort nextCode = Marker.NEW;
uint nextShift = 512;
ushort code = inp[0];
inp = inp[1 .. \$]; // popFront.
while (--len) {
ushort c = inp[0];
inp = inp[1 .. \$]; // popFront.
ushort nc = d[code][c];
if (nc) {
code = nc;
} else {
writeBits(code);
nc = d[code][c] = nextCode;
nextCode++;
code = c;
}

// Next new code would be too long for current table.
if (nextCode == nextShift) {
// Either reset table back to 9 bits.
bits++;
if (bits > maxBits) {
// Table clear marker must occur before bit reset.
writeBits(Marker.CLR);

bits = 9;
nextShift = 512;
nextCode = Marker.NEW;
d[] = LZWenc.init;
} else { // Or extend table.
nextShift *= 2;
d.length = nextShift;
}
}
}

writeBits(code);
writeBits(Marker.EOD);
if (tmp) {
assert(tmp <= ushort.max);
writeBits(cast(ushort)tmp);
}

return result[0 .. outLen];
}

ubyte[] lzwDecode(scope const(ubyte)[] inp) pure {
// For decoding, dictionary contains index of whatever prefix
// index plus trailing ubyte.  i.e. like previous example,
//     dict[1022] = { c: 'c', prev: 387 },
//     dict[387]  = { c: 'b', prev: 97 },
//     dict[97]   = { c: 'a', prev: 0 }
// the "back" element is used for temporarily chaining indices
// when resolving a code to bytes.
static struct LZWdec {
ushort prev, back;
ubyte c;
}

auto result = new ubyte[4];
uint outLen = 0;

void writeOut(in ubyte c) nothrow {
while (outLen >= result.length)
result.length *= 2;
result[outLen] = c;
outLen++;
}

auto d = new LZWdec[512];
ushort code = 0;
uint bits = 9;
uint len = 0;
uint nBits = 0;
uint tmp = 0;

void getCode() nothrow {
while (nBits < bits) {
if (len > 0) {
len--;
tmp = (tmp << 8) | inp[0];
inp = inp[1 .. \$]; // popFront.
nBits += 8;
} else {
tmp = tmp << (bits - nBits);
nBits = bits;
}
}

nBits -= bits;
assert(tmp >> nBits <= ushort.max);
code = cast(ushort)(tmp >> nBits);
tmp &= (1 << nBits) - 1;
}

uint nextShift = 512;
ushort nextCode = Marker.NEW;

void clearTable() nothrow {
d[] = LZWdec.init;
foreach (immutable ubyte j; 0 .. 256)
d[j].c = j;
nextCode = Marker.NEW;
nextShift = 512;
bits = 9;
}

for (len = inp.length; len;) {
getCode();
if (code == Marker.EOD)
break;
if (code == Marker.CLR) {
clearTable();
continue;
}

if (code >= nextCode)

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;

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;
}
```

{{out}}

```Input size:   206403
Encoded size: 97633
Decoded size: 206403
Decoded OK.
```

## Dylan

```Module:   LZW
Synopsis: LZW implementation for Rosetta code

define method output(n :: <integer>)
format-out("%d ", n);
end;

define method contains?(dict, var)
let x = element(dict, var, default: #f);
x ~= #f;
end;

define method byte->string(c)
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)
if (contains?(dict, wc))
cur-seq := wc;
else
dict[wc] := next-code;
next-code := next-code + 1;
end
end;
unless (empty?(cur-seq))
end;
result
end;

format-out("%=\n", compress("TOBEORNOTTOBEORTOBEORNOT"))
```

## Eiffel

```
class
APPLICATION

create
make

feature {NONE}

make
local
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

--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

-- 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

```

{{out}}

```
84 79 66 69 79 82 78 79 84 257 259 261 266 260 262 264
TOBEORNOTTOBEORTOBEORNOT

```

## Elixir

{{trans|Erlang}}

```defmodule LZW do
@encode_map  Enum.into(0..255, Map.new, &{[&1],&1})
@decode_map  Enum.into(0..255, Map.new, &{&1,[&1]})

def encode(str), do: encode(to_char_list(str), @encode_map, 256, [])

defp encode([h], d, _, out), do: Enum.reverse([d[[h]] | out])
defp encode([h|t], d, free, out) do
val = d[[h]]
find_match(t, [h], val, d, free, out)
end

defp find_match([h|t], l, lastval, d, free, out) do
case Map.fetch(d, [h|l]) do
{:ok, val} -> find_match(t, [h|l], val, d, free, out)
:error     -> d1 = Map.put(d, [h|l], free)
encode([h|t], d1, free+1, [lastval | out])
end
end
defp find_match([], _, lastval, _, _, out), do: Enum.reverse([lastval | out])

def decode([h|t]) do
val = @decode_map[h]
decode(t, val, 256, @decode_map, val)
end

defp decode([], _, _, _, l), do: Enum.reverse(l) |> to_string
defp decode([h|t], old, free, d, l) do
val = if h == free, do: old ++ [List.first(old)], else: d[h]
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
```

{{out}}

```
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
"TOBEORNOTTOBEORTOBEORNOT"
true

```

## 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),
decode(T, Val, Free+1, D1, Val ++ L).

init(D) -> init(255, D).

init(0, D) ->  D;
init(N, D) ->  D1 = dict:store([N],N,D),  init(N-1, D1).

init1(D) -> init1(255, D).

init1(0, D) ->  D;
init1(N, D) ->  D1 = dict:store(N,[N],D),  init1(N-1, D1).
```

## Forth

{{works with|GNU Forth|0.6.2}}

```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 )
256 - cells symtab + @ count ;

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
then then
entry count type  \ output
entry 1+ c@ w+c
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
```

## 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
```

{{out}}

```  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 `[https://golang.org/pkg/compress/lzw compress/lzw]` package in the standard library. {{trans|Java}}

This handles any series of bytes in the input string, not just ASCII or valid UTF8 encoding (tested with [https://github.com/dvyukov/go-fuzz go-fuzz]).

```package main

import (
"fmt"
"log"
"strings"
)

// Compress a string to a list of output symbols.
func compress(uncompressed string) []int {
// Build the dictionary.
dictSize := 256
// We actually want a map of []byte -> int but
// slices are not acceptable map key types.
dictionary := make(map[string]int, dictSize)
for i := 0; i < dictSize; i++ {
// Ugly mess to work around not having a []byte key type.
// Using `string(i)` would do utf8 encoding for i>127.
dictionary[string([]byte{byte(i)})] = i
}

var result []int
var w []byte
for i := 0; i < len(uncompressed); i++ {
c := uncompressed[i]
wc := append(w, c)
if _, ok := dictionary[string(wc)]; ok {
w = wc
} else {
result = append(result, dictionary[string(w)])
// Add wc to the dictionary.
dictionary[string(wc)] = dictSize
dictSize++
//w = []byte{c}, but re-using wc
wc[0] = c
w = wc[:1]
}
}

if len(w) > 0 {
// Output the code for w.
result = append(result, dictionary[string(w)])
}
return result
}

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 {
}
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)
}
```

{{out}}

```
[84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263]
TOBEORNOTTOBEORTOBEORNOT

```

## Groovy

```def compress = { text ->
def dictionary = (0..<256).inject([:]) { map, ch -> map."\${(char)ch}" = ch; map }
def w = '', compressed = []
text.each { ch ->
def wc = "\$w\$ch"
if (dictionary[wc]) {
w = wc
} else {
compressed << dictionary[w]
dictionary[wc] = dictionary.size()
w = "\$ch"
}
}
if (w) { compressed << dictionary[w] }
compressed
}

def decompress = { compressed ->
def dictionary = (0..<256).inject([:])  { map, ch -> map[ch] = "\${(char)ch}"; map }
int dictSize = 128;
String w = "\${(char)compressed[0]}"
StringBuffer result = new StringBuffer(w)

compressed.drop(1).each { k ->
String entry = dictionary[k]
if (!entry) {
if (k != dictionary.size()) throw new IllegalArgumentException("Bad compressed k \$k")
entry = "\$w\${w[0]}"
}
result << entry

dictionary[dictionary.size()] = "\$w\${entry[0]}"
w = entry
}

result.toString()
}
```

Testing:

```def plaintext = 'TOBEORNOTTOBEORTOBEORNOT'
def compressed = compress(plaintext)
def result = decompress(compressed)

println """\
Plaintext:    '\$plaintext'
Compressed:   \$compressed
Uncompressed: '\$result'""".stripIndent()
```

{{out}}

```Plaintext:    'TOBEORNOTTOBEORTOBEORNOT'
Compressed:   [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
Uncompressed: 'TOBEORNOTTOBEORTOBEORNOT'
```

```import Data.List (elemIndex, tails)
import Data.Maybe (fromJust)

doLZW :: Eq a => [a] -> [a] -> [Int]
doLZW _ [] = []
doLZW as (x:xs) = lzw (return <\$> as) [x] xs
where
lzw a w [] = [fromJust \$ elemIndex w a]
lzw a w (x:xs)
| w_ `elem` a = lzw a w_ xs
| otherwise = fromJust (elemIndex w a) : lzw (a ++ [w_]) [x] xs
where
w_ = w ++ [x]

undoLZW :: [a] -> [Int] -> [a]
undoLZW _ [] = []
undoLZW a cs =
cs >>=
(!!)
(foldl
((.) <\$> (++) <*>
(\x xs -> return (((++) <\$> head <*> take 1 . last) ((x !!) <\$> xs))))
(return <\$> a)
(take2 cs))

take2 :: [a] -> [[a]]
take2 xs = filter ((2 ==) . length) (take 2 <\$> tails xs)

main :: IO ()
main = do
print \$ doLZW ['\0' .. '\255'] "TOBEORNOTTOBEORTOBEORNOT"
print \$
undoLZW
['\0' .. '\255']
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
print \$
((==) <*> ((.) <\$> undoLZW <*> doLZW) ['\NUL' .. '\255'])
"TOBEORNOTTOBEORTOBEORNOT"
```

{{Out}}

```[84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263]
"TOBEORNOTTOBEORTOBEORNOT"
True
```

## J

Straightforward implementations of encoding and decoding:

```encodeLZW =: 4 : 0
d=. ;/x
r=.0\$0
wc=.w=.{.y
for_c. }.y do.
wc=.w,c
if. d e.~ <wc do. w=.wc else.
r=. r, d i.<w
d=.d,<wc
w=.c
end.
end.
r, d i.<w
)
```

Test:

```   a. encodeLZW 'TOBEORNOTTOBEORTOBEORNOT'
84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
```

Decoding:

```decodeLZW =: 4 : 0
d=.;/x
w=.r=. >d{~{.y
ds=. #d
for_c. }.y do.
select. * c-ds
case. _1 do. r=.r,e=.>c{d
case.  0 do. r=.r,e=.w,{.w
case.    do. 'error' return.
end.
d=.d,< w,{.e
w=.e
ds=.ds+1
end.
;r
)
```

Test:

```   a. decodeLZW 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT
```

encode --> decode --> compare with original:

```   a. (] -: [ decodeLZW encodeLZW) 'TOBEORNOTTOBEORTOBEORNOT'
1
```

Error test:

```   a. decodeLZW 84 79 66 69 79 82 78 79 84 256 258 456 260 265 259 261 263
error
```

Tacit J expression for decoding:

```decodeLZW=:[:;]{[:;[:(],<@(>@{.,{.@>@{:)@:{)&.>/<@(;/@[),~|.@(2<\])
```
```   a. decodeLZW 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT
```

## Java

{{works with|Java|1.5+}}

```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 {
// Add wc to the dictionary.
dictionary.put(wc, dictSize++);
w = "" + c;
}
}

// Output the code for w.
if (!w.equals(""))
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);
}
}
```

{{out}} (Command Line direct output):

```[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT
```

## 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);
```

### ES6 Version

This is the the same thing, but for ES6. The code has been refactored and cleaned up a bit to look neater.

```'use strict';
/**
Namespace for LZW compression and decompression.
Methods:
LZW.compress(uncompressed)
LZW.decompress(compressed)
*/
class LZW
{
/**
Perform the LZW compression
uncompressed - String. The string on which to perform the compression.
*/
static compress(uncompressed)
{
// Initialize dictionary
let dictionary = {};
for (let i = 0; i < 256; i++)
{
dictionary[String.fromCharCode(i)] = i;
}

let word = '';
let result = [];
let dictSize = 256;

for (let i = 0, len = uncompressed.length; i < len; i++)
{
let curChar = uncompressed[i];
let joinedWord = word + curChar;

// Do not use dictionary[joinedWord] because javascript objects
// will return values for myObject['toString']
if (dictionary.hasOwnProperty(joinedWord))
{
word = joinedWord;
}
else
{
result.push(dictionary[word]);
// Add wc to the dictionary.
dictionary[joinedWord] = dictSize++;
word = curChar;
}
}

if (word !== '')
{
result.push(dictionary[word]);
}

return result;
}

/**
Decompress LZW array generated by LZW.compress()
compressed - Array. The array that holds LZW compressed data.
*/
static decompress(compressed)
{
// Initialize Dictionary (inverse of compress)
let dictionary = {};
for (let i = 0; i < 256; i++)
{
dictionary[i] = String.fromCharCode(i);
}

let word = String.fromCharCode(compressed[0]);
let result = word;
let entry = '';
let dictSize = 256;

for (let i = 1, len = compressed.length; i < len; i++)
{
let curNumber = compressed[i];

if (dictionary[curNumber] !== undefined)
{
entry = dictionary[curNumber];
}
else
{
if (curNumber === dictSize)
{
entry = word + word[0];
}
else
{
throw 'Error in processing';
return null;
}
}

result += entry;

// Add word + entry[0] to dictionary
dictionary[dictSize++] = word + entry[0];

word = entry;
}

return result;
}
}

let comp = LZW.compress('TOBEORNOTTOBEORTOBEORNOT');
let decomp = LZW.decompress(comp);

console.log(`\${comp}
\${decomp}`);
```

{{out}}

```84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263
TOBEORNOTTOBEORTOBEORNOT
```

## jq

{{ works with|jq|1.4}} {{trans|JavaScript}}

```# LZW compression/decompression for strings
def lzw_compress:
def decode: [.] | implode;
# Build the dictionary:
256 as \$dictSize
| (reduce range(0; \$dictSize) as \$i ({}; .[ \$i | decode ] = \$i)) as \$dictionary
| reduce explode[] as \$i
( [\$dictionary, \$dictSize, "", []];        # state: [dictionary, dictSize, w, result]
.[0] as \$dictionary
| .[1] as \$dictSize
| .[2] as \$w
| (\$i | decode) as \$c
| (\$w + \$c ) as \$wc
| if \$dictionary[\$wc] then .[2] = \$wc
else
.[2] =  \$c                         # w = c
| .[3] += [\$dictionary[\$w]]          # result += dictionary[w]
| .[0][\$wc] = \$dictSize              # Add wc to the dictionary
| .[1] += 1                          # dictSize ++
end
)
# Output the code for w unless w == "":
| if .[2] == "" then .[3]
else .[3] + [.[0][.[2]]]
end
;

def lzw_decompress:
def decode: [.] | implode;
# Build the dictionary - an array of strings
256 as \$dictSize
| (reduce range(0; \$dictSize) as \$i ([]; .[ \$i ] = (\$i|decode))) as \$dictionary
| (.[0]|decode) as \$w
| reduce .[1:][] as \$k
( [ \$dictionary, \$dictSize, \$w, \$w];   # state: [dictionary, dictSize, w, result]
.[0][\$k] as \$entry
| (if \$entry then \$entry
elif \$k == .[1] then .[2] + .[2][0:1]
else error("lzw_decompress: k=\(\$k)")
end) as \$entry
| .[3] += \$entry                     # result += entry
| .[0][.[1]] = .[2] + \$entry[0:1]    # dictionary[dictSize] = w + entry.charAt(0);
| .[1] += 1                          # dictSize++
| .[2] = \$entry                      # w = entry
) | .[3]
;
```

'''Example''':

```"TOBEORNOTTOBEORTOBEORNOT" | lzw_compress| lzw_decompress
```

{{Out}} \$ jq -n -f LZW.jq "TOBEORNOTTOBEORTOBEORNOT"

## Julia

{{works with|Julia|1.1.1}}

```function compressLZW(decompressed::String)
dictsize = 256
dict     = Dict{String,Int}(string(Char(i)) => i for i in 0:dictsize)
result   = Vector{Int}(undef, 0)
w        = ""
for c in decompressed
wc = string(w, c)
w = wc
else
push!(result, dict[w])
dict[wc]  = dictsize
dictsize += 1
w        = string(c)
end
end
if !isempty(w) push!(result, dict[w]) end
return result
end

function decompressLZW(compressed::Vector{Int})
dictsize = 256
dict     = Dict{Int,String}(i => string('\0' + i) for i in 0:dictsize)
result   = IOBuffer()
w        = string(Char(popfirst!(compressed)))
write(result, w)
for k in compressed
entry = dict[k]
elseif k == dictsize
entry = string(w, w[1])
else
end
write(result, entry)
dict[dictsize] = string(w, entry[1])
dictsize += 1
w = entry
end
return String(take!(result))
end

original     = ["0123456789", "TOBEORNOTTOBEORTOBEORNOT", "dudidudidudida"]
compressed   = compressLZW.(original)
decompressed = decompressLZW.(compressed)

for (word, comp, decomp) in zip(original, compressed, decompressed)
comprate = (length(word) - length(comp)) / length(word) * 100
println("Original: \$word\n-> Compressed: \$comp (compr.rate: \$(round(comprate, digits=2))%)\n-> Decompressed: \$decomp")
end
```

{{out}}

```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

{{trans|Java}}

```// 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 {
// Add wc to the dictionary.
dictionary.put(wc, dictSize++)
w = c.toString()
}
}

// Output the code for 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
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)
}
```

{{out}}

```
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT

```

## Liberty BASIC

The encoder features variable-bit output, a 12 to 21 bit rotating dictionary (that can also be set to "Static"), and an unbalanced binary search tree that assures a worst-case-scenario maximum of 256 searches to find any given index, regardless of the dictionary's size. It uses both read and write buffers so is capable of handling files of any size, and it adds a settings-byte to the beginning of the encoded file to retain the maximum bit-width and rotating status of the dictionary. It also has the option to write the encoding/decoding dictionaries to file so the encoder can be checked for accuracy. This code directly follows the methodology described in an excellent web article by Juha Nieminen entitled "An efficient LZW implementation". DIM LZW(1, 1) DIM JDlzw(1) DIM JDch\$(1) LET maxBits = 20 ' maximum bit width of the dictionary: minimum=12; maximum=21 LET resetDictionary = 1 ' flag to reset the dictionary when it gets full: 1=TRUE; 0=FALSE LET printDictionary = 0 ' output encoding and decoding dictionaries to files LET maxChunkSize = 2 ^ 14 ' maximum size of the data buffer LET dSize = 2 ^ maxBits ' maximum dictionary size LET JDext\$ = ".lzw" ' file extension used for created archives FILEDIALOG "Select a file to test LZW...", ".", inputName\$ IF inputName\$ = "" THEN END DO ' get fullPath\ and fileName.ext P = X X = INSTR(inputName\$, "", (X + 1)) LOOP UNTIL X = 0 filePath\$ = LEFT\$(inputName\$, P) fileName\$ = MID\$(inputName\$, (P + 1)) DO ' get fileName and .ext P = X X = INSTR(fileName\$, ".", (X + 1)) LOOP UNTIL X = 0 fileExt\$ = MID\$(fileName\$, P) fileName\$ = LEFT\$(fileName\$, (P - 1))

GOSUB [lzwEncode] GOSUB [lzwDecode]

END

'''''''''''''''''''''''''''''''''''''''' ' Start LZW Encoder '''''''''''''''''''' [lzwEncode] REDIM LZW(dSize, 4) LET EMPTY=-1:PREFIX=0:BYTE=1:FIRST=2:LESS=3:MORE=4:bmxCorrect=1 LET bitsRemain=0:remainIndex=0:tagCount=0:currentBitSize=8:fileTag\$="" FOR dNext = 0 TO 255 ' initialize dictionary for LZW ' LZW(dNext, PREFIX) = EMPTY ' prefix index of '' ' LZW(dNext, BYTE) = dNext ' byte value of '' LZW(dNext, FIRST) = EMPTY ' first index to use 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 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 in the dictionary WHILE (dSearch > EMPTY) ' while is present in the dictionary IF LZW(dSearch, BYTE) = newByte THEN EXIT WHILE ' if is found IF newByte < LZW(dSearch, BYTE) THEN ' else if new is less than savIndex = LESS ' follow lesser binary tree ELSE savIndex = MORE ' else follow greater binary tree END IF prvIndex = dSearch ' set previous dSearch = LZW(dSearch, savIndex) ' read next search from binary tree WEND IF dSearch = EMPTY THEN ' if was not found in the dictionary GOSUB [WriteIndex] ' write to the output IF dNext < dSize THEN ' save 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 = ELSE ' if was found in the dictionary, orgIndex = dSearch ' then set = END IF LOOP WHILE chnkPoint < chunk ' loop until the chunk has been processed GOSUB [GetFileChunk] ' refill the buffer WEND ' loop until the buffer is empty GOSUB [WriteIndex] IF bitsRemain > 0 THEN #lzwOUT, CHR\$(remainIndex); CLOSE #lzwOUT CLOSE #lzwIN IF bmxCorrect THEN ' correct the settings, if needed IF (currentBitSize < maxBits) OR resetDictionary THEN IF currentBitSize < 12 THEN currentBitSize = 12 OPEN fileName\$ + fileExt\$ + JDext\$ FOR BINARY AS #lzwOUT #lzwOUT, CHR\$(currentBitSize - 12); CLOSE #lzwOUT END IF END IF GOSUB [PrintEncode] REDIM LZW(1, 1) RETURN

[WriteIndex] X = orgIndex ' add remaining bits to input IF bitsRemain > 0 THEN X = remainIndex + (X * (2 ^ bitsRemain)) bitsRemain = bitsRemain + currentBitSize ' add current bit size to output stack WHILE bitsRemain > 7 ' if 8 or more bits are to be written #lzwOUT, CHR\$(X MOD 256); ' attatch lower 8 bits to output string X = INT(X / 256) ' shift input value down by 2^8 bitsRemain = bitsRemain - 8 ' adjust counters WEND remainIndex = X ' retain trailing bits for next write RETURN

' End LZW Encoder '''''''''''''''''''''' ''''''''''''''''''''''''''''''''''''''''

[StartFileChunk] sizeOfFile = LOF(#lzwIN) ' set EOF marker bytesRemaining = sizeOfFile ' set EOF counter chunk = maxChunkSize ' set max buffer size [GetFileChunk] fileChunk\$ = "" IF bytesRemaining < 1 THEN RETURN IF chunk > bytesRemaining THEN chunk = bytesRemaining bytesRemaining = bytesRemaining - chunk fileChunk\$ = INPUT\$(#lzwIN, chunk) chnkPoint = 0 RETURN

'''''''''''''''''''''''''''''''''''''''' ' Start LZW Decoder '''''''''''''''''''' [lzwDecode] LET EMPTY=-1:bitsRemain=0:tagCount=0:fileTag\$="" OPEN fileName\$ + fileExt\$ + JDext\$ FOR INPUT AS #lzwIN OPEN fileName\$ + ".Copy" + fileExt\$ FOR OUTPUT AS #lzwOUT GOSUB [StartFileChunk] chnkPoint = 2 settings = ASC(fileChunk\$) maxBits = VAL(RIGHT\$(STR\$(settings), 1)) + 12 dSize = 2 ^ maxBits IF settings > 99 THEN resetDictionary = 1 GOSUB [ResetLZW] oldIndex = orgIndex WHILE fileChunk\$ <> "" ' decode current index and write to file GOSUB [GetIndex] IF JDch\$(orgIndex) = "" THEN tmpIndex = oldIndex tmp\$ = JDch\$(tmpIndex) WHILE JDlzw(tmpIndex) > EMPTY tmpIndex = JDlzw(tmpIndex) tmp\$ = JDch\$(tmpIndex) + tmp\$ WEND tmp\$ = tmp\$ + LEFT\$(tmp\$, 1) ELSE tmpIndex = orgIndex tmp\$ = JDch\$(tmpIndex) WHILE JDlzw(tmpIndex) > EMPTY tmpIndex = JDlzw(tmpIndex) tmp\$ = JDch\$(tmpIndex) + tmp\$ WEND END IF #lzwOUT, tmp\$; ' add next dictionary entry or reset dictionary IF dNext < dSize THEN JDlzw(dNext) = oldIndex JDch\$(dNext) = LEFT\$(tmp\$, 1) dNext = dNext + 1 IF dNext = (2 ^ currentBitSize) THEN IF maxBits > currentBitSize THEN currentBitSize = currentBitSize + 1 ELSE IF resetDictionary THEN GOSUB [PrintDecode] GOSUB [ResetLZW] END IF END IF END IF END IF oldIndex = orgIndex WEND CLOSE #lzwOUT CLOSE #lzwIN GOSUB [PrintDecode] REDIM JDlzw(1) REDIM JDch\$(1) RETURN

[GetIndex] byteCount = 0:orgIndex = 0 bitsToGrab = currentBitSize - bitsRemain IF bitsRemain > 0 THEN orgIndex = lastByte byteCount = 1 END IF WHILE bitsToGrab > 0 lastByte = ASC(MID\$(fileChunk\$, chnkPoint, 1)) orgIndex = orgIndex + (lastByte * (2 ^ (byteCount * 8))) IF chnkPoint = chunk THEN GOSUB [GetFileChunk] chnkPoint = chnkPoint + 1 byteCount = byteCount + 1 bitsToGrab = bitsToGrab - 8 WEND IF bitsRemain > 0 THEN orgIndex = orgIndex / (2 ^ (8 - bitsRemain)) orgIndex = orgIndex AND ((2 ^ currentBitSize) - 1) bitsRemain = bitsToGrab * (-1) RETURN

[ResetLZW] REDIM JDlzw(dSize) REDIM JDch\$(dSize) FOR dNext = 0 TO 255 JDlzw(dNext) = EMPTY ' Prefix index JDch\$(dNext) = CHR\$(dNext) ' New byte value NEXT dNext currentBitSize = 8 GOSUB [GetIndex] #lzwOUT, JDch\$(orgIndex); currentBitSize = 9 RETURN

' End LZW Decoder '''''''''''''''''''''' ''''''''''''''''''''''''''''''''''''''''

'''''''''''''''''''''''''''''''''''''''' [PrintEncode] IF printDictionary < 1 THEN RETURN OPEN "Encode_" + fileTag\$ + fileName\$ + ".txt" FOR OUTPUT AS #dictOUT FOR X = 0 TO 255 LZW(X, PREFIX) = EMPTY LZW(X, BYTE) = X NEXT X FOR X = dNext TO 0 STEP -1 tmpIndex = X tmp\$ = CHR\$(LZW(tmpIndex, BYTE)) WHILE LZW(tmpIndex, PREFIX) > EMPTY tmpIndex = LZW(tmpIndex, PREFIX) tmp\$ = CHR\$(LZW(tmpIndex, BYTE)) + tmp\$ WEND #dictOUT, X; ":"; tmp\$ NEXT X CLOSE #dictOUT tagCount = tagCount + 1 fileTag\$ = STR\$(tagCount) + "_" RETURN

[PrintDecode] IF printDictionary < 1 THEN RETURN OPEN "Decode_" + fileTag\$ + fileName\$ + ".txt" FOR OUTPUT AS #dictOUT FOR X = dNext TO 0 STEP -1 tmpIndex = X tmp\$ = JDch\$(tmpIndex) WHILE JDlzw(tmpIndex) > EMPTY tmpIndex = JDlzw(tmpIndex) tmp\$ = JDch\$(tmpIndex) + tmp\$ WEND #dictOUT, X; ":"; tmp\$ NEXT X CLOSE #dictOUT tagCount = tagCount + 1 fileTag\$ = STR\$(tagCount) + "_" RETURN ''''''''''''''''''''''''''''''''''''''''

```

## Lua

```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 = {}, 0, "", {}, ""
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
table.insert(result, entry)
dictionary[dictSize] = w .. string.sub(entry, 1, 1)
dictSize = dictSize + 1
w = entry
end
return table.concat(result)
end

local example = "TOBEORNOTTOBEORTOBEORNOT"
local com = compress(example)
local dec = decompress(com)
print(table.concat(com, ", "))
print(dec)
```

{{Out}}

```
84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263
TOBEORNOTTOBEORTOBEORNOT

```

## M2000 Interpreter

{{trans|BBC BASIC}}

```
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

```

And here a change for using Inventories, where we have hash function, and we find entry in O(1).

```
Module FastM2000 {
plaintext\$="TOBEORNOTTOBEORTOBEORNOT"
Function encodeLZW\$(i\$) {
Def long c, d, i, l, o\$, w\$
Inventory dict
For i = 0 to 255 {Append dict , Chr\$(i):=i}
l = i
i = 1
w\$ = LEFT\$(i\$,1)
REPEAT{
d = 0
Repeat {
c = d
IF i > Len(i\$) Then Exit
if exist(dict, w\$) Then {
d=eval(dict)
} Else  Append dict, w\$:=l: Exit
if d<l Then i += 1 : w\$ += Mid\$(i\$, i, 1)
} Until d >= l
l += 1 : w\$ = Right\$(w\$, 1)
o\$ += Chr\$(c Mod 256) + Chr\$(c div 256)
} Until i > Len(i\$)
= o\$
}
encodeLZW\$ = encodeLZW\$(plaintext\$)
Document Doc\$
For i = 1 to Len(encodeLZW\$) STEP 2
Doc\$= Str\$(Asc(Mid\$(encodeLZW\$,i)) + 256*Asc(Mid\$(encodeLZW\$,i+1)))
Next i
Doc\$={
}
Function decodeLZW\$(i\$) {
Def c, i, l, o\$, t\$, w\$
Inventory Dict
For i = 0 to 255 {Append dict , i:=chr\$(i)}
l = i
c = Asc(i\$) + 256*Asc(Mid\$(i\$,2))
w\$ = dict\$(c)
o\$ = w\$
IF Len(i\$) < 4 Then = o\$
For i = 3 to Len(i\$) STEP 2 {
c = Asc(Mid\$(i\$,i)) + 256*Asc(Mid\$(i\$,i+1))
IF c < l Then {
t\$ = dict\$(c)
} Else t\$ = w\$ + LEFT\$(w\$,1)
o\$ += t\$
Append dict, l:=w\$ + LEFT\$(t\$,1)
l += 1 : w\$ = t\$
}
= o\$
}
Doc\$=decodeLZW\$(encodeLZW\$)+{
}
Clipboard Doc\$
Report Doc\$
}
FastM2000

```

{{out}}

``` 84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT

```

## Mathematica

{{trans|Ruby}}
compress[uncompressed_] :=
Module[{dictsize, dictionary, w, result, wc},
dictsize = 256;
dictionary = # -> # & /@ FromCharacterCode /@ Range@dictsize;
w = "";
result = {};
Do[wc = w <> c;
If[MemberQ[dictionary[[All, 1]], wc],
w = wc,
AppendTo[result, w /. dictionary];
AppendTo[dictionary, wc -> dictsize];
dictsize++;
w = c],
{c, Characters[uncompressed]}];
AppendTo[result, w /. dictionary];
result];
decompress[compressed_] :=
Module[{dictsize, dictionary, w, result, entry},
dictsize = 256;
dictionary = # -> # & /@ FromCharacterCode /@ Range@dictsize;
w = result = compressed[[1]];
Do[Which[MemberQ[dictionary[[All, 1]], k],
entry = k /. dictionary,
k == dictsize,
entry = w <> StringTake[w, 1],
True,
Message[decompress::bc, k]];
result = result <> entry;
AppendTo[dictionary, dictsize -> w <> StringTake[entry, 1]];
dictsize++;
w = entry,
{k, compressed[[2 ;;]]}];
result];
(*How to use:*)
compress["TOBEORNOTTOBEORTOBEORNOT"]
decompress[%]
```

{{Out}}

```txt
{"T", "O", "B", "E", "O", "R", "N", "O", "T", 256, 258, 260, 265, 259, 261, 263}

"TOBEORNOTTOBEORTOBEORNOT"
```

## Nim

import tables

proc compress*(uncompressed: string): seq[int] =
## build the dictionary
var dictionary = initTable[string, int]()
for i in 0..255:

var w: string = newString(0)
var compressed = newSeq[int]()

for c in uncompressed:
var wc = w & c
w = wc
else:
# writes w to output
# wc is a new sequence; add it to the dictionary
w = \$c

# write remaining output if necessary
if(w != nil):

result = compressed

proc decompress*(compressed: var seq[int]): string =
# build the dictionary
var dictionary = initTable[int, string]()
for i in 0..255:

var w: string = dictionary[compressed[0]]

compressed.delete(0)

var decompressed = w

for k in compressed:
var entry: string = newString(0)
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

w = entry

result = decompressed

when isMainModule:
var compressed = compress("TOBEORNOTTOBEORTOBEORNOT")
echo compressed
var decompressed = decompress(compressed)
echo decompressed

```

{{Out}}

```txt

@[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT

```

## Objeck

{{trans|Java}}

```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);
# 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);
};

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();
}
}
```

```txt
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT
```

{{works with|GNUstep}}

The class for the LZW compression algorithm:

```objc>#import

@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 {
dict[wc] = @(codemark);
codemark++;
w = [string subdataWithRange: NSMakeRange(i, 1)];
}
}
if ( [w length] != 0 )
{
}
return YES;
}

-(NSArray *) getArray
{
return iostream;
}

@end
```

Usage example:

```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;
}
```

{{out}} (reformatted by hand):

```txt

84  79  66  69  79  82  78  79
84 256 258 260 265 259 261 263

```

## OCaml

```ocaml
#directory "+extlib"  (* or maybe "+site-lib/extlib/" *)
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
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 *)
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
done;

let w, compressed =
match compressed with
| hd::tl -> (String.make 1 (char_of_int hd)), tl
| [] -> failwith "empty input"
in

let result = [w] in

let result, _, _ =
List.fold_left (fun (result, w, dict_size) k ->
let entry =
if Hashtbl.mem dictionary k then
Hashtbl.find dictionary k
else if k = Hashtbl.length dictionary then
w ^ (String.make 1 w.[0])
else
raise(ValueError(Printf.sprintf "Bad compressed k: %d" k))
in
let result = entry :: result in

(* add (w ^ entry.[0]) to the dictionary *)
Hashtbl.add dictionary dict_size (w ^ (String.make 1 entry.[0]));
(result, entry, dict_size + 1)
) (result, w, dict_size) compressed
in
(List.rev result)
;;
```

here is the interface:

```ocaml
val compress : uncompressed:string -> int list
val decompress : compressed:int list -> string list
```

How to use:
The compressed datas are a list of symbols (of type int) that will require more than 8 bits to be saved.
So to know how many bits are required, you need to know how many bits are required for the greatest symbol in the list.

```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 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
(Buffer.contents buf)
;;
```

## Ol

This version use lazy streams which is pair (symbol . function-to-get-next-symbol).

```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)

```

And decoder (runes->string used to unify functions - both used string iterators):

```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

```

## Perl

In this version the hashes contain mixed typed data:

```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 {
}
\$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";
```

{{out}}

```txt

T O B E O R N O T 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT

```

## Perl 6

{{trans|Perl}}

```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;
```

{{out}}

```txt

T O B E O R N O T 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT

```

## Phix

{{trans|Lua}}

```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)

```

{{out}}

```txt

{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

{{trans|JavaScript}}

```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;

```

{{out}}

```txt

84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263
TOBEORNOTTOBEORTOBEORNOT

```

## PicoLisp

```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)
(idx 'Dict (cons WC (inc 'Codes)) T)
(setq W C) ) ) )
(and W (link (cdr (lup Dict W)))) ) ) ) )

(de lzwDecompress (Lst)
(let (Codes 255  Dict)
(balance 'Dict
(make
(for C Codes
(link (list C (char C))) ) ) )
(make
(let W NIL
(for N Lst
(let WC (if (lup Dict N) (cdr @) (cons (last W) W))
(chain (reverse WC))
(when W
(idx 'Dict (cons (inc 'Codes) (cons (last WC) W)) T) )
(setq W WC) ) ) ) ) ) )
```

Test:

```txt
: (lzwCompress (chop "TOBEORNOTTOBEORTOBEORNOT"))
-> (84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)

: (pack (lzwDecompress @))
-> "TOBEORNOTTOBEORTOBEORNOT"
```

## PL/I

{{trans|REXX}}
The interesting point is the implementation of REXX's associative array (compound variable).

```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))!!', ';
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;

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
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;
```

{{out}}

```txt
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.

```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
result() = dict(w)
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
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
```

Sample output:

```txt
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:

```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)
```

Output:

```txt

[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT

```

## Racket

```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))
(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"))))
(set! w entry)
entry)))

(def uncompressed "TOBEORNOTTOBEORTOBEORNOT")
(displayln uncompressed)
(def compressed (compress uncompressed))
(displayln compressed)
(def decompressed (decompress compressed))
(displayln decompressed)

```

Output:

```txt

TOBEORNOTTOBEORTOBEORNOT
(T O B E O R N O T 256 258 260 265 259 261 263)
TOBEORNOTTOBEORTOBEORNOT

```

## REXX

### version 1

{{trans|Java}}

```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
```

'''Output:'''

```txt
str=TOBEORNOTTOBEORTOBEORNOT
[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
compression ok
dec=TOBEORNOTTOBEORTOBEORNOT
decompression ok

str=To be or not to be that is the question!
[84, 111, 32, 98, 101, 32, 111, 114, 32, 110, 111, 116, 32, 116, 257, 259, 268, 104, 97, 267, 105, 115, 272, 260, 113, 117, 101, 115, 116, 105, 111, 110, 33]
dec=To be or not to be that is the question!
decompression ok

```

### version 2

(Based on the '''Java''' program.)

This REXX version can execute on   '''ASCII'''   or   '''EBCDIC'''   systems.

```rexx
/*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 \$
```

{{out|output|text=  when using the default input:}}

```txt

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

```ring

# Project : LZW compression

plaintext = "TOBEORNOTTOBEORTOBEORNOT"
result = []
encode = encodelzw(plaintext)
for i = 1 to len(encode) step 2
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

```

Output:

```txt

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:

```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
```

Output:

```txt

["T", "O", "B", "E", "O", "R", "N", "O", "T", 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT

```

## Scala

```scala

def compress(tc:String) = {
//initial dictionary
val startDict = (1 to 255).map(a=>(""+a.toChar,a)).toMap
val (fullDict, result, remain) = tc.foldLeft ((startDict, List[Int](), "")) {
case ((dict,res,leftOver),nextChar) =>
if (dict.contains(leftOver + nextChar)) // current substring already in dict
(dict, res, leftOver+nextChar)
else if (dict.size < 4096) // add to dictionary
(dict + ((leftOver+nextChar, dict.size+1)), dict(leftOver) :: res, ""+nextChar)
else // dictionary is full
(dict, dict(leftOver) :: res, ""+nextChar)
}
if (remain.isEmpty) result.reverse else (fullDict(remain) :: result).reverse
}

def decompress(ns: List[Int]): String = {
val startDict = (1 to 255).map(a=>(a,""+a.toChar)).toMap
val (_, result, _) =
ns.foldLeft[(Map[Int, String], List[String], Option[(Int, String)])]((startDict, Nil, None)) {
case ((dict, result, conjecture), n) => {
dict.get(n) match {
case Some(output) => {
val (newDict, newCode) = conjecture match {
case Some((code, prefix)) => ((dict + (code -> (prefix + output.head))), code + 1)
case None => (dict, dict.size + 1)
}
(newDict, output :: result, Some(newCode -> output))
}
case None => {
// conjecture being None would be an encoding error
val (code, prefix) = conjecture.get
val output = prefix + prefix.head
(dict + (code -> output), output :: result, Some(code + 1 -> output))
}
}
}
}
result.reverse.mkString("")
}
// test
val text = "TOBEORNOTTOBEORTOBEORNOT"
val compressed = compress(text)
println(compressed)
val result = decompress(compressed)
println(result)

```

## Scheme

```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)
(if (> (length compressed) (+ ci 1))
(begin
(set! kn (list-ref compressed (+ ci 1)))
(if (< kn (length dictionary))
(set! dictionary
(append dictionary
(list (string-append
(list-ref dictionary k)
(string (string-ref (list-ref dictionary kn) 0)))))))))

;; Build the resulting string
(set! result (string-append result (list-ref dictionary k)))

(if (not (eqv? ci (- (length compressed) 1)))
(decompress kn (+ ci 1))))
result)

(define compressed (lzw-compress "TOBEORNOTTOBEORTOBEORNOT"))
(display compressed) (newline)
(define decompressed (lzw-decompress compressed))
(display decompressed) (newline)
```

Output:
```txt
(84 79 66 69 79 82 78 79 84 256 258 260 265 259 261 263)
TOBEORNOTTOBEORTOBEORNOT
```

## Seed7

```seed7
\$ include "seed7_05.s7i";

const func string: lzwCompress (in string: uncompressed) is func
result
var string: compressed is "";
local
var char: ch is ' ';
var hash [string] char: mydict is (hash [string] char).value;
var string: buffer is "";
var string: xstr is "";
begin
for ch range chr(0) to chr(255) do
mydict @:= [str(ch)] ch;
end for;
for ch range uncompressed do
xstr := buffer & str(ch);
if xstr in mydict then
buffer &:= str(ch)
else
compressed &:= str(mydict[buffer]);
mydict @:= [xstr] chr(length(mydict));
buffer := str(ch);
end if;
end for;
if buffer <> "" then
compressed &:= str(mydict[buffer]);
end if;
end func;

const func string: lzwDecompress (in string: compressed) is func
result
var string: uncompressed is "";
local
var char: ch is ' ';
var hash [char] string: mydict is (hash [char] string).value;
var string: buffer is "";
var string: current is "";
var string: chain is "";
begin
for ch range chr(0) to chr(255) do
mydict @:= [ch] str(ch);
end for;
for ch range compressed do
if buffer = "" then
buffer := mydict[ch];
uncompressed &:= buffer;
elsif ch <= chr(255) then
current := mydict[ch];
uncompressed &:= current;
chain := buffer & current;
mydict @:= [chr(length(mydict))] chain;
buffer := current;
else
if ch in mydict then
chain := mydict[ch];
else
chain := buffer & str(buffer[1]);
end if;
uncompressed &:= chain;
mydict @:= [chr(length(mydict))] buffer & str(chain[1]);
buffer := chain;
end if;
end for;
end func;

const proc: main is func
local
var string: compressed is "";
var string: uncompressed is "";
begin
compressed := lzwCompress("TOBEORNOTTOBEORTOBEORNOT");
writeln(literal(compressed));
uncompressed := lzwDecompress(compressed);
writeln(uncompressed);
end func;
```

Output:

```txt

"TOBEORNOT\256;\258;\260;\265;\259;\261;\263;"
TOBEORNOTTOBEORTOBEORNOT

```

Original source: [http://seed7.sourceforge.net/algorith/string.htm#lzwCompress] and [http://seed7.sourceforge.net/algorith/string.htm#lzwDecompress]

## Sidef

{{trans|Perl}}

```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 {
}
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
```

{{out}}

```txt
T O B E O R N O T 256 258 260 265 259 261 263
TOBEORNOTTOBEORTOBEORNOT
```

## Swift

{{trans|JavaScript}}

```swift
class LZW {
class func compress(_ uncompressed:String) -> [Int] {
var dict = [String : Int]()

for i in 0 ..< 256 {
let s = String(Unicode.Scalar(UInt8(i)))
dict[s] = i
}

var dictSize = 256
var w = ""
var result = [Int]()
for c in uncompressed {
let wc = w + String(c)
if dict[wc] != nil {
w = wc
} else {
result.append(dict[w]!)
dict[wc] = dictSize + 1
w = String(c)
}
}

if w != "" {
result.append(dict[w]!)
}
return result
}

class func decompress(_ compressed:[Int]) -> String? {
var dict = [Int : String]()

for i in 0 ..< 256 {
dict[i] = String(Unicode.Scalar(UInt8(i)))
}

var dictSize = 256
var w = String(Unicode.Scalar(UInt8(compressed[0])))
var result = w
for k in compressed[1 ..< compressed.count] {
let entry : String
if let x = dict[k] {
entry = x
} else if k == dictSize {
entry = w + String(w[w.startIndex])
} else {
return nil
}

result += entry
dict[dictSize+1] = w + String(entry[entry.startIndex])
w = entry
}
return result
}
}

let comp = LZW.compress("TOBEORNOTTOBEORTOBEORNOT")
print(comp)

if let decomp = LZW.decompress(comp) {
print(decomp)
}
```

{{out}}

```txt

[84, 79, 66, 69, 79, 82, 78, 79, 84, 257, 257, 257, 257, 257, 257, 257]
TOBEORNOTOTTOOTTTOOOTTTTOOOOTTTT

```

## Tcl

```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
```

## Xojo

{{trans|PHP}}

```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

```

Test:

```txt

Dim str as String = "TOBEORNOTTOBEORTOBEORNOT"
Dim com as string = LZW.compress(str)
print com
print LZW.decompress(com)

```

Output:

```txt

84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263
TOBEORNOTTOBEORTOBEORNOT

```

## zkl

```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);
w=entry;
}
decommpressed.text
}
```

```zkl
compressed:=lzwCompress("TOBEORNOTTOBEORTOBEORNOT");
compressed.toString(*).println();

lzwUncompress(compressed).println();
```

{{out}}

```txt

L(84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263)
TOBEORNOTTOBEORTOBEORNOT

```

{{omit from|Lilypond}}

```