Compression LZSS

• Executable de compression et de décompression LZSS

Un fichier compressé en LZSS commence toujours avec 4 octets qui donnent la taille du contenu du fichier compressé. Le reste c'est le contenu compressé.

Code expansé :

Bonjour, je vous présente ici le concept du LZSS. Le LZSS est un concept présenté pour la première fois en 1982.

Code compressé :

Bonjour, je vous présente ici le concept du LZSS. Le(44,5) est un(33,9)(17,7)é pour la première fois en 1982.

Pour savoir quels sont les octets compressés (ceux qui donnent (adresse,taille)) et quels sont les octets expansés, le fichier est divisés en petits bouts. Chaque petit bout commence par un octet de 8 bits (:kefka:). Chaque bit dit, parmi les octets suivant, lesquels sont compressé, lesquels sont décompressés. (0= compressé ; 1= expansé).

exemple :

0xFC 0x5D 0x6F 0x54 0x21 0xA3 0x5F 0xA3 0x5F 0xA3 0x5F

• Premier octet : 0xFC = 11111100 donc on a six “1” et deux “0”. Donc ! Dans les octets qui suivent, il y a deux*2 octets (donc quatre) compressés et six octets expansés.
• Les octets compressés : Soit, par exemple, 0x5D 0x6F. On fait des petits bidouillages : 0x65D (=adresse) et 0xF (=taille)
• Les octets expansés : on y touche pas

• Premier octet suivant…

L'adresse désigne un emplacement dans les 4096 derniers octets. Ce que je propose c'est de créer un buffer de 4096 octets qui va se remplir des 4096 derniers octets du résultat de la décompression. Au départ, le buffer est vide. Et le curseur dans le buffer commence à 4078.

La taille réelle est en fait ''taille indiquée + 3“.

Algorithme original (compression et décompression) par Haruhiko Okumura en langage c :

/**************************************************************
	LZSS.C -- A Data Compression Program
	(tab = 4 spaces)
***************************************************************
	4/6/1989 Haruhiko Okumura
	Use, distribute, and modify this program freely.
	Please send me your improved versions.
		PC-VAN		SCIENCE
		NIFTY-Serve	PAF01022
		CompuServe	74050,1022
**************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
 
#define N		 4096	/* size of ring buffer */
#define F		   18	/* upper limit for match_length */
#define THRESHOLD	2   /* encode string into position and length
						   if match_length is greater than this */
#define NIL			N	/* index for root of binary search trees */
 
unsigned long int
		textsize = 0,	/* text size counter */
		codesize = 0,	/* code size counter */
		printcount = 0;	/* counter for reporting progress every 1K bytes */
unsigned char
		text_buf[N + F - 1];	/* ring buffer of size N,
			with extra F-1 bytes to facilitate string comparison */
int		match_position, match_length,  /* of longest match.  These are
			set by the InsertNode() procedure. */
		lson[N + 1], rson[N + 257], dad[N + 1];  /* left & right children &
			parents -- These constitute binary search trees. */
FILE	*infile, *outfile;  /* input & output files */
 
void InitTree(void)  /* initialize trees */
{
	int  i;
 
	/* For i = 0 to N - 1, rson[i] and lson[i] will be the right and
	   left children of node i.  These nodes need not be initialized.
	   Also, dad[i] is the parent of node i.  These are initialized to
	   NIL (= N), which stands for 'not used.'
	   For i = 0 to 255, rson[N + i + 1] is the root of the tree
	   for strings that begin with character i.  These are initialized
	   to NIL.  Note there are 256 trees. */
 
	for (i = N + 1; i <= N + 256; i++) rson[i] = NIL;
	for (i = 0; i < N; i++) dad[i] = NIL;
}
 
void InsertNode(int r)
	/* Inserts string of length F, text_buf[r..r+F-1], into one of the
	   trees (text_buf[r]'th tree) and returns the longest-match position
	   and length via the global variables match_position and match_length.
	   If match_length = F, then removes the old node in favor of the new
	   one, because the old one will be deleted sooner.
	   Note r plays double role, as tree node and position in buffer. */
{
	int  i, p, cmp;
	unsigned char  *key;
 
	cmp = 1;  key = &text_buf[r];  p = N + 1 + key[0];
	rson[r] = lson[r] = NIL;  match_length = 0;
	for ( ; ; ) {
		if (cmp >= 0) {
			if (rson[p] != NIL) p = rson[p];
			else {  rson[p] = r;  dad[r] = p;  return;  }
		} else {
			if (lson[p] != NIL) p = lson[p];
			else {  lson[p] = r;  dad[r] = p;  return;  }
		}
		for (i = 1; i < F; i++)
			if ((cmp = key[i] - text_buf[p + i]) != 0)  break;
		if (i > match_length) {
			match_position = p;
			if ((match_length = i) >= F)  break;
		}
	}
	dad[r] = dad[p];  lson[r] = lson[p];  rson[r] = rson[p];
	dad[lson[p]] = r;  dad[rson[p]] = r;
	if (rson[dad[p]] == p) rson[dad[p]] = r;
	else                   lson[dad[p]] = r;
	dad[p] = NIL;  /* remove p */
}
 
void DeleteNode(int p)  /* deletes node p from tree */
{
	int  q;
 
	if (dad[p] == NIL) return;  /* not in tree */
	if (rson[p] == NIL) q = lson[p];
	else if (lson[p] == NIL) q = rson[p];
	else {
		q = lson[p];
		if (rson[q] != NIL) {
			do {  q = rson[q];  } while (rson[q] != NIL);
			rson[dad[q]] = lson[q];  dad[lson[q]] = dad[q];
			lson[q] = lson[p];  dad[lson[p]] = q;
		}
		rson[q] = rson[p];  dad[rson[p]] = q;
	}
	dad[q] = dad[p];
	if (rson[dad[p]] == p) rson[dad[p]] = q;  else lson[dad[p]] = q;
	dad[p] = NIL;
}
 
void Encode(void)
{
	int  i, c, len, r, s, last_match_length, code_buf_ptr;
	unsigned char  code_buf[17], mask;
 
	InitTree();  /* initialize trees */
	code_buf[0] = 0;  /* code_buf[1..16] saves eight units of code, and
		code_buf[0] works as eight flags, "1" representing that the unit
		is an unencoded letter (1 byte), "0" a position-and-length pair
		(2 bytes).  Thus, eight units require at most 16 bytes of code. */
	code_buf_ptr = mask = 1;
	s = 0;  r = N - F;
	for (i = s; i < r; i++) text_buf[i] = ' ';  /* Clear the buffer with
		any character that will appear often. */
	for (len = 0; len < F && (c = getc(infile)) != EOF; len++)
		text_buf[r + len] = c;  /* Read F bytes into the last F bytes of
			the buffer */
	if ((textsize = len) == 0) return;  /* text of size zero */
	for (i = 1; i <= F; i++) InsertNode(r - i);  /* Insert the F strings,
		each of which begins with one or more 'space' characters.  Note
		the order in which these strings are inserted.  This way,
		degenerate trees will be less likely to occur. */
	InsertNode(r);  /* Finally, insert the whole string just read.  The
		global variables match_length and match_position are set. */
	do {
		if (match_length > len) match_length = len;  /* match_length
			may be spuriously long near the end of text. */
		if (match_length <= THRESHOLD) {
			match_length = 1;  /* Not long enough match.  Send one byte. */
			code_buf[0] |= mask;  /* 'send one byte' flag */
			code_buf[code_buf_ptr++] = text_buf[r];  /* Send uncoded. */
		} else {
			code_buf[code_buf_ptr++] = (unsigned char) match_position;
			code_buf[code_buf_ptr++] = (unsigned char)
				(((match_position >> 4) & 0xf0)
			  | (match_length - (THRESHOLD + 1)));  /* Send position and
					length pair. Note match_length > THRESHOLD. */
		}
		if ((mask <<= 1) == 0) {  /* Shift mask left one bit. */
			for (i = 0; i < code_buf_ptr; i++)  /* Send at most 8 units of */
				putc(code_buf[i], outfile);     /* code together */
			codesize += code_buf_ptr;
			code_buf[0] = 0;  code_buf_ptr = mask = 1;
		}
		last_match_length = match_length;
		for (i = 0; i < last_match_length &&
				(c = getc(infile)) != EOF; i++) {
			DeleteNode(s);		/* Delete old strings and */
			text_buf[s] = c;	/* read new bytes */
			if (s < F - 1) text_buf[s + N] = c;  /* If the position is
				near the end of buffer, extend the buffer to make
				string comparison easier. */
			s = (s + 1) & (N - 1);  r = (r + 1) & (N - 1);
				/* Since this is a ring buffer, increment the position
				   modulo N. */
			InsertNode(r);	/* Register the string in text_buf[r..r+F-1] */
		}
		if ((textsize += i) > printcount) {
			printf("%12ld\r", textsize);  printcount += 1024;
				/* Reports progress each time the textsize exceeds
				   multiples of 1024. */
		}
		while (i++ < last_match_length) {	/* After the end of text, */
			DeleteNode(s);					/* no need to read, but */
			s = (s + 1) & (N - 1);  r = (r + 1) & (N - 1);
			if (--len) InsertNode(r);		/* buffer may not be empty. */
		}
	} while (len > 0);	/* until length of string to be processed is zero */
	if (code_buf_ptr > 1) {		/* Send remaining code. */
		for (i = 0; i < code_buf_ptr; i++) putc(code_buf[i], outfile);
		codesize += code_buf_ptr;
	}
	printf("In : %ld bytes\n", textsize);	/* Encoding is done. */
	printf("Out: %ld bytes\n", codesize);
	printf("Out/In: %.3f\n", (double)codesize / textsize);
}
 
void Decode(void)	/* Just the reverse of Encode(). */
{
	int  i, j, k, r, c;
	unsigned int  flags;
 
	for (i = 0; i < N - F; i++) text_buf[i] = ' ';
	r = N - F;  flags = 0;
	for ( ; ; ) {
		if (((flags >>= 1) & 256) == 0) {
			if ((c = getc(infile)) == EOF) break;
			flags = c | 0xff00;		/* uses higher byte cleverly */
		}							/* to count eight */
		if (flags & 1) {
			if ((c = getc(infile)) == EOF) break;
			putc(c, outfile);  text_buf[r++] = c;  r &= (N - 1);
		} else {
			if ((i = getc(infile)) == EOF) break;
			if ((j = getc(infile)) == EOF) break;
			i |= ((j & 0xf0) << 4);  j = (j & 0x0f) + THRESHOLD;
			for (k = 0; k <= j; k++) {
				c = text_buf[(i + k) & (N - 1)];
				putc(c, outfile);  text_buf[r++] = c;  r &= (N - 1);
			}
		}
	}
}
 
int main(int argc, char *argv[])
{
	char  *s;
 
	if (argc != 4) {
		printf("'lzss e file1 file2' encodes file1 into file2.\n"
			   "'lzss d file2 file1' decodes file2 into file1.\n");
		return EXIT_FAILURE;
	}
	if ((s = argv[1], s[1] || strpbrk(s, "DEde") == NULL)
	 || (s = argv[2], (infile  = fopen(s, "rb")) == NULL)
	 || (s = argv[3], (outfile = fopen(s, "wb")) == NULL)) {
		printf("??? %s\n", s);  return EXIT_FAILURE;
	}
	if (toupper(*argv[1]) == 'E') Encode();  else Decode();
	fclose(infile);  fclose(outfile);
	return EXIT_SUCCESS;
}

• {en} Tuto sur le LZSS