lzss.c

#include "lzss.h"
#include <stdlib.h>
#include <string.h>
#include <errno.h>

lzss_error_t lzss_errno = LZSS_OK;

const char *lzss_strerror(lzss_error_t error) {
	switch (error) {
		case LZSS_NOMEM:
			return "Memory allocation failure";
			
		case LZSS_NODATA:
			return "Provided data is too short";
			
		case LZSS_INVARG:
			return "Invalid argument";
			
		case LZSS_OK:
			return "No error";
			
		default:
			return NULL;
	}
}

/**************************************************************
	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
 **************************************************************/

#define N         4096  /* size of ring buffer - must be power of 2 */
#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 */

ssize_t lzss_decompress(uint8_t *dst, unsigned int dstlen, uint8_t *src, unsigned int srclen)
{
	if (dst && src && dstlen && srclen) {
		/* ring buffer of size N, with extra F-1 bytes to aid string comparison */
		uint8_t text_buf[N + F - 1];
		uint8_t *dststart = dst;
		uint8_t *srcend = src + srclen;
		uint8_t *dstend = dst + dstlen;
		int  i, j, k, r, c;
		unsigned int flags;
		
		srcend = src + srclen;
		dst = dststart;
		for (i = 0; i < N - F; i++)
			text_buf[i] = ' ';
		r = N - F;
		flags = 0;
		
		while (1) {
			if (((flags >>= 1) & 0x100) == 0) {
				if (src < srcend) c = *src++; else break;
				flags = c | 0xFF00;  /* uses higher byte cleverly */
			}   /* to count eight */
			if (flags & 1) {
				if (src < srcend) c = *src++; else break;
				if (dst < dstend)
					*dst++ = c;
				else {
					lzss_errno = LZSS_NOMEM;
					return -1;
				}
				text_buf[r++] = c;
				r &= (N - 1);
			} else {
				if (src < srcend) i = *src++; else break;
				if (src < srcend) j = *src++; else break;
				i |= ((j & 0xF0) << 4);
				j  =  (j & 0x0F) + THRESHOLD;
				for (k = 0; k <= j; k++) {
					c = text_buf[(i + k) & (N - 1)];
					if (dst <= dstend)
						*dst++ = c;
					else {
						lzss_errno = LZSS_NOMEM;
						return -1;
					}
					text_buf[r++] = c;
					r &= (N - 1);
				}
			}
		}
		
		lzss_errno = LZSS_OK;
		return (ssize_t)dst - (ssize_t)dststart;
	} else {
		lzss_errno = LZSS_INVARG;
		return -1;
	}
}

struct encode_state {
	/* left & right children & parent. These constitute binary search trees. */
	int lchild[N + 1], rchild[N + 257], parent[N + 1];
	
	/* ring buffer of size N, with extra F-1 bytes to aid string comparison */
	uint8_t text_buf[N + F - 1];
	
	/* match_length of longest match.
	 * These are set by the insert_node() procedure.
	 */
	int match_position, match_length;
};

/* initialize state, mostly the trees
 *
 * For i = 0 to N - 1, rchild[i] and lchild[i] will be the right and left
 * children of node i.  These nodes need not be initialized.  Also, parent[i]
 * is the parent of node i.  These are initialized to NIL (= N), which stands
 * for 'not used.'  For i = 0 to 255, rchild[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. */
static void init_state(struct encode_state *sp) {
	int  i;
	
	memset(sp, 0, sizeof(*sp));
	
	for (i = 0; i < N - F; i++)
		sp->text_buf[i] = ' ';
	for (i = N + 1; i <= N + 256; i++)
		sp->rchild[i] = NIL;
	for (i = 0; i < N; i++)
		sp->parent[i] = NIL;
}

/* 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.
 */
static void insert_node(struct encode_state *sp, int r) {
	int  i, p, cmp;
	uint8_t  *key;
	
	cmp = 1;
	key = &sp->text_buf[r];
	p = N + 1 + key[0];
	sp->rchild[r] = sp->lchild[r] = NIL;
	sp->match_length = 0;
	for ( ; ; ) {
		if (cmp >= 0) {
			if (sp->rchild[p] != NIL)
				p = sp->rchild[p];
			else {
				sp->rchild[p] = r;
				sp->parent[r] = p;
				return;
			}
		} else {
			if (sp->lchild[p] != NIL)
				p = sp->lchild[p];
			else {
				sp->lchild[p] = r;
				sp->parent[r] = p;
				return;
			}
		}
		for (i = 1; i < F; i++) {
			if ((cmp = key[i] - sp->text_buf[p + i]) != 0)
				break;
		}
		if (i > sp->match_length) {
			sp->match_position = p;
			if ((sp->match_length = i) >= F)
				break;
		}
	}
	sp->parent[r] = sp->parent[p];
	sp->lchild[r] = sp->lchild[p];
	sp->rchild[r] = sp->rchild[p];
	sp->parent[sp->lchild[p]] = r;
	sp->parent[sp->rchild[p]] = r;
	if (sp->rchild[sp->parent[p]] == p)
		sp->rchild[sp->parent[p]] = r;
	else
		sp->lchild[sp->parent[p]] = r;
	sp->parent[p] = NIL;  /* remove p */
}

/* deletes node p from tree */
static void delete_node(struct encode_state *sp, int p) {
	int  q;
	
	if (sp->parent[p] == NIL)
		return;  /* not in tree */
	if (sp->rchild[p] == NIL)
		q = sp->lchild[p];
	else if (sp->lchild[p] == NIL)
		q = sp->rchild[p];
	else {
		q = sp->lchild[p];
		if (sp->rchild[q] != NIL) {
			do {
				q = sp->rchild[q];
			} while (sp->rchild[q] != NIL);
			sp->rchild[sp->parent[q]] = sp->lchild[q];
			sp->parent[sp->lchild[q]] = sp->parent[q];
			sp->lchild[q] = sp->lchild[p];
			sp->parent[sp->lchild[p]] = q;
		}
		sp->rchild[q] = sp->rchild[p];
		sp->parent[sp->rchild[p]] = q;
	}
	sp->parent[q] = sp->parent[p];
	if (sp->rchild[sp->parent[p]] == p)
		sp->rchild[sp->parent[p]] = q;
	else
		sp->lchild[sp->parent[p]] = q;
	sp->parent[p] = NIL;
}

ssize_t lzss_compress(uint8_t *dst, unsigned int dstlen, uint8_t *src, unsigned int srclen) {
	if (dst && src && dstlen && srclen) {
		/* Encoding state, mostly tree but some current match stuff */
		struct encode_state *sp;
		
		int  i, c, len, r, s, last_match_length, code_buf_ptr;
		uint8_t code_buf[17], mask;
		uint8_t *srcend = src + srclen;
		uint8_t *dstend = dst + dstlen;
		uint8_t *dststart = dst;
		
		/* initialize trees */
		sp = (struct encode_state *) malloc(sizeof(*sp));
		init_state(sp);
		
		/* 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), "" a position-and-length pair (2 bytes).
		 * Thus, eight units require at most 16 bytes of code.
		 */
		code_buf[0] = 0;
		code_buf_ptr = mask = 1;
		
		/* Clear the buffer with any character that will appear often. */
		s = 0;  r = N - F;
		
		/* Read F bytes into the last F bytes of the buffer */
		for (len = 0; len < F && src < srcend; len++)
			sp->text_buf[r + len] = *src++;
		if (!len) {
			free(sp);
			lzss_errno = LZSS_NODATA;
			return -1;
		}
		/*
		 * 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.
		 */
		for (i = 1; i <= F; i++)
			insert_node(sp, r - i);
		
		/* Finally, insert the whole string just read.
		 * The global variables match_length and match_position are set.
		 */
		insert_node(sp, r);
		do {
			/* match_length may be spuriously long near the end of text. */
			if (sp->match_length > len)
				sp->match_length = len;
			if (sp->match_length <= THRESHOLD) {
				sp->match_length = 1;  /* Not long enough match.  Send one byte. */
				code_buf[0] |= mask;  /* 'send one byte' flag */
				code_buf[code_buf_ptr++] = sp->text_buf[r];  /* Send uncoded. */
			} else {
				/* Send position and length pair. Note match_length > THRESHOLD. */
				code_buf[code_buf_ptr++] = (uint8_t) sp->match_position;
				code_buf[code_buf_ptr++] = (uint8_t)
				( ((sp->match_position >> 4) & 0xF0)
				 |  (sp->match_length - (THRESHOLD + 1)) );
			}
			if ((mask <<= 1) == 0) {  /* Shift mask left one bit. */
				/* Send at most 8 units of code together */
				for (i = 0; i < code_buf_ptr; i++)
					if (dst < dstend)
						*dst++ = code_buf[i];
					else {
						free(sp);
						lzss_errno = LZSS_NOMEM;
						return -1;
					}
				code_buf[0] = 0;
				code_buf_ptr = mask = 1;
			}
			last_match_length = sp->match_length;
			for (i = 0; i < last_match_length && src < srcend; i++) {
				delete_node(sp, s);    /* Delete old strings and */
				c = *src++;
				sp->text_buf[s] = c;    /* read new bytes */
				
				/* If the position is near the end of buffer, extend the buffer
				 * to make string comparison easier.
				 */
				if (s < F - 1)
					sp->text_buf[s + N] = c;
				
				/* Since this is a ring buffer, increment the position modulo N. */
				s = (s + 1) & (N - 1);
				r = (r + 1) & (N - 1);
				
				/* Register the string in text_buf[r..r+F-1] */
				insert_node(sp, r);
			}
			while (i++ < last_match_length) {
				delete_node(sp, s);
				
				/* After the end of text, no need to read, */
				s = (s + 1) & (N - 1);
				r = (r + 1) & (N - 1);
				/* but buffer may not be empty. */
				if (--len)
					insert_node(sp, r);
			}
		} 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++)
				if (dst < dstend)
					*dst++ = code_buf[i];
				else {
					free(sp);
					lzss_errno = LZSS_NOMEM;
					return -1;
				}
		}
		
		free(sp);
		lzss_errno = LZSS_OK;
		return (ssize_t)dst - (ssize_t)dststart;
	} else {
		lzss_errno = LZSS_INVARG;
		return -1;
	}
}