arclzw.c

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/*
 * $Header: arclzw.c,v 1.4 88/06/01 16:27:59 hyc Locked $
 */

/*
 * ARC - Archive utility - ARCLZW
 * 
 * Version 2.03, created on 10/24/86 at 11:46:22
 * 
 * (C) COPYRIGHT 1985,86 by System Enhancement Associates; ALL RIGHTS RESERVED
 * 
 * By:  Thom Henderson
 * 
 * Description: This file contains the routines used to implement Lempel-Zev
 * data compression, which calls for building a coding table on the fly.
 * This form of compression is especially good for encoding files which
 * contain repeated strings, and can often give dramatic improvements over
 * traditional Huffman SQueezing.
 * 
 * Language: Computer Innovations Optimizing C86
 * 
 * Programming notes: In this section I am drawing heavily on the COMPRESS
 * program from UNIX.  The basic method is taken from "A Technique for High
 * Performance Data Compression", Terry A. Welch, IEEE Computer Vol 17, No 6
 * (June 1984), pp 8-19.  Also see "Knuth's Fundamental Algorithms", Donald
 * Knuth, Vol 3, Section 6.4.
 * 
 * As best as I can tell, this method works by tracing down a hash table of code
 * strings where each entry has the property:
 * 
 * if <string> <char> is in the table then <string> is in the table.
 */
#include <stdio.h>
#include "arc.h"

void	putc_pak(), abort(), putc_ncr();
int	getc_unp();
#if	MSDOS
char	*setmem();
#else
char	*memset();
#endif

static void	putcode();
/* definitions for older style crunching */

#define FALSE    0
#define TRUE     !FALSE
#define TABSIZE  4096
#define NO_PRED  0xFFFF
#define EMPTY    0xFFFF
#define NOT_FND  0xFFFF

static unsigned short inbuf;	/* partial input code storage */
static int      sp;		/* current stack pointer */

struct entry {		/* string table entry format */
	char            used;	/* true when this entry is in use */
	unsigned char   follower;	/* char following string */
	unsigned short  next;	/* ptr to next in collision list */
	unsigned short  predecessor;	/* code for preceeding string */
};            /* string_tab[TABSIZE];	/* the code string table */


/* definitions for the new dynamic Lempel-Zev crunching */

#define BITS   12		/* maximum bits per code */
#define HSIZE  5003		/* 80% occupancy */
#define INIT_BITS 9		/* initial number of bits/code */

static int      n_bits;		/* number of bits/code */
static int      maxcode;	/* maximum code, given n_bits */
#define MAXCODE(n)      ((1<<(n)) - 1)	/* maximum code calculation */
static int      maxcodemax = 1 << BITS;	/* largest possible code (+1) */

static char     buf[BITS];	/* input/output buffer */

static unsigned char lmask[9] =	/* left side masks */
{
 0xff, 0xfe, 0xfc, 0xf8, 0xf0, 0xe0, 0xc0, 0x80, 0x00
};
static unsigned char rmask[9] =	/* right side masks */
{
 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff
};

static int      offset;		/* byte offset for code output */
static long     in_count;	/* length of input */
static long     bytes_out;	/* length of compressed output */
static long     bytes_ref;	/* output quality reference */
static long     bytes_last;	/* output size at last checkpoint */
static unsigned short ent;

/*
 * To save much memory (which we badly need at this point), we overlay the
 * table used by the previous version of Lempel-Zev with those used by the
 * new version.  Since no two of these routines will be used together, we can
 * safely do this.
 */

extern long     htab[HSIZE];		/* hash code table   (crunch) */
extern unsigned short codetab[HSIZE];	/* string code table (crunch) */
static struct	entry *string_tab=(struct entry *)htab;	/* old crunch string table */

static unsigned short *prefix=codetab;	/* prefix code table (uncrunch) */
static unsigned char *suffix=(unsigned char *)htab;	/* suffix table (uncrunch) */

static int      free_ent;	/* first unused entry */
static int      firstcmp;	/* true at start of compression */
extern unsigned char stack[HSIZE];	/* local push/pop stack */

/*
 * block compression parameters -- after all codes are used up, and
 * compression rate changes, start over.
 */

static int      clear_flg;
#define CHECK_GAP 2048		/* ratio check interval */
static long     checkpoint;
void            upd_tab();

/*
 * the next two codes should not be changed lightly, as they must not lie
 * within the contiguous general code space.
 */
#define FIRST   257		/* first free entry */
#define CLEAR   256		/* table clear output code */

/*
 * The cl_block() routine is called at each checkpoint to determine if
 * compression would likely improve by resetting the code table.  The method
 * chosen to determine this is based on empirical observation that, in
 * general, every 2k of input data should compress at least as well as the
 * first 2k of input.
 */

static          void
cl_block(t)			/* table clear for block compress */
	FILE           *t;	/* our output file */
{
	checkpoint = in_count + CHECK_GAP;

	if (bytes_ref) {
		if (bytes_out - bytes_last > bytes_ref) {
			setmem(htab, HSIZE * sizeof(long), 0xff);
			free_ent = FIRST;
			clear_flg = 1;
			putcode(CLEAR, t);
			bytes_ref = 0;
		}
	} else
		bytes_ref = bytes_out - bytes_last;

	bytes_last = bytes_out;	/* remember where we were */
}

/*****************************************************************
 *
 * Output a given code.
 * Inputs:
 *      code:   A n_bits-bit integer.  If == -1, then EOF.  This assumes
 *              that n_bits =< (LONG)wordsize - 1.
 * Outputs:
 *      Outputs code to the file.
 * Assumptions:
 *      Chars are 8 bits long.
 * Algorithm:
 *      Maintain a BITS character long buffer (so that 8 codes will
 * fit in it exactly).  When the buffer fills up empty it and start over.
 */

static          void
putcode(code, t)		/* output a code */
	int             code;	/* code to output */
	FILE           *t;	/* where to put it */
{
	int             r_off = offset;	/* right offset */
	int             bits = n_bits;	/* bits to go */
	char           *bp = buf;	/* buffer pointer */
	int             n;	/* index */

	register int	ztmp;

	if (code >= 0) {	/* if a real code *//* Get to the first byte. */
		bp += (r_off >> 3);
		r_off &= 7;

		/*
		 * Since code is always >= 8 bits, only need to mask the
		 * first hunk on the left. 
		 */
		ztmp = (code << r_off) & lmask[r_off];
		*bp = (*bp & rmask[r_off]) | ztmp;
		bp++;
		bits -= (8 - r_off);
		code >>= (8 - r_off);

		/* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
		if (bits >= 8) {
			*bp++ = code;
			code >>= 8;
			bits -= 8;
		}
		/* Last bits. */
		if (bits)
			*bp = code;
		offset += n_bits;

		if (offset == (n_bits << 3)) {
			bp = buf;
			bits = n_bits;
			bytes_out += bits;
			do
				putc_pak(*bp++, t);
			while (--bits);
			offset = 0;
		}
		/*
		 * If the next entry is going to be too big for the code
		 * size, then increase it, if possible. 
		 */
		if (free_ent > maxcode || clear_flg > 0) {
			/*
			 * Write the whole buffer, because the input side
			 * won't discover the size increase until after
			 * it has read it. 
			 */
			if (offset > 0) {
				bp = buf;	/* reset pointer for writing */
				bytes_out += n = n_bits;
				while (n--)
					putc_pak(*bp++, t);
			}
			offset = 0;

			if (clear_flg) {	/* reset if clearing */
				maxcode = MAXCODE(n_bits = INIT_BITS);
				clear_flg = 0;
			} else {/* else use more bits */
				n_bits++;
				if (n_bits == BITS)
					maxcode = maxcodemax;
				else
					maxcode = MAXCODE(n_bits);
			}
		}
	} else {		/* dump the buffer on EOF */
		bytes_out += n = (offset + 7) / 8;

		if (offset > 0)
			while (n--)
				putc_pak(*bp++, t);
		offset = 0;
	}
}

/*****************************************************************
 *
 * Read one code from the standard input.  If EOF, return -1.
 * Inputs:
 *      cmpin
 * Outputs:
 *      code or -1 is returned.
 */

static          int
getcode(f)			/* get a code */
	FILE           *f;	/* file to get from */
{
	int             code;
	static int      offset = 0, size = 0;
	int             r_off, bits;
	unsigned char  *bp = (unsigned char *) buf;

	if (clear_flg > 0 || offset >= size || free_ent > maxcode) {
		/*
		 * If the next entry will be too big for the current code
		 * size, then we must increase the size. This implies
		 * reading a new buffer full, too. 
		 */
		if (free_ent > maxcode) {
			n_bits++;
			if (n_bits == BITS)
				maxcode = maxcodemax;	/* won't get any bigger
							 * now */
			else
				maxcode = MAXCODE(n_bits);
		}
		if (clear_flg > 0) {
			maxcode = MAXCODE(n_bits = INIT_BITS);
			clear_flg = 0;
		}
		for (size = 0; size < n_bits; size++) {
			if ((code = getc_unp(f)) == EOF)
				break;
			else
				buf[size] = (char) code;
		}
		if (size <= 0)
			return -1;	/* end of file */

		offset = 0;
		/* Round size down to integral number of codes */
		size = (size << 3) - (n_bits - 1);
	}
	r_off = offset;
	bits = n_bits;

	/*
	 * Get to the first byte. 
	 */
	bp += (r_off >> 3);
	r_off &= 7;

	/* Get first part (low order bits) */
	code = (*bp++ >> r_off);
	bits -= 8 - r_off;
	r_off = 8 - r_off;	/* now, offset into code word */

	/* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
	if (bits >= 8) {
		code |= *bp++ << r_off;
		r_off += 8;
		bits -= 8;
	}
	/* high order bits. */
	code |= (*bp & rmask[bits]) << r_off;
	offset += n_bits;

	return code & MAXCODE(BITS);
}

/*
 * compress a file
 * 
 * Algorithm:  use open addressing double hashing (no chaining) on the prefix
 * code / next character combination.  We do a variant of Knuth's algorithm D
 * (vol. 3, sec. 6.4) along with G. Knott's relatively-prime secondary probe.
 * Here, the modular division first probe is gives way to a faster
 * exclusive-or manipulation.  Also do block compression with an adaptive
 * reset, where the code table is cleared when the compression ratio
 * decreases, but after the table fills.  The variable-length output codes
 * are re-sized at this point, and a special CLEAR code is generated for the
 * decompressor.
 */

void
init_cm(t)			/* initialize for compression */
	FILE           *t;	/* where compressed file goes */
{
	offset = 0;
	bytes_out = bytes_last = 1;
	bytes_ref = 0;
	clear_flg = 0;
	in_count = 1;
	checkpoint = CHECK_GAP;
	maxcode = MAXCODE(n_bits = INIT_BITS);
	free_ent = FIRST;
	setmem(htab, HSIZE * sizeof(long), 0xff);
	n_bits = INIT_BITS;	/* set starting code size */

	putc_pak(BITS, t);	/* note our max code length */

	firstcmp = 1;		/* next byte will be first */
}

void
putc_cm(c, t)			/* compress a character */
	unsigned char   c;	/* character to compress */
	FILE           *t;	/* where to put it */
{
	static long     fcode;
	static int      hshift;
	int             i;
	int             disp;

	if (firstcmp) {		/* special case for first byte */
		ent = c;	/* remember first byte */

		hshift = 0;
		for (fcode = (long) HSIZE; fcode < 65536L; fcode *= 2L)
			hshift++;
		hshift = 8 - hshift;	/* set hash code range bound */

		firstcmp = 0;	/* no longer first */
		return;
	}
	in_count++;

	fcode = (long) (((long) c << BITS) + ent);
	i = (c << hshift) ^ ent;/* xor hashing */

	if (htab[i] == fcode) {
		ent = codetab[i];
		return;
	} else if (htab[i] < 0)	/* empty slot */
		goto nomatch;
	disp = HSIZE - i;	/* secondary hash (after G.Knott) */
	if (i == 0)
		disp = 1;

probe: