bsd-comp.c

经典的ppp程序

/* Because this code is derived from the 4.3BSD compress source:
 *
 *
 * Copyright (c) 1985, 1986 The Regents of the University of California.
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * James A. Woods, derived from original work by Spencer Thomas
 * and Joseph Orost.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * $Id: bsd-comp.c,v 1.3 1999/04/16 11:35:59 paulus Exp $
 */

#include <sys/types.h>
#include <stddef.h>
#include <stdlib.h>
#include "ppp_defs.h"
#include "ppp-comp.h"

#if DO_BSD_COMPRESS

/*
 * PPP "BSD compress" compression
 *  The differences between this compression and the classic BSD LZW
 *  source are obvious from the requirement that the classic code worked
 *  with files while this handles arbitrarily long streams that
 *  are broken into packets.  They are:
 *
 *	When the code size expands, a block of junk is not emitted by
 *	    the compressor and not expected by the decompressor.
 *
 *	New codes are not necessarily assigned every time an old
 *	    code is output by the compressor.  This is because a packet
 *	    end forces a code to be emitted, but does not imply that a
 *	    new sequence has been seen.
 *
 *	The compression ratio is checked at the first end of a packet
 *	    after the appropriate gap.	Besides simplifying and speeding
 *	    things up, this makes it more likely that the transmitter
 *	    and receiver will agree when the dictionary is cleared when
 *	    compression is not going well.
 */

/*
 * A dictionary for doing BSD compress.
 */
struct bsd_db {
    int	    totlen;			/* length of this structure */
    u_int   hsize;			/* size of the hash table */
    u_char  hshift;			/* used in hash function */
    u_char  n_bits;			/* current bits/code */
    u_char  maxbits;
    u_char  debug;
    u_char  unit;
    u_short seqno;			/* sequence number of next packet */
    u_int   hdrlen;			/* header length to preallocate */
    u_int   mru;
    u_int   maxmaxcode;			/* largest valid code */
    u_int   max_ent;			/* largest code in use */
    u_int   in_count;			/* uncompressed bytes, aged */
    u_int   bytes_out;			/* compressed bytes, aged */
    u_int   ratio;			/* recent compression ratio */
    u_int   checkpoint;			/* when to next check the ratio */
    u_int   clear_count;		/* times dictionary cleared */
    u_int   incomp_count;		/* incompressible packets */
    u_int   incomp_bytes;		/* incompressible bytes */
    u_int   uncomp_count;		/* uncompressed packets */
    u_int   uncomp_bytes;		/* uncompressed bytes */
    u_int   comp_count;			/* compressed packets */
    u_int   comp_bytes;			/* compressed bytes */
    u_short *lens;			/* array of lengths of codes */
    struct bsd_dict {
	union {				/* hash value */
	    u_int32_t	fcode;
	    struct {
#ifdef BSD_LITTLE_ENDIAN
		u_short prefix;		/* preceding code */
		u_char	suffix;		/* last character of new code */
		u_char	pad;
#else
		u_char	pad;
		u_char	suffix;		/* last character of new code */
		u_short prefix;		/* preceding code */
#endif
	    } hs;
	} f;
	u_short codem1;			/* output of hash table -1 */
	u_short cptr;			/* map code to hash table entry */
    } dict[1];
};

#define BSD_OVHD	2		/* BSD compress overhead/packet */
#define BSD_INIT_BITS	BSD_MIN_BITS

static void	*bsd_decomp_alloc __P((u_char *options, int opt_len));
static void	bsd_free __P((void *state));
static int	bsd_decomp_init __P((void *state, u_char *options, int opt_len,
				     int unit, int hdrlen, int mru, int debug));
static void	bsd_incomp __P((void *state, u_char *dmsg, int len));
static int	bsd_decompress __P((void *state, u_char *cmp, int inlen,
				    u_char *dmp, int *outlen));
static void	bsd_reset __P((void *state));
static void	bsd_comp_stats __P((void *state, struct compstat *stats));

/*
 * Exported procedures.
 */
struct compressor ppp_bsd_compress = {
    CI_BSD_COMPRESS,		/* compress_proto */
    bsd_decomp_alloc,		/* decomp_alloc */
    bsd_free,			/* decomp_free */
    bsd_decomp_init,		/* decomp_init */
    bsd_reset,			/* decomp_reset */
    bsd_decompress,		/* decompress */
    bsd_incomp,			/* incomp */
    bsd_comp_stats,		/* decomp_stat */
};

/*
 * the next two codes should not be changed lightly, as they must not
 * lie within the contiguous general code space.
 */
#define CLEAR	256			/* table clear output code */
#define FIRST	257			/* first free entry */
#define LAST	255

#define MAXCODE(b)	((1 << (b)) - 1)
#define BADCODEM1	MAXCODE(BSD_MAX_BITS)

#define BSD_HASH(prefix,suffix,hshift)	((((u_int32_t)(suffix)) << (hshift)) \
					 ^ (u_int32_t)(prefix))
#define BSD_KEY(prefix,suffix)		((((u_int32_t)(suffix)) << 16) \
					 + (u_int32_t)(prefix))

#define CHECK_GAP	10000		/* Ratio check interval */

#define RATIO_SCALE_LOG	8
#define RATIO_SCALE	(1<<RATIO_SCALE_LOG)
#define RATIO_MAX	(0x7fffffff>>RATIO_SCALE_LOG)

/*
 * clear the dictionary
 */
static void
bsd_clear(db)
    struct bsd_db *db;
{
    db->clear_count++;
    db->max_ent = FIRST-1;
    db->n_bits = BSD_INIT_BITS;
    db->ratio = 0;
    db->bytes_out = 0;
    db->in_count = 0;
    db->checkpoint = CHECK_GAP;
}

/*
 * If the dictionary is full, then see if it is time to reset it.
 *
 * Compute the compression ratio using fixed-point arithmetic
 * with 8 fractional bits.
 *
 * Since we have an infinite stream instead of a single file,
 * watch only the local compression ratio.
 *
 * Since both peers must reset the dictionary at the same time even in
 * the absence of CLEAR codes (while packets are incompressible), they
 * must compute the same ratio.
 */
static int				/* 1=output CLEAR */
bsd_check(db)
    struct bsd_db *db;
{
    u_int new_ratio;

    if (db->in_count >= db->checkpoint) {
	/* age the ratio by limiting the size of the counts */
	if (db->in_count >= RATIO_MAX
	    || db->bytes_out >= RATIO_MAX) {
	    db->in_count -= db->in_count/4;
	    db->bytes_out -= db->bytes_out/4;
	}

	db->checkpoint = db->in_count + CHECK_GAP;

	if (db->max_ent >= db->maxmaxcode) {
	    /* Reset the dictionary only if the ratio is worse,
	     * or if it looks as if it has been poisoned
	     * by incompressible data.
	     *
	     * This does not overflow, because
	     *	db->in_count <= RATIO_MAX.
	     */
	    new_ratio = db->in_count << RATIO_SCALE_LOG;
	    if (db->bytes_out != 0)
		new_ratio /= db->bytes_out;

	    if (new_ratio < db->ratio || new_ratio < 1 * RATIO_SCALE) {
		bsd_clear(db);
		return 1;
	    }
	    db->ratio = new_ratio;
	}
    }
    return 0;
}

/*
 * Return statistics.
 */
static void
bsd_comp_stats(state, stats)
    void *state;
    struct compstat *stats;
{
    struct bsd_db *db = (struct bsd_db *) state;
    u_int out;

    stats->unc_bytes = db->uncomp_bytes;
    stats->unc_packets = db->uncomp_count;
    stats->comp_bytes = db->comp_bytes;
    stats->comp_packets = db->comp_count;
    stats->inc_bytes = db->incomp_bytes;
    stats->inc_packets = db->incomp_count;
    stats->ratio = db->in_count;
    out = db->bytes_out;
    if (stats->ratio <= 0x7fffff)
	stats->ratio <<= 8;
    else
	out >>= 8;
    if (out != 0)
	stats->ratio /= out;
}

/*
 * Reset state, as on a CCP ResetReq.
 */
static void
bsd_reset(state)
    void *state;
{
    struct bsd_db *db = (struct bsd_db *) state;

    db->seqno = 0;
    bsd_clear(db);
    db->clear_count = 0;
}

/*
 * Allocate space for a (de) compressor.
 */
static void *
bsd_alloc(options, opt_len, decomp)
    u_char *options;
    int opt_len, decomp;
{
    int bits;
    u_int newlen, hsize, hshift, maxmaxcode;
    struct bsd_db *db;

    if (opt_len != 3 || options[0] != CI_BSD_COMPRESS || options[1] != 3
	|| BSD_VERSION(options[2]) != BSD_CURRENT_VERSION)
	return NULL;

    bits = BSD_NBITS(options[2]);
    switch (bits) {
    case 9:			/* needs 82152 for both directions */
    case 10:			/* needs 84144 */
    case 11:			/* needs 88240 */
    case 12:			/* needs 96432 */
	hsize = 5003;
	hshift = 4;
	break;
    case 13:			/* needs 176784 */
	hsize = 9001;
	hshift = 5;
	break;
    case 14:			/* needs 353744 */
	hsize = 18013;
	hshift = 6;
	break;
    case 15:			/* needs 691440 */
	hsize = 35023;
	hshift = 7;
	break;
    case 16:			/* needs 1366160--far too much, */
	/* hsize = 69001; */	/* and 69001 is too big for cptr */
	/* hshift = 8; */	/* in struct bsd_db */
	/* break; */
    default:
	return NULL;
    }

    maxmaxcode = MAXCODE(bits);
    newlen = sizeof(*db) + (hsize-1) * (sizeof(db->dict[0]));
    db = (struct bsd_db *) malloc(newlen);
    if (!db)
	return NULL;
    memset(db, 0, sizeof(*db) - sizeof(db->dict));

    if (!decomp) {
	db->lens = NULL;
    } else {
	db->lens = (u_short *) malloc((maxmaxcode+1) * sizeof(db->lens[0]));
	if (!db->lens) {
	    free(db);
	    return NULL;
	}
    }

    db->totlen = newlen;
    db->hsize = hsize;
    db->hshift = hshift;
    db->maxmaxcode = maxmaxcode;
    db->maxbits = bits;

    return (void *) db;
}

static void
bsd_free(state)
    void *state;
{
    struct bsd_db *db = (struct bsd_db *) state;

    if (db->lens)
	free(db->lens);
    free(db);
}

static void *
bsd_decomp_alloc(options, opt_len)
    u_char *options;
    int opt_len;
{
    return bsd_alloc(options, opt_len, 1);
}

/*
 * Initialize the database.
 */
static int
bsd_init(db, options, opt_len, unit, hdrlen, mru, debug, decomp)
    struct bsd_db *db;
    u_char *options;
    int opt_len, unit, hdrlen, mru, debug, decomp;