[apple/xnu.git] / bsd / net / bsd_comp.c

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * Copyright (c) 1999-2003 Apple Computer, Inc.  All Rights Reserved.
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */
/* 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.
 */

/*
 * This version is for use with mbufs on BSD-derived systems.
 *
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <net/ppp_defs.h>

#define PACKETPTR       struct mbuf *
#include <net/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_int16_t seqno;                    /* sequence # 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_int16_t *lens;                    /* array of lengths of codes */
    struct bsd_dict {
        union {                         /* hash value */
            u_int32_t   fcode;
            struct {
#if BYTE_ORDER == LITTLE_ENDIAN
                u_int16_t 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_int16_t prefix;       /* preceding code */
#endif
            } hs;
        } f;
        u_int16_t codem1;               /* output of hash table -1 */
        u_int16_t 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_clear __P((struct bsd_db *db));
static int      bsd_check __P((struct bsd_db *db));
static void     *bsd_alloc __P((u_char *options, int opt_len, int decomp));
static int      bsd_init_comp_db __P((struct bsd_db *db, u_char *options, int opt_len,
                              int unit, int hdrlen, int mru, int debug,
                              int decomp));
static void     *bsd_comp_alloc __P((u_char *options, int opt_len));
static void     *bsd_decomp_alloc __P((u_char *options, int opt_len));
static void     bsd_free __P((void *state));
static int      bsd_comp_init __P((void *state, u_char *options, int opt_len,
                                   int unit, int hdrlen, int debug));
static int      bsd_decomp_init __P((void *state, u_char *options, int opt_len,
                                     int unit, int hdrlen, int mru, int debug));
static int      bsd_compress __P((void *state, struct mbuf **mret,
                                  struct mbuf *mp, int slen, int maxolen));
static void     bsd_incomp __P((void *state, struct mbuf *dmsg));
static int      bsd_decompress __P((void *state, struct mbuf *cmp,
                                    struct mbuf **dmpp));
static void     bsd_reset __P((void *state));
static void     bsd_comp_stats __P((void *state, struct compstat *stats));

/*
 * Procedures exported to if_ppp.c.
 */
struct compressor ppp_bsd_compress = {
    CI_BSD_COMPRESS,            /* compress_proto */
    bsd_comp_alloc,             /* comp_alloc */
    bsd_free,                   /* comp_free */
    bsd_comp_init,              /* comp_init */
    bsd_reset,                  /* comp_reset */
    bsd_compress,               /* compress */
    bsd_comp_stats,             /* comp_stat */
    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 < CILEN_BSD_COMPRESS || options[0] != CI_BSD_COMPRESS
        || options[1] != CILEN_BSD_COMPRESS
        || 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]));
    MALLOC(db, struct bsd_db *, newlen, M_DEVBUF, M_NOWAIT);
    if (!db)
        return NULL;
    bzero(db, sizeof(*db) - sizeof(db->dict));

    if (!decomp) {
        db->lens = NULL;
    } else {
        MALLOC(db->lens, u_int16_t *, (maxmaxcode+1) * sizeof(db->lens[0]),
               M_DEVBUF, M_NOWAIT);
        if (!db->lens) {
            FREE(db, M_DEVBUF);
            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, M_DEVBUF);
    FREE(db, M_DEVBUF);
}

static void *
bsd_comp_alloc(options, opt_len)
    u_char *options;
    int opt_len;
{
    return bsd_alloc(options, opt_len, 0);
}

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_comp_db(db, options, opt_len, unit, hdrlen, mru, debug, decomp)
    struct bsd_db *db;
    u_char *options;
    int opt_len, unit, hdrlen, mru, debug, decomp;
{
    int i;

    if (opt_len < CILEN_BSD_COMPRESS || options[0] != CI_BSD_COMPRESS
        || options[1] != CILEN_BSD_COMPRESS
        || BSD_VERSION(options[2]) != BSD_CURRENT_VERSION
        || BSD_NBITS(options[2]) != db->maxbits
        || (decomp && db->lens == NULL))
        return 0;

    if (decomp) {
        i = LAST+1;
        while (i != 0)
            db->lens[--i] = 1;
    }
    i = db->hsize;
    while (i != 0) {
        db->dict[--i].codem1 = BADCODEM1;
        db->dict[i].cptr = 0;
    }

    db->unit = unit;
    db->hdrlen = hdrlen;
    db->mru = mru;
#ifndef DEBUG
    if (debug)
#endif
        db->debug = 1;

    bsd_reset(db);

    return 1;
}

static int
bsd_comp_init(state, options, opt_len, unit, hdrlen, debug)
    void *state;
    u_char *options;
    int opt_len, unit, hdrlen, debug;
{
    return bsd_init_comp_db((struct bsd_db *) state, options, opt_len,
                    unit, hdrlen, 0, debug, 0);
}

static int
bsd_decomp_init(state, options, opt_len, unit, hdrlen, mru, debug)
    void *state;
    u_char *options;
    int opt_len, unit, hdrlen, mru, debug;
{
    return bsd_init_comp_db((struct bsd_db *) state, options, opt_len,
                    unit, hdrlen, mru, debug, 1);
}


/*
 * compress a packet
 *      One change from the BSD compress command is that when the
 *      code size expands, we do not output a bunch of padding.
 */
int                                     /* new slen */
bsd_compress(state, mret, mp, slen, maxolen)
    void *state;
    struct mbuf **mret;         /* return compressed mbuf chain here */
    struct mbuf *mp;            /* from here */
    int slen;                   /* uncompressed length */
    int maxolen;                /* max compressed length */
{
    struct bsd_db *db = (struct bsd_db *) state;
    int hshift = db->hshift;
    u_int max_ent = db->max_ent;
    u_int n_bits = db->n_bits;
    u_int bitno = 32;
    u_int32_t accm = 0, fcode;
    struct bsd_dict *dictp;
    u_char c;
    int hval, disp, ent, ilen;
    u_char *rptr, *wptr;
    u_char *cp_end;
    int olen;
    struct mbuf *m;

#define PUTBYTE(v) {                                    \
    ++olen;                                             \
    if (wptr) {                                         \
        *wptr++ = (v);                                  \
        if (wptr >= cp_end) {                           \
            m->m_len = wptr - mtod(m, u_char *);        \
            MGET(m->m_next, M_DONTWAIT, MT_DATA);       \
            m = m->m_next;                              \
            if (m) {                                    \
                m->m_len = 0;                           \
                if (maxolen - olen > MLEN)              \
                    MCLGET(m, M_DONTWAIT);              \
                wptr = mtod(m, u_char *);               \
                cp_end = wptr + M_TRAILINGSPACE(m);     \
            } else                                      \
                wptr = NULL;                            \
        }                                               \
    }                                                   \
}

#define OUTPUT(ent) {                                   \
    bitno -= n_bits;                                    \
    accm |= ((ent) << bitno);                           \
    do {                                                \
        PUTBYTE(accm >> 24);                            \
        accm <<= 8;                                     \
        bitno += 8;                                     \
    } while (bitno <= 24);                              \
}

    /*
     * If the protocol is not in the range we're interested in,
     * just return without compressing the packet.  If it is,
     * the protocol becomes the first byte to compress.
     */
    rptr = mtod(mp, u_char *);
    ent = PPP_PROTOCOL(rptr);
    if (ent < 0x21 || ent > 0xf9) {
        *mret = NULL;
        return slen;
    }

    /* Don't generate compressed packets which are larger than
       the uncompressed packet. */
    if (maxolen > slen)
        maxolen = slen;

    /* Allocate one mbuf to start with. */
    MGET(m, M_DONTWAIT, MT_DATA);
    *mret = m;
    if (m != NULL) {
        m->m_len = 0;
        if (maxolen + db->hdrlen > MLEN)
            MCLGET(m, M_DONTWAIT);
        m->m_data += db->hdrlen;
        wptr = mtod(m, u_char *);
        cp_end = wptr + M_TRAILINGSPACE(m);
    } else
        wptr = cp_end = NULL;

    /*
     * Copy the PPP header over, changing the protocol,
     * and install the 2-byte packet sequence number.
     */
    if (wptr) {
        *wptr++ = PPP_ADDRESS(rptr);    /* assumes the ppp header is */
        *wptr++ = PPP_CONTROL(rptr);    /* all in one mbuf */
        *wptr++ = 0;                    /* change the protocol */
        *wptr++ = PPP_COMP;
        *wptr++ = db->seqno >> 8;
        *wptr++ = db->seqno;
    }
    ++db->seqno;

    olen = 0;
    rptr += PPP_HDRLEN;
    slen = mp->m_len - PPP_HDRLEN;
    ilen = slen + 1;
    for (;;) {
        if (slen <= 0) {
            mp = mp->m_next;
            if (!mp)
                break;
            rptr = mtod(mp, u_char *);
            slen = mp->m_len;
            if (!slen)
                continue;   /* handle 0-length buffers */
            ilen += slen;
        }

        slen--;
        c = *rptr++;
        fcode = BSD_KEY(ent, c);
        hval = BSD_HASH(ent, c, hshift);
        dictp = &db->dict[hval];

        /* Validate and then check the entry. */
        if (dictp->codem1 >= max_ent)
            goto nomatch;
        if (dictp->f.fcode == fcode) {
            ent = dictp->codem1+1;
            continue;   /* found (prefix,suffix) */
        }

        /* continue probing until a match or invalid entry */
        disp = (hval == 0) ? 1 : hval;
        do {
            hval += disp;
            if (hval >= db->hsize)
                hval -= db->hsize;
            dictp = &db->dict[hval];
            if (dictp->codem1 >= max_ent)
                goto nomatch;
        } while (dictp->f.fcode != fcode);
        ent = dictp->codem1 + 1;        /* finally found (prefix,suffix) */
        continue;

    nomatch:
        OUTPUT(ent);            /* output the prefix */

        /* code -> hashtable */
        if (max_ent < db->maxmaxcode) {
            struct bsd_dict *dictp2;
            /* expand code size if needed */
            if (max_ent >= MAXCODE(n_bits))
                db->n_bits = ++n_bits;

            /* Invalidate old hash table entry using
             * this code, and then take it over.
             */
            dictp2 = &db->dict[max_ent+1];
            if (db->dict[dictp2->cptr].codem1 == max_ent)
                db->dict[dictp2->cptr].codem1 = BADCODEM1;
            dictp2->cptr = hval;
            dictp->codem1 = max_ent;
            dictp->f.fcode = fcode;

            db->max_ent = ++max_ent;
        }
        ent = c;
    }

    OUTPUT(ent);                /* output the last code */
    db->bytes_out += olen;
    db->in_count += ilen;
    if (bitno < 32)
        ++db->bytes_out;        /* count complete bytes */

    if (bsd_check(db))
        OUTPUT(CLEAR);          /* do not count the CLEAR */

    /*
     * Pad dribble bits of last code with ones.
     * Do not emit a completely useless byte of ones.
     */
    if (bitno != 32)
        PUTBYTE((accm | (0xff << (bitno-8))) >> 24);

    if (m != NULL) {
        m->m_len = wptr - mtod(m, u_char *);
        m->m_next = NULL;
    }

    /*
     * Increase code size if we would have without the packet
     * boundary and as the decompressor will.
     */
    if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode)
        db->n_bits++;

    db->uncomp_bytes += ilen;
    ++db->uncomp_count;
    if (olen + PPP_HDRLEN + BSD_OVHD > maxolen) {
        /* throw away the compressed stuff if it is longer than uncompressed */
        if (*mret != NULL) {
            m_freem(*mret);
            *mret = NULL;
        }
        ++db->incomp_count;
        db->incomp_bytes += ilen;
    } else {
        ++db->comp_count;
        db->comp_bytes += olen + BSD_OVHD;
    }

    return olen + PPP_HDRLEN + BSD_OVHD;
#undef OUTPUT
#undef PUTBYTE
}


/*
 * Update the "BSD Compress" dictionary on the receiver for
 * incompressible data by pretending to compress the incoming data.
 */
static void
bsd_incomp(state, dmsg)
    void *state;
    struct mbuf *dmsg;
{
    struct bsd_db *db = (struct bsd_db *) state;
    u_int hshift = db->hshift;
    u_int max_ent = db->max_ent;
    u_int n_bits = db->n_bits;
    struct bsd_dict *dictp;
    u_int32_t fcode;
    u_char c;
    u_int32_t hval, disp;
    int slen, ilen;
    u_int bitno = 7;
    u_char *rptr;
    u_int ent;

    /*
     * If the protocol is not in the range we're interested in,
     * just return without looking at the packet.  If it is,
     * the protocol becomes the first byte to "compress".
     */
    rptr = mtod(dmsg, u_char *);
    ent = PPP_PROTOCOL(rptr);
    if (ent < 0x21 || ent > 0xf9)
        return;

    db->seqno++;
    ilen = 1;           /* count the protocol as 1 byte */
    rptr += PPP_HDRLEN;
    slen = dmsg->m_len - PPP_HDRLEN;
    for (;;) {
        if (slen <= 0) {
            dmsg = dmsg->m_next;
            if (!dmsg)
                break;
            rptr = mtod(dmsg, u_char *);
            slen = dmsg->m_len;
            continue;
        }
        ilen += slen;

        do {
            c = *rptr++;
            fcode = BSD_KEY(ent, c);
            hval = BSD_HASH(ent, c, hshift);
            dictp = &db->dict[hval];

            /* validate and then check the entry */
            if (dictp->codem1 >= max_ent)
                goto nomatch;
            if (dictp->f.fcode == fcode) {
                ent = dictp->codem1+1;
                continue;   /* found (prefix,suffix) */
            }

            /* continue probing until a match or invalid entry */
            disp = (hval == 0) ? 1 : hval;
            do {
                hval += disp;
                if (hval >= db->hsize)
                    hval -= db->hsize;
                dictp = &db->dict[hval];
                if (dictp->codem1 >= max_ent)
                    goto nomatch;
            } while (dictp->f.fcode != fcode);
            ent = dictp->codem1+1;
            continue;   /* finally found (prefix,suffix) */

        nomatch:                /* output (count) the prefix */
            bitno += n_bits;

            /* code -> hashtable */
            if (max_ent < db->maxmaxcode) {
                struct bsd_dict *dictp2;
                /* expand code size if needed */
                if (max_ent >= MAXCODE(n_bits))
                    db->n_bits = ++n_bits;

                /* Invalidate previous hash table entry
                 * assigned this code, and then take it over.
                 */
                dictp2 = &db->dict[max_ent+1];
                if (db->dict[dictp2->cptr].codem1 == max_ent)
                    db->dict[dictp2->cptr].codem1 = BADCODEM1;
                dictp2->cptr = hval;
                dictp->codem1 = max_ent;
                dictp->f.fcode = fcode;

                db->max_ent = ++max_ent;
                db->lens[max_ent] = db->lens[ent]+1;
            }
            ent = c;
        } while (--slen != 0);
    }
    bitno += n_bits;            /* output (count) the last code */
    db->bytes_out += bitno/8;
    db->in_count += ilen;
    (void)bsd_check(db);

    ++db->incomp_count;
    db->incomp_bytes += ilen;
    ++db->uncomp_count;
    db->uncomp_bytes += ilen;

    /* Increase code size if we would have without the packet
     * boundary and as the decompressor will.
     */
    if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode)
        db->n_bits++;
}


/*
 * Decompress "BSD Compress".
 *
 * Because of patent problems, we return DECOMP_ERROR for errors
 * found by inspecting the input data and for system problems, but
 * DECOMP_FATALERROR for any errors which could possibly be said to
 * be being detected "after" decompression.  For DECOMP_ERROR,
 * we can issue a CCP reset-request; for DECOMP_FATALERROR, we may be
 * infringing a patent of Motorola's if we do, so we take CCP down
 * instead.
 *
 * Given that the frame has the correct sequence number and a good FCS,
 * errors such as invalid codes in the input most likely indicate a
 * bug, so we return DECOMP_FATALERROR for them in order to turn off
 * compression, even though they are detected by inspecting the input.
 */
int
bsd_decompress(state, cmp, dmpp)
    void *state;
    struct mbuf *cmp, **dmpp;
{
    struct bsd_db *db = (struct bsd_db *) state;
    u_int max_ent = db->max_ent;
    u_int32_t accm = 0;
    u_int bitno = 32;           /* 1st valid bit in accm */
    u_int n_bits = db->n_bits;
    u_int tgtbitno = 32-n_bits; /* bitno when we have a code */
    struct bsd_dict *dictp;
    int explen, i, seq, len;
    u_int incode, oldcode, finchar;
    u_char *p, *rptr, *wptr;
    struct mbuf *m, *dmp, *mret;
    int adrs, ctrl, ilen;
    int space, codelen, extra;

    /*
     * Save the address/control from the PPP header
     * and then get the sequence number.
     */
    *dmpp = NULL;
    rptr = mtod(cmp, u_char *);
    adrs = PPP_ADDRESS(rptr);
    ctrl = PPP_CONTROL(rptr);
    rptr += PPP_HDRLEN;
    len = cmp->m_len - PPP_HDRLEN;
    seq = 0;
    for (i = 0; i < 2; ++i) {
        while (len <= 0) {
            cmp = cmp->m_next;
            if (cmp == NULL)
                return DECOMP_ERROR;
            rptr = mtod(cmp, u_char *);
            len = cmp->m_len;
        }
        seq = (seq << 8) + *rptr++;
        --len;
    }

    /*
     * Check the sequence number and give up if it differs from
     * the value we're expecting.
     */
    if (seq != db->seqno) {
        if (db->debug)
            printf("bsd_decomp%d: bad sequence # %d, expected %d\n",
                   db->unit, seq, db->seqno - 1);
        return DECOMP_ERROR;
    }
    ++db->seqno;

    /*
     * Allocate one mbuf to start with.
     */
    MGETHDR(dmp, M_DONTWAIT, MT_DATA);
    if (dmp == NULL)
        return DECOMP_ERROR;
    mret = dmp;
    dmp->m_len = 0;
    dmp->m_next = NULL;
    MCLGET(dmp, M_DONTWAIT);
    dmp->m_data += db->hdrlen;
    wptr = mtod(dmp, u_char *);
    space = M_TRAILINGSPACE(dmp) - PPP_HDRLEN + 1;

    /*
     * Fill in the ppp header, but not the last byte of the protocol
     * (that comes from the decompressed data).
     */
    wptr[0] = adrs;
    wptr[1] = ctrl;
    wptr[2] = 0;
    wptr += PPP_HDRLEN - 1;

    ilen = len;
    oldcode = CLEAR;
    explen = 0;
    for (;;) {
        if (len == 0) {
            cmp = cmp->m_next;
            if (!cmp)           /* quit at end of message */
                break;
            rptr = mtod(cmp, u_char *);
            len = cmp->m_len;
            ilen += len;
            continue;           /* handle 0-length buffers */
        }

        /*
         * Accumulate bytes until we have a complete code.
         * Then get the next code, relying on the 32-bit,
         * unsigned accm to mask the result.
         */
        bitno -= 8;
        accm |= *rptr++ << bitno;
        --len;
        if (tgtbitno < bitno)
            continue;
        incode = accm >> tgtbitno;
        accm <<= n_bits;
        bitno += n_bits;

        if (incode == CLEAR) {
            /*
             * The dictionary must only be cleared at
             * the end of a packet.  But there could be an
             * empty mbuf at the end.
             */
            if (len > 0 || cmp->m_next != NULL) {
                while ((cmp = cmp->m_next) != NULL)
                    len += cmp->m_len;
                if (len > 0) {
                    m_freem(mret);
                    if (db->debug)
                        printf("bsd_decomp%d: bad CLEAR\n", db->unit);
                    return DECOMP_FATALERROR;   /* probably a bug */
                }
            }
            bsd_clear(db);
            explen = ilen = 0;
            break;
        }

        if (incode > max_ent + 2 || incode > db->maxmaxcode
            || (incode > max_ent && oldcode == CLEAR)) {
            m_freem(mret);
            if (db->debug) {
                printf("bsd_decomp%d: bad code 0x%x oldcode=0x%x ",
                       db->unit, incode, oldcode);
                printf("max_ent=0x%x explen=%d seqno=%d\n",
                       max_ent, explen, db->seqno);
            }
            return DECOMP_FATALERROR;   /* probably a bug */
        }

        /* Special case for KwKwK string. */
        if (incode > max_ent) {
            finchar = oldcode;
            extra = 1;
        } else {
            finchar = incode;
            extra = 0;
        }

        codelen = db->lens[finchar];
        explen += codelen + extra;
        if (explen > db->mru + 1) {
            m_freem(mret);
            if (db->debug) {
                printf("bsd_decomp%d: ran out of mru\n", db->unit);
#ifdef DEBUG
                while ((cmp = cmp->m_next) != NULL)
                    len += cmp->m_len;
                printf("  len=%d, finchar=0x%x, codelen=%d, explen=%d\n",
                       len, finchar, codelen, explen);
#endif
            }
            return DECOMP_FATALERROR;
        }

        /*
         * For simplicity, the decoded characters go in a single mbuf,
         * so we allocate a single extra cluster mbuf if necessary.
         */
        if ((space -= codelen + extra) < 0) {
            dmp->m_len = wptr - mtod(dmp, u_char *);
            MGET(m, M_DONTWAIT, MT_DATA);
            if (m == NULL) {
                m_freem(mret);
                return DECOMP_ERROR;
            }
            m->m_len = 0;
            m->m_next = NULL;
            dmp->m_next = m;
            MCLGET(m, M_DONTWAIT);
            space = M_TRAILINGSPACE(m) - (codelen + extra);
            if (space < 0) {
                /* now that's what I call *compression*. */
                m_freem(mret);
                return DECOMP_ERROR;
            }
            dmp = m;
            wptr = mtod(dmp, u_char *);
        }

        /*
         * Decode this code and install it in the decompressed buffer.
         */
        p = (wptr += codelen);
        while (finchar > LAST) {
            dictp = &db->dict[db->dict[finchar].cptr];
#ifdef DEBUG
            if (--codelen <= 0 || dictp->codem1 != finchar-1)
                goto bad;
#endif
            *--p = dictp->f.hs.suffix;
            finchar = dictp->f.hs.prefix;
        }
        *--p = finchar;

#ifdef DEBUG
        if (--codelen != 0)
            printf("bsd_decomp%d: short by %d after code 0x%x, max_ent=0x%x\n",
                   db->unit, codelen, incode, max_ent);
#endif

        if (extra)              /* the KwKwK case again */
            *wptr++ = finchar;

        /*
         * If not first code in a packet, and
         * if not out of code space, then allocate a new code.
         *
         * Keep the hash table correct so it can be used
         * with uncompressed packets.
         */
        if (oldcode != CLEAR && max_ent < db->maxmaxcode) {
            struct bsd_dict *dictp2;
            u_int32_t fcode;
            u_int32_t hval, disp;

            fcode = BSD_KEY(oldcode,finchar);
            hval = BSD_HASH(oldcode,finchar,db->hshift);
            dictp = &db->dict[hval];

            /* look for a free hash table entry */
            if (dictp->codem1 < max_ent) {
                disp = (hval == 0) ? 1 : hval;
                do {
                    hval += disp;
                    if (hval >= db->hsize)
                        hval -= db->hsize;
                    dictp = &db->dict[hval];
                } while (dictp->codem1 < max_ent);
            }

            /*
             * Invalidate previous hash table entry
             * assigned this code, and then take it over
             */
            dictp2 = &db->dict[max_ent+1];
            if (db->dict[dictp2->cptr].codem1 == max_ent) {
                db->dict[dictp2->cptr].codem1 = BADCODEM1;
            }
            dictp2->cptr = hval;
            dictp->codem1 = max_ent;
            dictp->f.fcode = fcode;

            db->max_ent = ++max_ent;
            db->lens[max_ent] = db->lens[oldcode]+1;

            /* Expand code size if needed. */
            if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode) {
                db->n_bits = ++n_bits;
                tgtbitno = 32-n_bits;
            }
        }
        oldcode = incode;
    }
    dmp->m_len = wptr - mtod(dmp, u_char *);

    /*
     * Keep the checkpoint right so that incompressible packets
     * clear the dictionary at the right times.
     */
    db->bytes_out += ilen;
    db->in_count += explen;
    if (bsd_check(db) && db->debug) {
        printf("bsd_decomp%d: peer should have cleared dictionary\n",
               db->unit);
    }

    ++db->comp_count;
    db->comp_bytes += ilen + BSD_OVHD;
    ++db->uncomp_count;
    db->uncomp_bytes += explen;

    *dmpp = mret;
    return DECOMP_OK;

#ifdef DEBUG
 bad:
    if (codelen <= 0) {
        printf("bsd_decomp%d: fell off end of chain ", db->unit);
        printf("0x%x at 0x%x by 0x%x, max_ent=0x%x\n",
               incode, finchar, db->dict[finchar].cptr, max_ent);
    } else if (dictp->codem1 != finchar-1) {
        printf("bsd_decomp%d: bad code chain 0x%x finchar=0x%x ",
               db->unit, incode, finchar);
        printf("oldcode=0x%x cptr=0x%x codem1=0x%x\n", oldcode,
               db->dict[finchar].cptr, dictp->codem1);
    }
    m_freem(mret);
    return DECOMP_FATALERROR;
#endif /* DEBUG */
}
#endif /* DO_BSD_COMPRESS */