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bsd_comp.c

/*
 * Update: The Berkeley copyright was changed, and the change
 * is retroactive to all "true" BSD software (ie everything
 * from UCB as opposed to other peoples code that just carried
 * the same license). The new copyright doesn't clash with the
 * GPL, so the module-only restriction has been removed..
 */

/* 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 contiguous buffers on Linux-derived systems.
 *
 *  ==FILEVERSION 20000226==
 *
 *  NOTE TO MAINTAINERS:
 *     If you modify this file at all, please set the number above to the
 *     date of the modification as YYMMDD (year month day).
 *     bsd_comp.c is shipped with a PPP distribution as well as with
 *     the kernel; if everyone increases the FILEVERSION number above,
 *     then scripts can do the right thing when deciding whether to
 *     install a new bsd_comp.c file. Don't change the format of that
 *     line otherwise, so the installation script can recognize it.
 *
 * From: bsd_comp.c,v 1.3 1994/12/08 01:59:58 paulus Exp
 */

#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/string.h>

#include <linux/ppp_defs.h>

#undef   PACKETPTR
#define  PACKETPTR 1
#include <linux/ppp-comp.h>
#undef   PACKETPTR

#include <asm/byteorder.h>

/*
 * 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.
 */

/*
 * Macros to extract protocol version and number of bits
 * from the third byte of the BSD Compress CCP configuration option.
 */

#define BSD_VERSION(x)  ((x) >> 5)
#define BSD_NBITS(x)    ((x) & 0x1F)

#define BSD_CURRENT_VERSION   1

/*
 * A dictionary for doing BSD compress.
 */

struct bsd_dict {
    union {                   /* hash value */
      unsigned long     fcode;
      struct {
#if defined(__LITTLE_ENDIAN)        /* Little endian order */
          unsigned short      prefix;     /* preceding code */
          unsigned char suffix; /* last character of new code */
          unsigned char pad;
#elif defined(__BIG_ENDIAN)         /* Big endian order */
          unsigned char pad;
          unsigned char suffix; /* last character of new code */
          unsigned short      prefix; /* preceding code */
#else
#error Endianness not defined...
#endif
      } hs;
    } f;
    unsigned short codem1;          /* output of hash table -1 */
    unsigned short cptr;            /* map code to hash table entry */
};

struct bsd_db {
    int         totlen;             /* length of this structure */
    unsigned int   hsize;           /* size of the hash table */
    unsigned char  hshift;          /* used in hash function */
    unsigned char  n_bits;          /* current bits/code */
    unsigned char  maxbits;         /* maximum bits/code */
    unsigned char  debug;           /* non-zero if debug desired */
    unsigned char  unit;            /* ppp unit number */
    unsigned short seqno;           /* sequence # of next packet */
    unsigned int   mru;             /* size of receive (decompress) bufr */
    unsigned int   maxmaxcode;            /* largest valid code */
    unsigned int   max_ent;         /* largest code in use */
    unsigned int   in_count;        /* uncompressed bytes, aged */
    unsigned int   bytes_out;       /* compressed bytes, aged */
    unsigned int   ratio;           /* recent compression ratio */
    unsigned int   checkpoint;            /* when to next check the ratio */
    unsigned int   clear_count;           /* times dictionary cleared */
    unsigned int   incomp_count;    /* incompressible packets */
    unsigned int   incomp_bytes;    /* incompressible bytes */
    unsigned int   uncomp_count;    /* uncompressed packets */
    unsigned int   uncomp_bytes;    /* uncompressed bytes */
    unsigned int   comp_count;            /* compressed packets */
    unsigned int   comp_bytes;            /* compressed bytes */
    unsigned short  *lens;          /* array of lengths of codes */
    struct bsd_dict *dict;          /* dictionary */
};

#define BSD_OVHD  2           /* BSD compress overhead/packet */
#define MIN_BSD_BITS    9
#define BSD_INIT_BITS   MIN_BSD_BITS
#define MAX_BSD_BITS    15

static void bsd_free (void *state);
static void *bsd_alloc(unsigned char *options, int opt_len, int decomp);
static void *bsd_comp_alloc (unsigned char *options, int opt_len);
static void *bsd_decomp_alloc (unsigned char *options, int opt_len);

static int  bsd_init        (void *db, unsigned char *options,
                           int opt_len, int unit, int debug, int decomp);
static int  bsd_comp_init   (void *state, unsigned char *options,
                           int opt_len, int unit, int opthdr, int debug);
static int  bsd_decomp_init (void *state, unsigned char *options,
                         int opt_len, int unit, int opthdr, int mru,
                         int debug);

static void bsd_reset (void *state);
static void bsd_comp_stats (void *state, struct compstat *stats);

static int  bsd_compress (void *state, unsigned char *rptr,
                        unsigned char *obuf, int isize, int osize);
static void bsd_incomp (void *state, unsigned char *ibuf, int icnt);

static int  bsd_decompress (void *state, unsigned char *ibuf, int isize,
                        unsigned char *obuf, int osize);

/* These are in ppp_generic.c */
extern int  ppp_register_compressor   (struct compressor *cp);
extern void ppp_unregister_compressor (struct compressor *cp);

/*
 * 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(MAX_BSD_BITS);

#define BSD_HASH(prefix,suffix,hshift) ((((unsigned long)(suffix))<<(hshift)) \
                               ^ (unsigned long)(prefix))
#define BSD_KEY(prefix,suffix)            ((((unsigned long)(suffix)) << 16) \
                               + (unsigned long)(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(struct bsd_db *db)
{
    db->clear_count++;
    db->max_ent      = FIRST-1;
    db->n_bits       = BSD_INIT_BITS;
    db->bytes_out    = 0;
    db->in_count     = 0;
    db->ratio          = 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 bsd_check (struct bsd_db *db)  /* 1=output CLEAR */
  {
    unsigned 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  >> 2);
          db->bytes_out -= (db->bytes_out >> 2);
        }

      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 (void *state, struct compstat *stats)
  {
    struct bsd_db *db = (struct bsd_db *) state;

    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->in_count     = db->in_count;
    stats->bytes_out    = db->bytes_out;
  }

/*
 * Reset state, as on a CCP ResetReq.
 */

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

    bsd_clear(db);

    db->seqno       = 0;
    db->clear_count = 0;
  }

/*
 * Release the compression structure
 */

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

      if (!db)
            return;

/*
 * Release the dictionary
 */
      vfree(db->dict);
      db->dict = NULL;
/*
 * Release the string buffer
 */
      vfree(db->lens);
      db->lens = NULL;
/*
 * Finally release the structure itself.
 */
      kfree(db);
}

/*
 * Allocate space for a (de) compressor.
 */

static void *bsd_alloc (unsigned char *options, int opt_len, int decomp)
  {
    int bits;
    unsigned int 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;
      }
/*
 * Allocate the main control structure for this instance.
 */
    maxmaxcode = MAXCODE(bits);
    db         = kzalloc(sizeof (struct bsd_db),
                                  GFP_KERNEL);
    if (!db)
      {
      return NULL;
      }

/*
 * Allocate space for the dictionary. This may be more than one page in
 * length.
 */
    db->dict = vmalloc(hsize * sizeof(struct bsd_dict));
    if (!db->dict)
      {
      bsd_free (db);
      return NULL;
      }

/*
 * If this is the compression buffer then there is no length data.
 */
    if (!decomp)
      {
      db->lens = NULL;
      }
/*
 * For decompression, the length information is needed as well.
 */
    else
      {
        db->lens = vmalloc((maxmaxcode + 1) * sizeof(db->lens[0]));
      if (!db->lens)
        {
          bsd_free (db);
          return (NULL);
        }
      }
/*
 * Initialize the data information for the compression code
 */
    db->totlen     = sizeof (struct bsd_db)   +
                      (sizeof (struct bsd_dict) * hsize);

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

    return (void *) db;
  }

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

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

/*
 * Initialize the database.
 */

static int bsd_init (void *state, unsigned char *options,
                 int opt_len, int unit, int debug, int decomp)
  {
    struct bsd_db *db = state;
    int indx;

    if ((opt_len != 3) || (options[0] != CI_BSD_COMPRESS) || (options[1] != 3)
      || (BSD_VERSION(options[2]) != BSD_CURRENT_VERSION)
      || (BSD_NBITS(options[2]) != db->maxbits)
      || (decomp && db->lens == NULL))
      {
      return 0;
      }

    if (decomp)
      {
      indx = LAST;
      do
        {
          db->lens[indx] = 1;
        }
      while (indx-- > 0);
      }

    indx = db->hsize;
    while (indx-- != 0)
      {
      db->dict[indx].codem1 = BADCODEM1;
      db->dict[indx].cptr   = 0;
      }

    db->unit = unit;
    db->mru  = 0;
#ifndef DEBUG
    if (debug)
#endif
      db->debug = 1;

    bsd_reset(db);

    return 1;
  }

static int bsd_comp_init (void *state, unsigned char *options,
                    int opt_len, int unit, int opthdr, int debug)
  {
    return bsd_init (state, options, opt_len, unit, debug, 0);
  }

static int bsd_decomp_init (void *state, unsigned char *options,
                      int opt_len, int unit, int opthdr, int mru,
                      int debug)
  {
    return bsd_init (state, options, opt_len, unit, debug, 1);
  }

/*
 * Obtain pointers to the various structures in the compression tables
 */

#define dict_ptrx(p,idx) &(p->dict[idx])
#define lens_ptrx(p,idx) &(p->lens[idx])

#ifdef DEBUG
static unsigned short *lens_ptr(struct bsd_db *db, int idx)
  {
    if ((unsigned int) idx > (unsigned int) db->maxmaxcode)
      {
      printk ("<9>ppp: lens_ptr(%d) > max\n", idx);
      idx = 0;
      }
    return lens_ptrx (db, idx);
  }

static struct bsd_dict *dict_ptr(struct bsd_db *db, int idx)
  {
    if ((unsigned int) idx >= (unsigned int) db->hsize)
      {
      printk ("<9>ppp: dict_ptr(%d) > max\n", idx);
      idx = 0;
      }
    return dict_ptrx (db, idx);
  }

#else
#define lens_ptr(db,idx) lens_ptrx(db,idx)
#define dict_ptr(db,idx) dict_ptrx(db,idx)
#endif

/*
 * compress a packet
 *
 *    The result of this function is the size of the compressed
 *    packet. A zero is returned if the packet was not compressed
 *    for some reason, such as the size being larger than uncompressed.
 *
 *    One change from the BSD compress command is that when the
 *    code size expands, we do not output a bunch of padding.
 */

static int bsd_compress (void *state, unsigned char *rptr, unsigned char *obuf,
                   int isize, int osize)
  {
    struct bsd_db *db;
    int hshift;
    unsigned int max_ent;
    unsigned int n_bits;
    unsigned int bitno;
    unsigned long accm;
    int ent;
    unsigned long fcode;
    struct bsd_dict *dictp;
    unsigned char c;
    int hval;
    int disp;
    int ilen;
    int mxcode;
    unsigned char *wptr;
    int olen;

#define PUTBYTE(v)                  \
  {                           \
    ++olen;                   \
    if (wptr)                       \
      {                             \
      *wptr++ = (unsigned char) (v);      \
      if (olen >= osize)            \
        {                     \
          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.
   */

    ent = PPP_PROTOCOL(rptr);
    if (ent < 0x21 || ent > 0xf9)
      {
      return 0;
      }

    db      = (struct bsd_db *) state;
    hshift  = db->hshift;
    max_ent = db->max_ent;
    n_bits  = db->n_bits;
    bitno   = 32;
    accm    = 0;
    mxcode  = MAXCODE (n_bits);

    /* Initialize the output pointers */
    wptr  = obuf;
    olen  = PPP_HDRLEN + BSD_OVHD;

    if (osize > isize)
      {
      osize = isize;
      }

    /* This is the PPP header information */
    if (wptr)
      {
      *wptr++ = PPP_ADDRESS(rptr);
      *wptr++ = PPP_CONTROL(rptr);
      *wptr++ = 0;
      *wptr++ = PPP_COMP;
      *wptr++ = db->seqno >> 8;
      *wptr++ = db->seqno;
      }

    /* Skip the input header */
    rptr  += PPP_HDRLEN;
    isize -= PPP_HDRLEN;
    ilen   = ++isize;   /* Low byte of protocol is counted as input */

    while (--ilen > 0)
      {
      c     = *rptr++;
      fcode = BSD_KEY  (ent, c);
      hval  = BSD_HASH (ent, c, hshift);
      dictp = dict_ptr (db, 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 = dict_ptr (db, 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;
          struct bsd_dict *dictp3;
          int    indx;

          /* expand code size if needed */
          if (max_ent >= mxcode)
            {
            db->n_bits = ++n_bits;
            mxcode     = MAXCODE (n_bits);
            }

          /* Invalidate old hash table entry using
           * this code, and then take it over.
           */

          dictp2 = dict_ptr (db, max_ent + 1);
          indx   = dictp2->cptr;
          dictp3 = dict_ptr (db, indx);

          if (dictp3->codem1 == max_ent)
            {
            dictp3->codem1 = BADCODEM1;
            }

          dictp2->cptr   = hval;
          dictp->codem1  = max_ent;
          dictp->f.fcode = fcode;
          db->max_ent    = ++max_ent;

          if (db->lens)
            {
            unsigned short *len1 = lens_ptr (db, max_ent);
            unsigned short *len2 = lens_ptr (db, ent);
            *len1 = *len2 + 1;
            }
        }
      ent = c;
      }

    OUTPUT(ent);        /* output the last code */

    db->bytes_out    += olen - PPP_HDRLEN - BSD_OVHD;
    db->uncomp_bytes += isize;
    db->in_count     += isize;
    ++db->uncomp_count;
    ++db->seqno;

    if (bitno < 32)
      {
      ++db->bytes_out; /* must be set before calling bsd_check */
      }

    /*
     * Generate the clear command if needed
     */

    if (bsd_check(db))
      {
      OUTPUT (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);
      }

    /*
     * Increase code size if we would have without the packet
     * boundary because the decompressor will do so.
     */

    if (max_ent >= mxcode && max_ent < db->maxmaxcode)
      {
      db->n_bits++;
      }

    /* If output length is too large then this is an incomplete frame. */
    if (wptr == NULL)
      {
      ++db->incomp_count;
      db->incomp_bytes += isize;
      olen              = 0;
      }
    else /* Count the number of compressed frames */
      {
      ++db->comp_count;
      db->comp_bytes += olen;
      }

    /* Return the resulting output length */
    return olen;
#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 (void *state, unsigned char *ibuf, int icnt)
  {
    (void) bsd_compress (state, ibuf, (char *) 0, icnt, 0);
  }

/*
 * 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.
 */

static int bsd_decompress (void *state, unsigned char *ibuf, int isize,
                     unsigned char *obuf, int osize)
  {
    struct bsd_db *db;
    unsigned int max_ent;
    unsigned long accm;
    unsigned int bitno;       /* 1st valid bit in accm */
    unsigned int n_bits;
    unsigned int tgtbitno;    /* bitno when we have a code */
    struct bsd_dict *dictp;
    int explen;
    int seq;
    unsigned int incode;
    unsigned int oldcode;
    unsigned int finchar;
    unsigned char *p;
    unsigned char *wptr;
    int adrs;
    int ctrl;
    int ilen;
    int codelen;
    int extra;

    db       = (struct bsd_db *) state;
    max_ent  = db->max_ent;
    accm     = 0;
    bitno    = 32;            /* 1st valid bit in accm */
    n_bits   = db->n_bits;
    tgtbitno = 32 - n_bits;   /* bitno when we have a code */

    /*
     * Save the address/control from the PPP header
     * and then get the sequence number.
     */

    adrs  = PPP_ADDRESS (ibuf);
    ctrl  = PPP_CONTROL (ibuf);

    seq   = (ibuf[4] << 8) + ibuf[5];

    ibuf += (PPP_HDRLEN + 2);
    ilen  = isize - (PPP_HDRLEN + 2);

    /*
     * Check the sequence number and give up if it differs from
     * the value we're expecting.
     */

    if (seq != db->seqno)
      {
      if (db->debug)
        {
          printk("bsd_decomp%d: bad sequence # %d, expected %d\n",
               db->unit, seq, db->seqno - 1);
        }
      return DECOMP_ERROR;
      }

    ++db->seqno;
    db->bytes_out += ilen;

    /*
     * Fill in the ppp header, but not the last byte of the protocol
     * (that comes from the decompressed data).
     */

    wptr    = obuf;
    *wptr++ = adrs;
    *wptr++ = ctrl;
    *wptr++ = 0;

    oldcode = CLEAR;
    explen  = 3;

    /*
     * Keep the checkpoint correctly so that incompressible packets
     * clear the dictionary at the proper times.
     */

    for (;;)
      {
      if (ilen-- <= 0)
        {
          db->in_count += (explen - 3); /* don't count the header */
          break;
        }

      /*
       * 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  |= *ibuf++ << bitno;
      if (tgtbitno < bitno)
        {
          continue;
        }

      incode = accm >> tgtbitno;
      accm <<= n_bits;
      bitno += n_bits;

      /*
       * The dictionary must only be cleared at the end of a packet.
       */

      if (incode == CLEAR)
        {
          if (ilen > 0)
            {
            if (db->debug)
              {
                printk("bsd_decomp%d: bad CLEAR\n", db->unit);
              }
            return DECOMP_FATALERROR;     /* probably a bug */
            }

          bsd_clear(db);
          break;
        }

      if ((incode > max_ent + 2) || (incode > db->maxmaxcode)
          || (incode > max_ent && oldcode == CLEAR))
        {
          if (db->debug)
            {
            printk("bsd_decomp%d: bad code 0x%x oldcode=0x%x ",
                   db->unit, incode, oldcode);
            printk("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 = *(lens_ptr (db, finchar));
      explen += codelen + extra;
      if (explen > osize)
        {
          if (db->debug)
            {
            printk("bsd_decomp%d: ran out of mru\n", db->unit);
#ifdef DEBUG
            printk("  len=%d, finchar=0x%x, codelen=%d, explen=%d\n",
                   ilen, finchar, codelen, explen);
#endif
            }
          return DECOMP_FATALERROR;
        }

      /*
       * Decode this code and install it in the decompressed buffer.
       */

      wptr += codelen;
      p     = wptr;
      while (finchar > LAST)
        {
          struct bsd_dict *dictp2 = dict_ptr (db, finchar);

          dictp = dict_ptr (db, dictp2->cptr);
#ifdef DEBUG
          if (--codelen <= 0 || dictp->codem1 != finchar-1)
            {
            if (codelen <= 0)
              {
                printk("bsd_decomp%d: fell off end of chain ", db->unit);
                printk("0x%x at 0x%x by 0x%x, max_ent=0x%x\n",
                     incode, finchar, dictp2->cptr, max_ent);
              }
            else
              {
                if (dictp->codem1 != finchar-1)
                  {
                  printk("bsd_decomp%d: bad code chain 0x%x "
                         "finchar=0x%x ",
                         db->unit, incode, finchar);

                  printk("oldcode=0x%x cptr=0x%x codem1=0x%x\n",
                         oldcode, dictp2->cptr, dictp->codem1);
                  }
              }
            return DECOMP_FATALERROR;
            }
#endif
          *--p    = dictp->f.hs.suffix;
          finchar = dictp->f.hs.prefix;
        }
      *--p = finchar;

#ifdef DEBUG
      if (--codelen != 0)
        {
          printk("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, *dictp3;
          unsigned short  *lens1,  *lens2;
          unsigned long fcode;
          int hval, disp, indx;

          fcode = BSD_KEY(oldcode,finchar);
          hval  = BSD_HASH(oldcode,finchar,db->hshift);
          dictp = dict_ptr (db, 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 = dict_ptr (db, hval);
              }
            while (dictp->codem1 < max_ent);
            }

          /*
           * Invalidate previous hash table entry
           * assigned this code, and then take it over
           */

          dictp2 = dict_ptr (db, max_ent + 1);
          indx   = dictp2->cptr;
          dictp3 = dict_ptr (db, indx);

          if (dictp3->codem1 == max_ent)
            {
            dictp3->codem1 = BADCODEM1;
            }

          dictp2->cptr   = hval;
          dictp->codem1  = max_ent;
          dictp->f.fcode = fcode;
          db->max_ent    = ++max_ent;

          /* Update the length of this string. */
          lens1  = lens_ptr (db, max_ent);
          lens2  = lens_ptr (db, oldcode);
          *lens1 = *lens2 + 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;
      }

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

    if (bsd_check(db))
      {
      if (db->debug)
        {
          printk("bsd_decomp%d: peer should have cleared dictionary on %d\n",
               db->unit, db->seqno - 1);
        }
      }
    return explen;
  }

/*************************************************************
 * Table of addresses for the BSD compression module
 *************************************************************/

static struct compressor ppp_bsd_compress = {
      .compress_proto = CI_BSD_COMPRESS,
      .comp_alloc =           bsd_comp_alloc,
      .comp_free =            bsd_free,
      .comp_init =            bsd_comp_init,
      .comp_reset =           bsd_reset,
      .compress =       bsd_compress,
      .comp_stat =            bsd_comp_stats,
      .decomp_alloc =         bsd_decomp_alloc,
      .decomp_free =          bsd_free,
      .decomp_init =          bsd_decomp_init,
      .decomp_reset =         bsd_reset,
      .decompress =           bsd_decompress,
      .incomp =         bsd_incomp,
      .decomp_stat =          bsd_comp_stats,
      .owner =          THIS_MODULE
};

/*************************************************************
 * Module support routines
 *************************************************************/

static int __init bsdcomp_init(void)
{
      int answer = ppp_register_compressor(&ppp_bsd_compress);
      if (answer == 0)
            printk(KERN_INFO "PPP BSD Compression module registered\n");
      return answer;
}

static void __exit bsdcomp_cleanup(void)
{
      ppp_unregister_compressor(&ppp_bsd_compress);
}

module_init(bsdcomp_init);
module_exit(bsdcomp_cleanup);
MODULE_LICENSE("Dual BSD/GPL");
MODULE_ALIAS("ppp-compress-" __stringify(CI_BSD_COMPRESS));

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