/*
   * 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
  ", 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
  ", 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
  ",
  		   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
  ", 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
  ",
  		       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
  ", db->unit);
  #ifdef DEBUG
  		printk("  len=%d, finchar=0x%x, codelen=%d, explen=%d
  ",
  		       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
  ",
  			   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
  ",
  			       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
  ",
  		   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
  ",
  		   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
  ");
  	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));