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sfdclzw.c

/***********************************************************************
*                                                                      *
*               This software is part of the ast package               *
*           Copyright (c) 1995-2007 AT&T Knowledge Ventures            *
*                                                                      *
* This software is provided 'as-is', without any express or implied    *
* warranty. In no event will the authors be held liable for any        *
* damages arising from the use of this software.                       *
*                                                                      *
* Permission is granted to anyone to use this software for any         *
* purpose, including commercial applications, and to alter it and      *
* redistribute it freely, subject to the following restrictions:       *
*                                                                      *
*  1. The origin of this software must not be misrepresented;          *
*     you must not claim that you wrote the original software. If      *
*     you use this software in a product, an acknowledgment in the     *
*     product documentation would be appreciated but is not            *
*     required.                                                        *
*                                                                      *
*  2. Altered source versions must be plainly marked as such,          *
*     and must not be misrepresented as being the original             *
*     software.                                                        *
*                                                                      *
*  3. This notice may not be removed or altered from any source        *
*     distribution.                                                    *
*                                                                      *
*              Information and Software Systems Research               *
*                            AT&T Research                             *
*                           Florham Park NJ                            *
*                                                                      *
*                           Jean-loup Gailly                           *
*                              Mark Adler                              *
*                                                                      *
***********************************************************************/
#pragma prototyped

/*
 * compress/zcat discipline snarfed from BSD zopen by
 * Glenn Fowler
 * AT&T Research
 * 1999-06-23
 */

/*-
 * Copyright (c) 1985, 1986, 1992, 1993
 *    The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Diomidis Spinellis and 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. 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.
 */

/*-
 * fcompress.c - File compression ala IEEE Computer, June 1984.
 *
 * Compress authors:
 *          Spencer W. Thomas (decvax!utah-cs!thomas)
 *          Jim McKie         (decvax!mcvax!jim)
 *          Steve Davies            (decvax!vax135!petsd!peora!srd)
 *          Ken Turkowski           (decvax!decwrl!turtlevax!ken)
 *          James A. Woods          (decvax!ihnp4!ames!jaw)
 *          Joe Orost         (decvax!vax135!petsd!joe)
 *
 * Cleaned up and converted to library returning I/O streams by
 * Diomidis Spinellis <dds@doc.ic.ac.uk>.
 */

#include <sfio_t.h>
#include <ast.h>
#include <sfdcgzip.h>

#define     BITS        16          /* Default bits. */
#define     HSIZE       69001       /* 95% occupancy */

/* A code_int must be able to hold 2**BITS values of type int, and also -1. */
typedef long code_int;
typedef long count_int;

typedef u_char char_type;
static char_type magic_header[] =
      { 0x1f, 0x9d };

#define     BIT_MASK    0x1f        /* Defines for third byte of header. */
#define     BLOCK_MASK  0x80

/*
 * Masks 0x40 and 0x20 are free.  I think 0x20 should mean that there is
 * a fourth header byte (for expansion).
 */
#define     INIT_BITS 9             /* Initial number of bits/code. */

#define     MAXCODE(n_bits)   ((1 << (n_bits)) - 1)

typedef struct s_zstate {
      Sfdisc_t disc;                /* sfio discipline */
      enum {
            S_START, S_MIDDLE, S_EOF
      } zs_state;             /* State of computation */
      int zs_n_bits;                /* Number of bits/code. */
      int zs_maxbits;               /* User settable max # bits/code. */
      code_int zs_maxcode;          /* Maximum code, given n_bits. */
      code_int zs_maxmaxcode;       /* Should NEVER generate this code. */
      count_int zs_htab [HSIZE];
      u_short zs_codetab [HSIZE];
      code_int zs_hsize;            /* For dynamic table sizing. */
      code_int zs_free_ent;         /* First unused entry. */
      /*
       * Block compression parameters -- after all codes are used up,
       * and compression rate changes, start over.
       */
      int zs_block_compress;
      int zs_clear_flg;
      long zs_ratio;
      count_int zs_checkpoint;
      int zs_offset;
      long zs_in_count;       /* Length of input. */
      long zs_bytes_out;            /* Length of compressed output. */
      long zs_out_count;            /* # of codes output (for debugging). */
      char_type zs_buf[BITS];
      union {
            struct {
                  long zs_fcode;
                  code_int zs_ent;
                  code_int zs_hsize_reg;
                  int zs_hshift;
            } w;              /* Write paramenters */
            struct {
                  char_type *zs_stackp;
                  int zs_finchar;
                  code_int zs_code, zs_oldcode, zs_incode;
                  int zs_roffset, zs_size;
                  char_type zs_gbuf[BITS];
            } r;              /* Read parameters */
      } u;
} LZW_t;

/* Definitions to retain old variable names */
#define     state       zs->zs_state
#define     n_bits            zs->zs_n_bits
#define     maxbits           zs->zs_maxbits
#define     maxcode           zs->zs_maxcode
#define     maxmaxcode  zs->zs_maxmaxcode
#define     htab        zs->zs_htab
#define     codetab           zs->zs_codetab
#define     hsize       zs->zs_hsize
#define     free_ent    zs->zs_free_ent
#define     block_compress    zs->zs_block_compress
#define     clear_flg   zs->zs_clear_flg
#define     ratio       zs->zs_ratio
#define     checkpoint  zs->zs_checkpoint
#define     offset            zs->zs_offset
#define     in_count    zs->zs_in_count
#define     bytes_out   zs->zs_bytes_out
#define     out_count   zs->zs_out_count
#define     buf         zs->zs_buf
#define     fcode       zs->u.w.zs_fcode
#define     hsize_reg   zs->u.w.zs_hsize_reg
#define     ent         zs->u.w.zs_ent
#define     hshift            zs->u.w.zs_hshift
#define     stackp            zs->u.r.zs_stackp
#define     finchar           zs->u.r.zs_finchar
#define     code        zs->u.r.zs_code
#define     oldcode           zs->u.r.zs_oldcode
#define     incode            zs->u.r.zs_incode
#define     roffset           zs->u.r.zs_roffset
#define     size        zs->u.r.zs_size
#define     gbuf        zs->u.r.zs_gbuf

/*
 * To save much memory, we overlay the table used by compress() with those
 * used by decompress().  The tab_prefix table is the same size and type as
 * the codetab.  The tab_suffix table needs 2**BITS characters.  We get this
 * from the beginning of htab.  The output stack uses the rest of htab, and
 * contains characters.  There is plenty of room for any possible stack
 * (stack used to be 8000 characters).
 */

#define     htabof(i)   htab[i]
#define     codetabof(i)      codetab[i]

#define     tab_prefixof(i)   codetabof(i)
#define     tab_suffixof(i)   ((char_type *)(htab))[i]
#define     de_stack    ((char_type *)&tab_suffixof(1 << BITS))

#define     CHECK_GAP 10000         /* Ratio check interval. */

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

/*-
 * Algorithm from "A Technique for High Performance Data Compression",
 * Terry A. Welch, IEEE Computer Vol 17, No 6 (June 1984), pp 8-19.
 *
 * Algorithm:
 *    Modified Lempel-Ziv method (LZW).  Basically finds common
 * substrings and replaces them with a variable size code.  This is
 * deterministic, and can be done on the fly.  Thus, the decompression
 * procedure needs no input table, but tracks the way the table was built.
 */

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

static char_type lmask[9] =
      {0xff, 0xfe, 0xfc, 0xf8, 0xf0, 0xe0, 0xc0, 0x80, 0x00};
static char_type rmask[9] =
      {0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};

static int
output(LZW_t* zs, Sfio_t* f, code_int ocode, Sfdisc_t* dp)
{
      register int bits, r_off;
      register char_type *bp;

      r_off = offset;
      bits = n_bits;
      bp = buf;
      if (ocode >= 0) {
            /* Get to the first byte. */
            bp += (r_off >> 3);
            r_off &= 7;
            /*
             * Since ocode is always >= 8 bits, only need to mask the first
             * hunk on the left.
             */
            *bp = (*bp & rmask[r_off]) | ((ocode << r_off) & lmask[r_off]);
            bp++;
            bits -= (8 - r_off);
            ocode >>= 8 - r_off;
            /* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
            if (bits >= 8) {
                  *bp++ = ocode;
                  ocode >>= 8;
                  bits -= 8;
            }
            /* Last bits. */
            if (bits)
                  *bp = ocode;
            offset += n_bits;
            if (offset == (n_bits << 3)) {
                  bp = buf;
                  bits = n_bits;
                  bytes_out += bits;
                  if (sfwr(f, bp, bits, dp) != bits)
                        return (-1);
                  bp += bits;
                  bits = 0;
                  offset = 0;
            }
            /*
             * If the next entry is going to be too big for the ocode size,
             * then increase it, if possible.
             */
            if (free_ent > maxcode || (clear_flg > 0)) {
                   /*
                  * Write the whole buffer, because the input side won't
                  * discover the size increase until after it has read it.
                  */
                  if (offset > 0) {
                        if (sfwr(f, buf, n_bits, dp) != n_bits)
                              return (-1);
                        bytes_out += n_bits;
                  }
                  offset = 0;

                  if (clear_flg) {
                        maxcode = MAXCODE(n_bits = INIT_BITS);
                        clear_flg = 0;
                  } else {
                        n_bits++;
                        if (n_bits == maxbits)
                              maxcode = maxmaxcode;
                        else
                              maxcode = MAXCODE(n_bits);
                  }
            }
      } else {
            /* At EOF, write the rest of the buffer. */
            if (offset > 0) {
                  offset = (offset + 7) / 8;
                  if (sfwr(f, buf, offset, dp) != offset)
                        return (-1);
                  bytes_out += offset;
            }
            offset = 0;
      }
      return (0);
}

/*-
 * Read one code from the standard input.  If EOF, return -1.
 * Inputs:
 *    stdin
 * Outputs:
 *    code or -1 is returned.
 */
static code_int
getcode(LZW_t* zs, Sfio_t* f, Sfdisc_t* dp)
{
      register code_int gcode;
      register int r_off, bits;
      register char_type *bp;

      bp = gbuf;
      if (clear_flg > 0 || roffset >= size || free_ent > maxcode) {
            /*
             * If the next entry will be too big for the current gcode
             * size, then we must increase the size.  This implies reading
             * a new buffer full, too.
             */
            if (free_ent > maxcode) {
                  n_bits++;
                  if (n_bits == maxbits)  /* Won't get any bigger now. */
                        maxcode = maxmaxcode;
                  else
                        maxcode = MAXCODE(n_bits);
            }
            if (clear_flg > 0) {
                  maxcode = MAXCODE(n_bits = INIT_BITS);
                  clear_flg = 0;
            }
            size = sfrd(f, gbuf, n_bits, dp);
            if (size <= 0)                /* End of file. */
                  return (-1);
            roffset = 0;
            /* Round size down to integral number of codes. */
            size = (size << 3) - (n_bits - 1);
      }
      r_off = roffset;
      bits = n_bits;

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

      /* Get first part (low order bits). */
      gcode = (*bp++ >> r_off);
      bits -= (8 - r_off);
      r_off = 8 - r_off;      /* Now, roffset into gcode word. */

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

      /* High order bits. */
      gcode |= (*bp & rmask[bits]) << r_off;
      roffset += n_bits;

      return (gcode);
}

static void
cl_hash(LZW_t* zs, register count_int cl_hsize)       /* Reset code table. */
{
      register count_int *htab_p;
      register long i, m1;

      m1 = -1;
      htab_p = htab + cl_hsize;
      i = cl_hsize - 16;
      do {              /* Might use Sys V memset(3) here. */
            *(htab_p - 16) = m1;
            *(htab_p - 15) = m1;
            *(htab_p - 14) = m1;
            *(htab_p - 13) = m1;
            *(htab_p - 12) = m1;
            *(htab_p - 11) = m1;
            *(htab_p - 10) = m1;
            *(htab_p - 9) = m1;
            *(htab_p - 8) = m1;
            *(htab_p - 7) = m1;
            *(htab_p - 6) = m1;
            *(htab_p - 5) = m1;
            *(htab_p - 4) = m1;
            *(htab_p - 3) = m1;
            *(htab_p - 2) = m1;
            *(htab_p - 1) = m1;
            htab_p -= 16;
      } while ((i -= 16) >= 0);
      for (i += 16; i > 0; i--)
            *--htab_p = m1;
}

static int
cl_block(LZW_t* zs, Sfio_t* f, Sfdisc_t* dp)/* Table clear for block compress. */
{
      register long rat;

      checkpoint = in_count + CHECK_GAP;

      if (in_count > 0x007fffff) {  /* Shift will overflow. */
            rat = bytes_out >> 8;
            if (rat == 0)           /* Don't divide by zero. */
                  rat = 0x7fffffff;
            else
                  rat = in_count / rat;
      } else
            rat = (in_count << 8) / bytes_out;  /* 8 fractional bits. */
      if (rat > ratio)
            ratio = rat;
      else {
            ratio = 0;
            cl_hash(zs, (count_int) hsize);
            free_ent = FIRST;
            clear_flg = 1;
            if (output(zs, f, (code_int) CLEAR, dp) == -1)
                  return (-1);
      }
      return (0);
}

/*
 * lzw sync/flush/getpos
 * only sync is implemented right now
 * see sfdcgzip() for full implementation
 */

static Sfoff_t
lzw_sync(LZW_t* zs, Sfio_t* f, Sfoff_t off, Sfdisc_t* dp)
{
#if 1
      static int  nosync;

      if (off == -1)
      {
            if (!nosync)
                  nosync = getenv("SFDCLZW_nosync") ? 1 : -1;
            if (nosync > 0)
                  return 0;
      }
#endif
      if (zs->disc.writef && cl_block(zs, f, dp))
            return -1;
      if (off == -1)
            return 0;
      return -1;
}

/*
 * lzw exception handler
 * free on close
 */

static int
lzw_except(Sfio_t* f, int op, void* val, Sfdisc_t* dp)
{
      register LZW_t*   zs = (LZW_t*)dp;
      int         flags;
      int         r;

      NoP(f);
      switch (op)
      {
      case SF_ATEXIT:
            sfdisc(f, SF_POPDISC);
            return 0;
      case SF_CLOSING:
      case SF_DPOP:
      case SF_FINAL:
            r = 0;
            if (dp->writef)
            {
                  SFDCNEXT(f, flags);
                  if (output(zs, f, (code_int) ent, dp) == -1)
                        r = -1;
                  else
                  {
                        out_count++;
                        if (output(zs, f, (code_int) -1, dp) == -1)
                              r = -1;
                  }
                  SFDCPREV(f, flags);
            }
            if (op != SF_CLOSING)
                  free(dp);
            return r;
      case SF_DBUFFER:
            return 1;
      case SF_READ:
      case SF_WRITE:
            return *((ssize_t*)val) < 0 ? -1 : 0;
      case SF_SYNC:
            return val ? 0 : lzw_sync(zs, f, -1, dp) == -1 ? -1 : 0;
      case SFGZ_HANDLE:
            return (*((LZW_t**)val) = zs) ? 1 : -1;
      case SFGZ_GETPOS:
            return (*((Sfoff_t*)val) = lzw_sync(zs, f, -1, dp)) == -1 ? -1 : 0;
      case SFGZ_SETPOS:
            return lzw_sync(zs, f, *((Sfoff_t*)val), dp) == -1 ? -1 : 0;
      }
      return 0;
}

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

static ssize_t
lzw_write(Sfio_t* f, const Void_t* wbp, size_t num, Sfdisc_t* dp)
{
      register code_int i;
      register int c, disp;
      LZW_t *zs;
      const u_char *bp;
      u_char tmp;
      int count;

      if (num == 0)
            return (0);

      zs = (LZW_t*)dp;
      count = num;
      bp = (u_char *)wbp;
      if (state == S_MIDDLE)
            goto middle;
      state = S_MIDDLE;

      maxmaxcode = 1L << maxbits;
      if (sfwr(f, magic_header, sizeof(magic_header), dp) != sizeof(magic_header))
            return (-1);
      tmp = (u_char)((maxbits) | block_compress);
      if (sfwr(f, &tmp, sizeof(tmp), dp) != sizeof(tmp))
            return (-1);

      offset = 0;
      bytes_out = 3;          /* Includes 3-byte header mojo. */
      out_count = 0;
      clear_flg = 0;
      ratio = 0;
      in_count = 1;
      checkpoint = CHECK_GAP;
      maxcode = MAXCODE(n_bits = INIT_BITS);
      free_ent = ((block_compress) ? FIRST : 256);

      ent = *bp++;
      --count;

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

      hsize_reg = hsize;
      cl_hash(zs, (count_int)hsize_reg);  /* Clear hash table. */

middle:     for (i = 0; count--;) {
            c = *bp++;
            in_count++;
            fcode = (long)(((long)c << maxbits) + ent);
            i = ((c << hshift) ^ ent);    /* Xor hashing. */

            if (htabof(i) == fcode) {
                  ent = codetabof(i);
                  continue;
            } else if ((long)htabof(i) < 0)     /* Empty slot. */
                  goto nomatch;
            disp = hsize_reg - i;   /* Secondary hash (after G. Knott). */
            if (i == 0)
                  disp = 1;
probe:            if ((i -= disp) < 0)
                  i += hsize_reg;

            if (htabof(i) == fcode) {
                  ent = codetabof(i);
                  continue;
            }
            if ((long)htabof(i) >= 0)
                  goto probe;
nomatch:    if (output(zs, f, (code_int) ent, dp) == -1)
                  return (-1);
            out_count++;
            ent = c;
            if (free_ent < maxmaxcode) {
                  codetabof(i) = free_ent++;    /* code -> hashtable */
                  htabof(i) = fcode;
            } else if ((count_int)in_count >=
                checkpoint && block_compress) {
                  if (cl_block(zs, f, dp) == -1)
                        return (-1);
            }
      }
      return (num);
}

/*
 * Decompress read.  This routine adapts to the codes in the file building
 * the "string" table on-the-fly; requiring no table to be stored in the
 * compressed file.  The tables used herein are shared with those of the
 * compress() routine.  See the definitions above.
 */
static ssize_t
lzw_read(Sfio_t* f, Void_t* rbp, size_t num, Sfdisc_t* dp)
{
      register u_int count;
      LZW_t *zs;
      u_char *bp, header[3];

      if (num == 0)
            return (0);

      zs = (LZW_t*)dp;
      count = num;
      bp = (u_char *)rbp;
      switch (state) {
      case S_START:
            state = S_MIDDLE;
            break;
      case S_MIDDLE:
            goto middle;
      case S_EOF:
            goto eof;
      }

      /* Check the magic number */
      if (sfrd(f, header, sizeof(header), dp) != sizeof(header) ||
          memcmp(header, magic_header, sizeof(magic_header)) != 0)
            return (-1);
      maxbits = header[2];    /* Set -b from file. */
      block_compress = maxbits & BLOCK_MASK;
      maxbits &= BIT_MASK;
      maxmaxcode = 1L << maxbits;
      if (maxbits > BITS)
            return (-1);
      /* As above, initialize the first 256 entries in the table. */
      maxcode = MAXCODE(n_bits = INIT_BITS);
      for (code = 255; code >= 0; code--) {
            tab_prefixof(code) = 0;
            tab_suffixof(code) = (char_type) code;
      }
      free_ent = block_compress ? FIRST : 256;

      finchar = oldcode = getcode(zs, f, dp);
      if (oldcode == -1)      /* EOF already? */
            return (0); /* Get out of here */

      /* First code must be 8 bits = char. */
      *bp++ = (u_char)finchar;
      count--;
      stackp = de_stack;

      while ((code = getcode(zs, f, dp)) > -1) {

            if ((code == CLEAR) && block_compress) {
                  for (code = 255; code >= 0; code--)
                        tab_prefixof(code) = 0;
                  clear_flg = 1;
                  free_ent = FIRST - 1;
                  if ((code = getcode(zs, f, dp)) == -1)    /* O, untimely death! */
                        break;
            }
            incode = code;

            /* Special case for KwKwK string. */
            if (code >= free_ent) {
                  *stackp++ = finchar;
                  code = oldcode;
            }

            /* Generate output characters in reverse order. */
            while (code >= 256) {
                  *stackp++ = tab_suffixof(code);
                  code = tab_prefixof(code);
            }
            *stackp++ = finchar = tab_suffixof(code);

            /* And put them out in forward order.  */
middle:           do {
                  if (count-- == 0)
                        return (num);
                  *bp++ = *--stackp;
            } while (stackp > de_stack);

            /* Generate the new entry. */
            if ((code = free_ent) < maxmaxcode) {
                  tab_prefixof(code) = (u_short) oldcode;
                  tab_suffixof(code) = finchar;
                  free_ent = code + 1;
            }

            /* Remember previous code. */
            oldcode = incode;
      }
      state = S_EOF;
eof:  return (num - count);
}

/*
 * create and push the sfio lzw discipline
 * for SF_READ streams only
 *
 * (flags&SFGZ_VERIFY) return
 *    >0    is an lzw file
 *     0    not an lzw file
 *    <0    error
 * otherwise return
 *    >0    discipline pushed (gzip or lzw)
 *     0    discipline not needed
 *    <0    error
 */

int
sfdclzw(Sfio_t* f, int flags)
{
      LZW_t*      zs;

      if (sfset(f, 0, 0) & SF_READ)
      {
            register unsigned char* s;
            register int            n;

            /*
             * peek the first 2 bytes to verify the magic
             *
             *    0x1f8b      sfdcgzip    gzip  
             *    0x1f9d      sfdclzw           compress
             */
            
            if (!(n = sfset(f, 0, 0) & SF_SHARE))
                  sfset(f, SF_SHARE, 1);
            s = (unsigned char*)sfreserve(f, 2, 1);
            if (!n)
                  sfset(f, SF_SHARE, 0);
            if (!s)
                  return -1;
            if (*s != 0x1f)
                  n = -1;
            else if (*(s + 1) == 0x8b)
                  n = 1;
            else if (*(s + 1) == 0x9d)
                  n = 0;
            else
                  n = -1;
            sfread(f, s, 0);
            if (n < 0)
                  return 0;
            if (flags & SFGZ_VERIFY)
                  return n >= 0;
            if (n > 0)
                  return sfdcgzip(f, flags);
      }
      if (!(zs = newof(0, LZW_t, 1, 0)))
            return -1;
      zs->disc.exceptf = lzw_except;
      if (sfset(f, 0, 0) & SF_READ)
            zs->disc.readf = lzw_read;
      else
            zs->disc.writef = lzw_write;

      maxbits = BITS;               /* fixed max # bits/code. */
      maxmaxcode = 1L << maxbits;   /* Should NEVER generate this code. */
      hsize = HSIZE;                /* For dynamic table sizing. */
      free_ent = 0;                 /* First unused entry. */
      block_compress = BLOCK_MASK;
      clear_flg = 0;
      ratio = 0;
      checkpoint = CHECK_GAP;
      in_count = 1;                 /* Length of input. */
      out_count = 0;                /* # of codes output (for debugging). */
      state = S_START;
      roffset = 0;
      size = 0;

      sfset(f, SF_SHARE|SF_PUBLIC, 0);
      if (sfdisc(f, &zs->disc) != &zs->disc)
      {     
            free(zs);
            return -1;
      }
      return 1;
}

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