lib/gifdecomp.c

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/* decode.c - An LZW decoder for GIF
 * Copyright (C) 1987, by Steven A. Bennett
 *
 * Permission is given by the author to freely redistribute and include
 * this code in any program as long as this credit is given where due.
 *
 * In accordance with the above, I want to credit Steve Wilhite who wrote
 * the code which this is heavily inspired by...
 *
 * GIF and 'Graphics Interchange Format' are trademarks (tm) of
 * Compuserve, Incorporated, an H&R Block Company.
 *
 * Release Notes: This file contains a decoder routine for GIF images
 * which is similar, structurally, to the original routine by Steve Wilhite.
 * It is, however, somewhat noticably faster in most cases.
 *
 */

#include "common.h"
#include "gifcodes.h"

static char const rcsid[] = "$Id: gifdecomp.c,v 1.3 2003/05/06 07:33:42 kate Exp $";


/* extern int gif_get_byte()
 *
 *   - This external (machine specific) function is expected to return
 * either the next byte from the GIF file, or a negative number, as
 * defined in gifcodes.h.
 */
extern int gif_get_byte();

/* extern int gif_out_line(pixels, linelen)
 *     UBYTE pixels[];
 *     int linelen;
 *
 *   - This function takes a full line of pixels (one byte per pixel) and
 * displays them (or does whatever your program wants with them...).  It
 * should return zero, or negative if an error or some other event occurs
 * which would require aborting the decode process...  Note that the length
 * passed will almost always be equal to the line length passed to the
 * decoder function, with the sole exception occurring when an ending code
 * occurs in an odd place in the GIF file...  In any case, linelen will be
 * equal to the number of pixels passed...
 */
extern int gif_out_line();

/* extern int bad_code_count;
 *
 * This value is the only other global required by the using program, and
 * is incremented each time an out of range code is read by the decoder.
 * When this value is non-zero after a decode, your GIF file is probably
 * corrupt in some way...
 */
int bad_code_count;

#define MAX_CODES   4095

/* Static variables */
static WORD curr_size;                     /* The current code size */
static WORD clear;                         /* Value for a clear code */
static WORD ending;                        /* Value for a ending code */
static WORD newcodes;                      /* First available code */
static WORD top_slot;                      /* Highest code for current size */
static WORD slot;                          /* Last read code */

/* The following static variables are used
 * for seperating out codes
 */
static WORD navail_bytes = 0;              /* # bytes left in block */
static WORD nbits_left = 0;                /* # bits left in current byte */
static UBYTE b1;                           /* Current byte */
static UBYTE gif_byte_buff[256+3];               /* Current block */
static UBYTE *pbytes;                      /* Pointer to next byte in block */

static long code_mask[13] = {
     0,
     0x0001, 0x0003,
     0x0007, 0x000F,
     0x001F, 0x003F,
     0x007F, 0x00FF,
     0x01FF, 0x03FF,
     0x07FF, 0x0FFF
     };


/* This function initializes the decoder for reading a new image.
 */
static WORD init_exp(size)
   WORD size;
   {
   curr_size = size + 1;
   top_slot = 1 << curr_size;
   clear = 1 << size;
   ending = clear + 1;
   slot = newcodes = ending + 1;
   navail_bytes = nbits_left = 0;
   return(0);
   }

/* get_next_code()
 * - gets the next code from the GIF file.  Returns the code, or else
 * a negative number in case of file errors...
 */
static WORD get_next_code()
   {
   WORD i, x;
   unsigned long ret;

   if (nbits_left == 0)
      {
      if (navail_bytes <= 0)
         {

         /* Out of bytes in current block, so read next block
          */
         pbytes = gif_byte_buff;
         if ((navail_bytes = gif_get_byte()) < 0)
            return(navail_bytes);
         else if (navail_bytes)
            {
            for (i = 0; i < navail_bytes; ++i)
               {
               if ((x = gif_get_byte()) < 0)
                  return(x);
               gif_byte_buff[i] = x;
               }
            }
         }
      b1 = *pbytes++;
      nbits_left = 8;
      --navail_bytes;
      }

   ret = b1 >> (8 - nbits_left);
   while (curr_size > nbits_left)
      {
      if (navail_bytes <= 0)
         {

         /* Out of bytes in current block, so read next block
          */
         pbytes = gif_byte_buff;
         if ((navail_bytes = gif_get_byte()) < 0)
            return(navail_bytes);
         else if (navail_bytes)
            {
            for (i = 0; i < navail_bytes; ++i)
               {
               if ((x = gif_get_byte()) < 0)
                  return(x);
               gif_byte_buff[i] = x;
               }
            }
         }
      b1 = *pbytes++;
      ret |= b1 << nbits_left;
      nbits_left += 8;
      --navail_bytes;
      }
   nbits_left -= curr_size;
   ret &= code_mask[curr_size];
   return((WORD)(ret));
   }



/* WORD decoder(linewidth)
 *    WORD linewidth;               * Pixels per line of image *
 *
 * - This function decodes an LZW image, according to the method used
 * in the GIF spec.  Every *linewidth* 'characters' (ie. pixels) decoded
 * will generate a call to gif_out_line(), which is a user specific function
 * to display a line of pixels.  The function gets it's codes from
 * get_next_code() which is responsible for reading blocks of data and
 * seperating them into the proper size codes.  Finally, gif_get_byte() is
 * the global routine to read the next byte from the GIF file.
 *
 * It is generally a good idea to have linewidth correspond to the actual
 * width of a line (as specified in the Image header) to make your own
 * code a bit simpler, but it isn't absolutely necessary.
 *
 * Returns: 0 if successful, else negative.  (See ERRS.H)
 *
 */

static WORD decoder(linewidth,buf,stack,suffix,prefix)
   WORD linewidth;
   UBYTE *buf;          /* food for gif_line_out, where the pixels go */
   UBYTE *stack;        /* Stack for storing pixels backwards */
   UBYTE *suffix;       /* Suffix table */
   UWORD *prefix;       /* Prefix linked list */
   {
   register UBYTE *sp, *bufptr;
   register WORD code, fc, oc, bufcnt;
   WORD c, size, ret;

   /* Initialize for decoding a new image...
    */
   if ((size = gif_get_byte()) < 0)
      return(size);
   if (size < 2 || 9 < size)
      return(BAD_CODE_SIZE);
   init_exp(size);

   /* Initialize in case they forgot to put in a clear code.
    * (This shouldn't happen, but we'll try and decode it anyway...)
    */
   oc = fc = 0;


   /* Set up the stack pointer and decode buffer pointer
    */
   sp = stack;
   bufptr = buf;
   bufcnt = linewidth;

   /* This is the main loop.  For each code we get we pass through the
    * linked list of prefix codes, pushing the corresponding 'character' for
    * each code onto the stack.  When the list reaches a single 'character'
    * we push that on the stack too, and then start unstacking each
    * character for output in the correct order.  Special handling is
    * included for the clear code, and the whole thing ends when we get
    * an ending code.
    */
   while ((c = get_next_code()) != ending)
      {

      /* If we had a file error, return without completing the decode
       */
      if (c < 0)
         {
         return(c);
         }

      /* If the code is a clear code, reinitialize all necessary items.
       */
      if (c == clear)
         {
         curr_size = size + 1;
         slot = newcodes;
         top_slot = 1 << curr_size;

         /* Continue reading codes until we get a non-clear code
          * (Another unlikely, but possible case...)
          */
         while ((c = get_next_code()) == clear)
            ;

         /* If we get an ending code immediately after a clear code
          * (Yet another unlikely case), then break out of the loop.
          */
         if (c == ending)
            break;

         /* Finally, if the code is beyond the range of already set codes,
          * (This one had better NOT happen...  I have no idea what will
          * result from this, but I doubt it will look good...) then set it
          * to color zero.
          */
         if (c >= slot)
            c = 0;

         oc = fc = c;

         /* And let us not forget to put the char into the buffer... And
          * if, on the off chance, we were exactly one pixel from the end
          * of the line, we have to send the buffer to the gif_out_line()
          * routine...
          */
         *bufptr++ = c;
         if (--bufcnt == 0)
            {
            if ((ret = gif_out_line(buf, linewidth)) < 0)
               {
               return(ret);
               }
            bufptr = buf;
            bufcnt = linewidth;
            }
         }
      else
         {

         /* In this case, it's not a clear code or an ending code, so
          * it must be a code code...  So we can now decode the code into
          * a stack of character codes. (Clear as mud, right?)
          */
         code = c;

         /* Here we go again with one of those off chances...  If, on the
          * off chance, the code we got is beyond the range of those already
          * set up (Another thing which had better NOT happen...) we trick
          * the decoder into thinking it actually got the last code read.
          * (Hmmn... I'm not sure why this works...  But it does...)
          */
         if (code >= slot)
            {
            if (code > slot)
               ++bad_code_count;
            code = oc;
            *sp++ = fc;
            }

         /* Here we scan back along the linked list of prefixes, pushing
          * helpless characters (ie. suffixes) onto the stack as we do so.
          */
         while (code >= newcodes)
            {
            *sp++ = suffix[code];
            code = prefix[code];
            }

         /* Push the last character on the stack, and set up the new
          * prefix and suffix, and if the required slot number is greater
          * than that allowed by the current bit size, increase the bit
          * size.  (NOTE - If we are all full, we *don't* save the new
          * suffix and prefix...  I'm not certain if this is correct...
          * it might be more proper to overwrite the last code...
          */
         *sp++ = code;
         if (slot < top_slot)
            {
            suffix[slot] = fc = code;
            prefix[slot++] = oc;
            oc = c;
            }
         if (slot >= top_slot)
            if (curr_size < 12)
               {
               top_slot <<= 1;
               ++curr_size;
               } 

         /* Now that we've pushed the decoded string (in reverse order)
          * onto the stack, lets pop it off and put it into our decode
          * buffer...  And when the decode buffer is full, write another
          * line...
          */
         while (sp > stack)
            {
            *bufptr++ = *(--sp);
            if (--bufcnt == 0)
               {
               if ((ret = gif_out_line(buf, linewidth)) < 0)
                  {
                  return(ret);
                  }
               bufptr = buf;
               bufcnt = linewidth;
               }
            }
         }
      }
   ret = 0;
   if (bufcnt != linewidth)
      ret = gif_out_line(buf, (linewidth - bufcnt));
   return(ret);
   }

/* basically just allocate memory for buffers and tables, and then
   call Steve B.'s decoder */
int gif_decoder(int linewidth)
{
UBYTE *buf, *stack, *suffix;
UWORD *prefix;
int ret;

ret = OUT_OF_MEMORY;
stack = NULL;
suffix = NULL;
prefix = NULL;
/* stack = suffix = (UBYTE *)prefix = NULL; */
if ((buf = (UBYTE *)needMem(linewidth + 1)) == NULL)
        goto OUT;
if ((stack = (UBYTE *)needMem(MAX_CODES+1)) == NULL)
        goto OUT;
if ((suffix = (UBYTE *)needMem(MAX_CODES+1)) == NULL)
        goto OUT;
if ((prefix = (UWORD *)needMem((MAX_CODES+1)*sizeof(UWORD) )) == NULL)
        goto OUT;
ret = decoder(linewidth,buf,stack,suffix,prefix);
OUT:
freeMem(buf);
freeMem(stack);
freeMem(prefix);
freeMem(suffix);
return(ret);
}

---