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decoder.cpp

/* 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 "std.h" */
#include "errs.h"
#include "image.hpp"
#include "macs.hpp"
#include <stdio.h>
FILE *ufp;

/*IMPORT TEXT *malloc();  */               /* Standard C library allocation */

/* IMPORT INT 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 ERRS.H.
 */

/* IMPORT INT 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...
 */
/*IMPORT INT out_line(); */

/* IMPORT 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 */
short curr_size;                     /* The current code size */
short clear;                         /* Value for a clear code */
short ending;                        /* Value for a ending code */
short newcodes;                      /* First available code */
short top_slot;                      /* Highest code for current size */
short slot;                          /* Last read code */

/* The following static variables are used
 * for seperating out codes
 */
short navail_bytes = 0;              /* # bytes left in block */
short nbits_left = 0;                /* # bits left in current byte */
  unsigned char b1;                           /* Current byte */
  unsigned char byte_buff[257];               /* Current block */
  unsigned char *pbytes;                      /* Pointer to next byte in block */

  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.
 */
short init_exp(short 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...
 */

short int get_byte()
{
  short int x=0;
  if (fread(&x,1,1,ufp)!=1)
    return READ_ERROR;
  else return x;
}

short get_next_code()
   {
   short i, x;
   unsigned long ret;

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

       /* Out of bytes in current block, so read next block
        */
       pbytes = byte_buff;
       if ((navail_bytes = get_byte()) < 0)
          return(navail_bytes);
       else if (navail_bytes)
          {
          for (i = 0; i < navail_bytes; ++i)
             {
             if ((x = get_byte()) < 0)
              return(x);
             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 = byte_buff;
       if ((navail_bytes = get_byte()) < 0)
          return(navail_bytes);
       else if (navail_bytes)
          {
          for (i = 0; i < navail_bytes; ++i)
             {
             if ((x = get_byte()) < 0)
              return(x);
             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((short)(ret));
   }


/* The reason we have these seperated like this instead of using
 * a structure like the original Wilhite code did, is because this
 * stuff generally produces significantly faster code when compiled...
 * This code is full of similar speedups...  (For a good book on writing
 * C for speed or for space optomisation, see Efficient C by Tom Plum,
 * published by Plum-Hall Associates...)
 */
  unsigned char stack[MAX_CODES + 1];            /* Stack for storing pixels */
  unsigned char suffix[MAX_CODES + 1];           /* Suffix table */
  unsigned short prefix[MAX_CODES + 1];           /* Prefix linked list */

/* short decoder(linewidth)
 *    short 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 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, 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)
 *
 */

short decode_gif_data(image *im, FILE *fp)

   {
   register unsigned char *sp, *bufptr;
   unsigned char *buf;
   register short code, fc, oc, bufcnt;
   short c, size, ret,y;
   ufp=fp;
   /* Initialize for decoding a new image...
    */
   if ((size = 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;

   /* Allocate space for the decode buffer
    */
   buf=im->scan_line(0);  y=0;
/*   if ((buf = (unsigned char *)malloc(linewidth + 1)) == NULL)
      return(OUT_OF_MEMORY); */

   /* Set up the stack pointer and decode buffer pointer
    */
   sp = stack;
   bufptr = buf;
   bufcnt = im->height();

   /* 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)
       {
//     free(buf);
       return(0);
       }

      /* 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.
        */
      CONDITION(c<slot,"Error occured while reading gif");
       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 out_line()
        * routine...
        */
       *bufptr++ = c;
       if (--bufcnt == 0)
          {

//        if ((ret = out_line(buf, linewidth)) < 0)
//           {
//           free(buf);
//           return(ret);
//           }
          y++;
          if (y<im->height())
            buf=im->scan_line(y);
          bufptr = buf;
          bufcnt = im->width()-1;
          }
       }
      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 = out_line(buf, linewidth)) < 0)
              {
              free(buf);
              return(ret);
              } */
             y++;
             if (y<im->height())
             buf=im->scan_line(y);

             bufptr = buf;
             bufcnt = im->width()-1;
             }
          }
       }
      }
   ret = 0;
   return(ret);
   }


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