/* DECODE.C - An LZW decoder for GIF
 * Copyright (C) 1987, by Steven A. Bennett
 * Copyright (C) 1994, C++ version by Alejandro Aguilar Sierra
*
 * 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 "stdafx.h"

#include "gifdecod.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

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.
 */
SHORT GIFDecoder::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 GIFDecoder::get_next_code()
	{
	SHORT i, x;
	ULONG 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...)
 */
LOCAL byte stack[MAX_CODES + 1];            /* Stack for storing pixels */
LOCAL byte suffix[MAX_CODES + 1];           /* Suffix table */
LOCAL USHORT 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 GIFDecoder::decoder(SHORT linewidth, INT&  bad_code_count)
	{
	FAST byte *sp, *bufptr;
	byte *buf;
	FAST SHORT code, fc, oc, bufcnt;
	SHORT c, size, ret;

	/* Initialize for decoding a new image...
	 */
	bad_code_count = 0;
	if ((size = get_byte()) < 0)
		return(size);
	if (size < 2 || 9 < size)
		return(BAD_CODE_SIZE);
/*   out_line = outline;*/
	init_exp(size);
//  printf("L %d %x\n",linewidth,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
    */
	if ((buf = new byte[linewidth + 1]) == NULL)
      return(OUT_OF_MEMORY);

   /* 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)
         {
	 delete[] 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.
          */
         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)
               {
			 delete[] buf;
               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 = out_line(buf, linewidth)) < 0)
                  {
		  delete[] buf;
                  return(ret);
                  }
               bufptr = buf;
               bufcnt = linewidth;
               }
            }
         }
      }
   ret = 0;
   if (bufcnt != linewidth)
      ret = out_line(buf, (linewidth - bufcnt));
   delete[] buf;
   return(ret);
   }

