Keen: Modding

Code: Select all

/*
==================
=
= CarmackCompress
=
= Compress a string of words
=
==================
*/

#define NEARTAG	0xa7
#define FARTAG	0xa8

long CarmackCompress (unsigned far *source,long length,
	unsigned far *dest)
{
	unsigned	ch,chhigh;
	unsigned	far *instart, far *inptr, far *inscan,
				far *stopscan, far *outptr;
	unsigned	far *bestscan, beststring;
	unsigned	dist,maxstring,string,outlength;
	unsigned	nearrepeats,farrepeats;
	unsigned	dups,count;
	unsigned	newwords;

//
// this compression method produces a stream of words
// If the words high byte is NEARTAG or FARTAG, the low byte is a word
// copy count from the a position specified by either the next byte
// or the next word, respectively.  A copy count of 0 means to insert the
// next byte as the low byte of the tag into the output
//


//
// set up
//
	instart = inptr = source;
	outptr = dest;

	outlength = 0;
	length = (length+1)/2;

	nearrepeats = farrepeats = dups = 0;
	count = 10;
	newwords = 0;
//
// compress
//
	do
	{
		ch = *inptr;

//
// scan from start for patterns that match current data
//
		beststring = 0;
		for (inscan = instart; inscan<inptr; inscan++)
		{
			if (*inscan != ch)
				continue;

			maxstring = inptr-inscan;
			if (maxstring > length)
				maxstring = length;
			if (maxstring > 255)
				maxstring = 255;

			string = 1;
			while (*(inscan+string) == *(inptr+string) && string<maxstring)
				string++;

			if (string >= beststring)
			{
				beststring = string;
				bestscan = inscan;
			}
		}

		if (beststring > 1 && inptr-bestscan <= 255)
		{
		// near string
			*outptr++ = beststring + (NEARTAG*256);
			*((unsigned char far *)outptr)++ = inptr-bestscan;
			outlength += 3;
			nearrepeats++;
			inptr += beststring;
			length -= beststring;
		}
		else if (beststring > 2)
		{
		// far string
			*outptr++ = beststring + (FARTAG*256);
			*outptr++ = bestscan-instart;
			outlength += 4;
			farrepeats++;
			inptr += beststring;
			length -= beststring;
		}
		else							// no compression
		{
			chhigh = ch>>8;
			if (chhigh == NEARTAG || chhigh == FARTAG)
			{
			// special case of encountering a
			// tag word in the data stream
			// send a length of 0, and follow it with the real low byte
				*outptr++ = ch & 0xff00;
				*((unsigned char far *)outptr)++ = ch&0xff;
				outlength += 3;
				dups++;
			}
			else
			{
				*outptr++ = ch;
				outlength += 2;
			}
			inptr++;
			length--;
			newwords++;
		}

		if (!--count)
		{
			static char cc[2]="-";
			cc[0]^='+'^'-';
			print(cc);
			sx--;

			count = 10;

		 if (keydown[1])
		 {
		  while(keydown[1]);
		  return 0;
		 }
		}
		if (length<0)
			Quit ("Length < 0!");
	} while (length);
	#if 0
	printf ("%u near patterns\n",nearrepeats);
	printf ("%u far patterns\n",farrepeats);
	printf ("%u new words\n",newwords);
	printf ("%u dups\n\n",dups);
	#endif
	return outlength;
}