xref: /plan9/sys/src/cmd/gs/src/slzwe.c (revision 593dc095aefb2a85c828727bbfa9da139a49bdf4)

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/* $Id: slzwe.c,v 1.4 2005/04/04 23:00:24 igor Exp $ */
/* LZW encoding filter */
#include "stdio_.h"	/* includes std.h */
#include "gdebug.h"
#include "strimpl.h"
#include "slzwx.h"

/********************************************************/
/* LZW routines are based on:				*/
/* Dr. Dobbs Journal --- Oct. 1989. 			*/
/* Article on LZW Data Compression by Mark R. Nelson 	*/
/********************************************************/

/* Define the special codes */
#define code_reset 256
#define code_eod 257
#define code_0 258			/* first assignable code */

/* Import the stream closing procedure */
extern stream_proc_release(s_LZW_release);

typedef struct lzw_encode_s {
	byte datum;			/* last byte of this code */
	ushort prefix;			/* code for prefix of this code */
} lzw_encode;

#define encode_max 4095		/* max # of codes, must be */
				/* > code_0 and <= 4095 */
#define hash_size (encode_max + encode_max / 4)

struct lzw_encode_table_s {
	lzw_encode encode[encode_max];
	ushort hashed[hash_size];
};
gs_private_st_simple(st_lzwe_table, lzw_encode_table, "lzw_encode_table");

/* Hashing function */
#define encode_hash(code, chr)\
  ((uint)((code) * 59 + (chr) * ((hash_size / 256) | 1)) % hash_size)

/* Internal routine to put a code into the output buffer. */
/* Let S = ss->code_size, M = ss->next_code, N = 1 << M. */
/* Relevant invariants: 9 <= S <= 12; N / 2 <= M < N; 0 <= code < N; */
/* 1 <= ss->bits_left <= 8; only the rightmost (8 - ss->bits_left) */
/* bits of ss->bits contain valid data. */
private byte *
lzw_put_code(register stream_LZW_state *ss, byte *q, uint code)
{	uint size = ss->code_size;
	byte cb = (ss->bits << ss->bits_left) +
		(code >> (size - ss->bits_left));
	if_debug2('W', "[w]writing 0x%x,%d\n", code, ss->code_size);
	*++q = cb;
	if ( (ss->bits_left += 8 - size) <= 0 )
	{	*++q = code >> -ss->bits_left;
		ss->bits_left += 8;
	}
	ss->bits = code;
	return q;
}

/* Internal routine to reset the encoding table */
private void
lzw_reset_encode(stream_LZW_state *ss)
{	register int c;
	lzw_encode_table *table = ss->table.encode;
	ss->next_code = code_0;
	ss->code_size = 9;
	ss->prev_code = code_eod;
	for ( c = 0; c < hash_size; c++ )
		table->hashed[c] = code_eod;
	for ( c = 0; c < 256; c++ )
	{	lzw_encode *ec = &table->encode[c];
		register ushort *tc = &table->hashed[encode_hash(code_eod, c)];
		while ( *tc != code_eod )
		  if ( ++tc == &table->hashed[hash_size] )
		    tc = &table->hashed[0];
		*tc = c;
		ec->datum = c, ec->prefix = code_eod;
	}
	table->encode[code_eod].prefix = code_reset;	/* guarantee no match */
}

#define ss ((stream_LZW_state *)st)

/* Initialize LZWEncode filter */
private int
s_LZWE_init(stream_state *st)
{	ss->bits_left = 8;
	ss->table.encode = gs_alloc_struct(st->memory,
			lzw_encode_table, &st_lzwe_table, "LZWEncode init");
	if ( ss->table.encode == 0 )
		return ERRC;		/****** WRONG ******/
	ss->first = true;
	lzw_reset_encode(ss);
	return 0;
}

/* Process a buffer */
private int
s_LZWE_process(stream_state *st, stream_cursor_read *pr,
  stream_cursor_write *pw, bool last)
{	register const byte *p = pr->ptr;
	const byte *rlimit = pr->limit;
	register byte *q = pw->ptr;
	byte *wlimit = pw->limit;
	int code = ss->prev_code;
	lzw_encode_table *table = ss->table.encode;
	ushort *table_end = &table->hashed[hash_size];
	int status = 0;
	int limit_code;
#define set_limit_code()\
  limit_code = (1 << ss->code_size) - ss->EarlyChange;\
  if ( limit_code > encode_max ) limit_code = encode_max
	set_limit_code();
	if ( ss->first )
	{	/* Emit the initial reset code. */
		if ( wlimit - q < 2 )
			return 1;
		q = lzw_put_code(ss, q, code_reset);
		ss->first = false;
	}
	while ( p < rlimit )
	{	byte c = p[1];
		ushort *tp;
		for ( tp = &table->hashed[encode_hash(code, c)]; ; )
		{	lzw_encode *ep = &table->encode[*tp];
			if ( ep->prefix == code && ep->datum == c )
			{	code = *tp;
				p++;
				break;
			}
			else if ( *tp != code_eod )
			{	if ( ++tp == table_end )
				  tp = &table->hashed[0]; /* wrap around */
			}
			else
			{	/* end of recognized sequence */
				if ( wlimit - q <= 4 )
				{	status = 1;
					goto out;
				}
				q = lzw_put_code(ss, q, code);
				if ( ss->next_code == limit_code )
				{	/* Reached power of 2 or limit. */
					/* Determine which. */
					if ( ss->next_code == encode_max )
					{	q = lzw_put_code(ss, q, code_reset);
						lzw_reset_encode(ss);
						set_limit_code();
						goto cx;
					}
					ss->code_size++;
					set_limit_code();
				}
				if_debug3('W', "[W]encoding 0x%x=0x%x+%c\n",
					  ss->next_code, code, c);
				*tp = ss->next_code++;
				ep = &table->encode[*tp];
				ep->datum = c;
				ep->prefix = code;
cx:				code = code_eod;
				break;
			}
		}
	}
	if ( last && status == 0 )
	{	if ( wlimit - q < 4 )
			status = 1;
		else
		  {	if ( code != code_eod )
			  {	q = lzw_put_code(ss, q, code);	/* put out final code */
			  }
			q = lzw_put_code(ss, q, code_eod);
			if ( ss->bits_left < 8 )
			  *++q = ss->bits << ss->bits_left;  /* final byte */
		  }
	}
out:	ss->prev_code = code;
	pr->ptr = p;
	pw->ptr = q;
	return status;
}

#undef ss

/* Stream template */
const stream_template s_LZWE_template =
{	&st_LZW_state, s_LZWE_init, s_LZWE_process, 1, 4, s_LZW_release,
	s_LZW_set_defaults
};