harvey/sys/src/cmd/gs/src/slzwce.c

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/*$Id: slzwce.c,v 1.2 2000/09/19 19:00:50 lpd Exp $ */
/* Simple encoder compatible with LZW decoding filter */
#include "stdio_.h"		/* includes std.h */
#include "gdebug.h"
#include "strimpl.h"
#include "slzwx.h"

/* ------ Alternate LZWEncode filter implementation ------ */

/*

   The encoded data stream produced by this implementation of the LZWEncode
   filter consists of a sequence of 9-bit data elements.  These elements are
   packed into bytes in big-endian order, e.g. the elements

   100000000 001100001

   occurring at the very beginning of the data stream would be packed into
   bytes as

   10000000 00011000 01......

   The first bit of each data element is a control bit.  If the control bit is
   0, the remaining 8 bits of the data element are a data byte.  If the control
   bit is 1, the remaining 8 bits of the data element define a control
   function:

   1 00000000   synchronization mark, see below
   1 00000001   end of data
   1 xxxxxxxx   not used (all other values)

   The synchronization mark occurs at the beginning of the data stream, and at
   least once every 254 data bytes thereafter.

   This format is derived from basic principles of data encoding (the use of a
   separate flag bit to distinguish out-of-band control information from data
   per se, and the use of a periodic synchronization mark to help verify the
   validity of a data stream); it has no relationship to data compression.  It
   is, however, compatible with LZW decompressors.  It produces output that is
   approximately 9/8 times the size of the input.

 */

/* Define the special codes, relative to 1 << InitialCodeLength. */
#define CODE_RESET 0
#define CODE_EOD 1
#define CODE_0 2		/* first assignable code */

/* Internal routine to put a code into the output buffer. */
/* Let S = ss->code_size. */
/* Relevant invariants: 9 <= S <= 15, 0 <= code < 1 << S; */
/* 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;
}

/* Initialize LZW-compatible encoding filter. */
private int
s_LZWE_reset(stream_state * st)
{
    stream_LZW_state *const ss = (stream_LZW_state *) st;

    ss->code_size = ss->InitialCodeLength + 1;
    ss->bits_left = 8;
    /* Force the first code emitted to be a reset. */
    ss->next_code = (1 << ss->code_size) - 2;
    return 0;
}
private int
s_LZWE_init(stream_state * st)
{
    stream_LZW_state *const ss = (stream_LZW_state *) st;

    ss->InitialCodeLength = 8;
    ss->table.encode = 0;
    return s_LZWE_reset(st);
}

/* Process a buffer */
private int
s_LZWE_process(stream_state * st, stream_cursor_read * pr,
	       stream_cursor_write * pw, bool last)
{
    stream_LZW_state *const ss = (stream_LZW_state *) st;
    register const byte *p = pr->ptr;
    const byte *rlimit = pr->limit;
    register byte *q = pw->ptr;
    byte *wlimit = pw->limit;
    int status = 0;
    int signal = 1 << (ss->code_size - 1);
    uint limit_code = (1 << ss->code_size) - 2;		/* reset 1 early */
    uint next_code = ss->next_code;

    while (p < rlimit) {
	if (next_code == limit_code) {	/* Emit a reset code. */
	    if (wlimit - q < 2) {
		status = 1;
		break;
	    }
	    q = lzw_put_code(ss, q, signal + CODE_RESET);
	    next_code = signal + CODE_0;
	}
	if (wlimit - q < 2) {
	    status = 1;
	    break;
	}
	q = lzw_put_code(ss, q, *++p);
	next_code++;
    }
    if (last && status == 0) {
	if (wlimit - q < 2)
	    status = 1;
	else {
	    q = lzw_put_code(ss, q, signal + CODE_EOD);
	    if (ss->bits_left < 8)
		*++q = ss->bits << ss->bits_left;	/* final byte */
	}
    }
    ss->next_code = next_code;
    pr->ptr = p;
    pw->ptr = q;
    return status;
}

/* Stream template */
const stream_template s_LZWE_template = {
    &st_LZW_state, s_LZWE_init, s_LZWE_process, 1, 2, NULL,
    s_LZW_set_defaults, s_LZWE_reset
};