Category : C Source Code
Archive   : FRAC151S.ZIP
Filename : DECODER.C

 
Output of file : DECODER.C contained in archive : FRAC151S.ZIP


/* 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.
*
== This routine was modified for use in FRACTINT in two ways.
==
== 1) The original #includes were folded into the routine strictly to hold
== down the number of files we were dealing with.
==
== 2) The 'stack', 'suffix', 'prefix', and 'buf' arrays were changed from
== static and 'malloc()'ed to external only so that the assembler
== program could use the same array space for several independent
== chunks of code. Also, 'stack' was renamed to 'dstack' for TASM
== compatibility.
==
== 3) The 'out_line()' external function has been changed to reference
== '*outln()' for flexibility (in particular, 3D transformations)
==
== 4) A call to 'keypressed()' has been added after the 'outln()' calls
== to check for the presenc of a key-press as a bail-out signal
==
== (Bert Tyler and Timothy Wegner)
*/

#define LOCAL static
#define IMPORT extern

#define FAST register

typedef short WORD;
typedef unsigned short UWORD;
typedef char TEXT;
typedef unsigned char UTINY;
typedef long LONG;
typedef unsigned long ULONG;
typedef int INT;


/* Various error codes used by decoder
* and my own routines... It's okay
* for you to define whatever you want,
* as long as it's negative... It will be
* returned intact up the various subroutine
* levels...
*/
#define OUT_OF_MEMORY -10
#define BAD_CODE_SIZE -20
#define READ_ERROR -1
#define WRITE_ERROR -2
#define OPEN_ERROR -3
#define CREATE_ERROR -4



/* 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 get_byte();

/* 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();
INT (*outln)(UTINY *,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...
*/
IMPORT INT bad_code_count;

/* whups, here are more globals, added by PB: */
IMPORT INT skipxdots; /* 0 to get every dot, 1 for every 2nd, 2 every 3rd, ... */
IMPORT INT skipydots; /* ditto for rows */

#define NULL 0L
#define MAX_CODES 4095

/* Static variables */
LOCAL WORD curr_size; /* The current code size */
LOCAL WORD clear; /* Value for a clear code */
LOCAL WORD ending; /* Value for a ending code */
LOCAL WORD newcodes; /* First available code */
LOCAL WORD top_slot; /* Highest code for current size */
LOCAL WORD slot; /* Last read code */

/* The following static variables are used
* for seperating out codes
*/
LOCAL WORD navail_bytes = 0; /* # bytes left in block */
LOCAL WORD nbits_left = 0; /* # bits left in current byte */
LOCAL UTINY b1; /* Current byte */
LOCAL UTINY byte_buff[257]; /* Current block, reuse shared mem */
LOCAL UTINY *pbytes; /* Pointer to next byte in block */

LOCAL 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.
*/
LOCAL WORD init_exp(size)
WORD 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...
*/
LOCAL WORD get_next_code()
{
WORD 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((WORD)(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...)
*/

/*
I removed the LOCAL identifiers in the arrays below and replaced them
with 'extern's so as to declare (and re-use) the space elsewhere.
The arrays are actually declared in the assembler source.
Bert Tyler
*/

extern UTINY dstack[MAX_CODES + 1]; /* Stack for storing pixels */
extern UTINY suffix[MAX_CODES + 1]; /* Suffix table */
extern UWORD prefix[MAX_CODES + 1]; /* Prefix linked list */
extern UTINY decoderline[2]; /* decoded line goes here */

/* WORD decoder(linewidth)
* WORD 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 its 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)
*
*/

WORD decoder(linewidth)
WORD linewidth;
{
FAST UTINY *sp, *bufptr;
UTINY *buf;
FAST WORD code, fc, oc, bufcnt;
WORD c, size, ret;
WORD xskip,yskip;

/* 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);
xskip = yskip = 0;

/* 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;

buf = decoderline;

/* Set up the stack pointer and decode buffer pointer
*/
sp = dstack;
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)
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... */
*sp++ = c; /* let the common code outside the if else stuff it */
}
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 > dstack)
{
--sp;
if (--xskip < 0)
{
xskip = skipxdots;
*bufptr++ = *sp;
}
if (--bufcnt == 0) /* finished an input row? */
{
if (--yskip < 0)
{
if ((ret = (*outln)(buf, bufptr - buf)) < 0)
return(ret);
yskip = skipydots;
}
if (keypressed())
return(-1);
bufptr = buf;
bufcnt = linewidth;
xskip = 0;
}
}
}
/* PB note that if last line is incomplete, we're not going to try
to emit it; original code did, but did so via out_line and therefore
couldn't have worked well in all cases... */
return(0);
}