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JDHUFF.C
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1992-12-03
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/*
* jdhuff.c
*
* Copyright (C) 1991, 1992, Thomas G. Lane.
* This file is part of the Independent JPEG Group's software.
* For conditions of distribution and use, see the accompanying README file.
*
* This file contains Huffman entropy decoding routines.
* These routines are invoked via the methods entropy_decode
* and entropy_decode_init/term.
*/
#include "jinclude.h"
/* Static variables to avoid passing 'round extra parameters */
static decompress_info_ptr dcinfo;
static INT32 get_buffer; /* current bit-extraction buffer */
static int bits_left; /* # of unused bits in it */
static boolean printed_eod; /* flag to suppress multiple end-of-data msgs */
LOCAL void
fix_huff_tbl (HUFF_TBL * htbl)
/* Compute derived values for a Huffman table */
{
int p, i, l, si;
char huffsize[257];
UINT16 huffcode[257];
UINT16 code;
/* Figure C.1: make table of Huffman code length for each symbol */
/* Note that this is in code-length order. */
p = 0;
for (l = 1; l <= 16; l++) {
for (i = 1; i <= (int) htbl->bits[l]; i++)
huffsize[p++] = (char) l;
}
huffsize[p] = 0;
/* Figure C.2: generate the codes themselves */
/* Note that this is in code-length order. */
code = 0;
si = huffsize[0];
p = 0;
while (huffsize[p]) {
while (((int) huffsize[p]) == si) {
huffcode[p++] = code;
code++;
}
code <<= 1;
si++;
}
/* We don't bother to fill in the encoding tables ehufco[] and ehufsi[], */
/* since they are not used for decoding. */
/* Figure F.15: generate decoding tables */
p = 0;
for (l = 1; l <= 16; l++) {
if (htbl->bits[l]) {
htbl->valptr[l] = p; /* huffval[] index of 1st sym of code len l */
htbl->mincode[l] = huffcode[p]; /* minimum code of length l */
p += htbl->bits[l];
htbl->maxcode[l] = huffcode[p-1]; /* maximum code of length l */
} else {
htbl->maxcode[l] = -1;
}
}
htbl->maxcode[17] = 0xFFFFFL; /* ensures huff_DECODE terminates */
}
/*
* Code for extracting the next N bits from the input stream.
* (N never exceeds 15 for JPEG data.)
* This needs to go as fast as possible!
*
* We read source bytes into get_buffer and dole out bits as needed.
* If get_buffer already contains enough bits, they are fetched in-line
* by the macros get_bits() and get_bit(). When there aren't enough bits,
* fill_bit_buffer is called; it will attempt to fill get_buffer to the
* "high water mark", then extract the desired number of bits. The idea,
* of course, is to minimize the function-call overhead cost of entering
* fill_bit_buffer.
* On most machines MIN_GET_BITS should be 25 to allow the full 32-bit width
* of get_buffer to be used. (On machines with wider words, an even larger
* buffer could be used.) However, on some machines 32-bit shifts are
* relatively slow and take time proportional to the number of places shifted.
* (This is true with most PC compilers, for instance.) In this case it may
* be a win to set MIN_GET_BITS to the minimum value of 15. This reduces the
* average shift distance at the cost of more calls to fill_bit_buffer.
*/
#ifdef SLOW_SHIFT_32
#define MIN_GET_BITS 15 /* minimum allowable value */
#else
#define MIN_GET_BITS 25 /* max value for 32-bit get_buffer */
#endif
static const int bmask[16] = /* bmask[n] is mask for n rightmost bits */
{ 0, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF,
0x01FF, 0x03FF, 0x07FF, 0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF };
LOCAL int
fill_bit_buffer (int nbits)
/* Load up the bit buffer and do get_bits(nbits) */
{
/* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
while (bits_left < MIN_GET_BITS) {
register int c = JGETC(dcinfo);
/* If it's 0xFF, check and discard stuffed zero byte */
if (c == 0xFF) {
int c2 = JGETC(dcinfo);
if (c2 != 0) {
/* Oops, it's actually a marker indicating end of compressed data. */
/* Better put it back for use later */
JUNGETC(c2,dcinfo);
JUNGETC(c,dcinfo);
/* There should be enough bits still left in the data segment; */
/* if so, just break out of the while loop. */
if (bits_left >= nbits)
break;
/* Uh-oh. Report corrupted data to user and stuff zeroes into
* the data stream, so we can produce some kind of image.
* Note that this will be repeated for each byte demanded for the
* rest of the segment; this is a bit slow but not unreasonably so.
* The main thing is to avoid getting a zillion warnings, hence:
*/
if (! printed_eod) {
WARNMS(dcinfo->emethods, "Corrupt JPEG data: premature end of data segment");
printed_eod = TRUE;
}
c = 0; /* insert a zero byte into bit buffer */
}
}
/* OK, load c into get_buffer */
get_buffer = (get_buffer << 8) | c;
bits_left += 8;
}
/* Having filled get_buffer, extract desired bits (this simplifies macros) */
bits_left -= nbits;
return ((int) (get_buffer >> bits_left)) & bmask[nbits];
}
/* Macros to make things go at some speed! */
/* NB: parameter to get_bits should be simple variable, not expression */
#define get_bits(nbits) \
(bits_left >= (nbits) ? \
((int) (get_buffer >> (bits_left -= (nbits)))) & bmask[nbits] : \
fill_bit_buffer(nbits))
#define get_bit() \
(bits_left ? \
((int) (get_buffer >> (--bits_left))) & 1 : \
fill_bit_buffer(1))
/* Figure F.16: extract next coded symbol from input stream */
INLINE
LOCAL int
huff_DECODE (HUFF_TBL * htbl)
{
register int l;
register INT32 code;
code = get_bit();
l = 1;
while (code > htbl->maxcode[l]) {
code = (code << 1) | get_bit();
l++;
}
/* With garbage input we may reach the sentinel value l = 17. */
if (l > 16) {
WARNMS(dcinfo->emethods, "Corrupt JPEG data: bad Huffman code");
return 0; /* fake a zero as the safest result */
}
return htbl->huffval[ htbl->valptr[l] + ((int) (code - htbl->mincode[l])) ];
}
/* Figure F.12: extend sign bit */
#define huff_EXTEND(x,s) ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))
static const int extend_test[16] = /* entry n is 2**(n-1) */
{ 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,
0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };
static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
{ 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };
/*
* Initialize for a Huffman-compressed scan.
* This is invoked after reading the SOS marker.
*/
METHODDEF void
huff_decoder_init (decompress_info_ptr cinfo)
{
short ci;
jpeg_component_info * compptr;
/* Initialize static variables */
dcinfo = cinfo;
bits_left = 0;
printed_eod = FALSE;
for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
compptr = cinfo->cur_comp_info[ci];
/* Make sure requested tables are present */
if (cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no] == NULL ||
cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no] == NULL)
ERREXIT(cinfo->emethods, "Use of undefined Huffman table");
/* Compute derived values for Huffman tables */
/* We may do this more than once for same table, but it's not a big deal */
fix_huff_tbl(cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no]);
fix_huff_tbl(cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no]);
/* Initialize DC predictions to 0 */
cinfo->last_dc_val[ci] = 0;
}
/* Initialize restart stuff */
cinfo->restarts_to_go = cinfo->restart_interval;
cinfo->next_restart_num = 0;
}
/*
* Check for a restart marker & resynchronize decoder.
*/
LOCAL void
process_restart (decompress_info_ptr cinfo)
{
int c, nbytes;
short ci;
/* Throw away any unused bits remaining in bit buffer */
nbytes = bits_left / 8; /* count any full bytes loaded into buffer */
bits_left = 0;
p