/* | |
* jdhuff.h | |
* | |
* Copyright (C) 1991-1997, 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 declarations for Huffman entropy decoding routines | |
* that are shared between the sequential decoder (jdhuff.c) and the | |
* progressive decoder (jdphuff.c). No other modules need to see these. | |
*/ | |
/* Short forms of external names for systems with brain-damaged linkers. */ | |
#ifdef NEED_SHORT_EXTERNAL_NAMES | |
#define jpeg_make_d_derived_tbl jMkDDerived | |
#define jpeg_fill_bit_buffer jFilBitBuf | |
#define jpeg_huff_decode jHufDecode | |
#endif /* NEED_SHORT_EXTERNAL_NAMES */ | |
/* Derived data constructed for each Huffman table */ | |
#define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */ | |
typedef struct { | |
/* Basic tables: (element [0] of each array is unused) */ | |
INT32 maxcode[18]; /* largest code of length k (-1 if none) */ | |
/* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ | |
INT32 valoffset[17]; /* huffval[] offset for codes of length k */ | |
/* valoffset[k] = huffval[] index of 1st symbol of code length k, less | |
* the smallest code of length k; so given a code of length k, the | |
* corresponding symbol is huffval[code + valoffset[k]] | |
*/ | |
/* Link to public Huffman table (needed only in jpeg_huff_decode) */ | |
JHUFF_TBL *pub; | |
/* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of | |
* the input data stream. If the next Huffman code is no more | |
* than HUFF_LOOKAHEAD bits long, we can obtain its length and | |
* the corresponding symbol directly from these tables. | |
*/ | |
int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */ | |
UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */ | |
} d_derived_tbl; | |
/* Expand a Huffman table definition into the derived format */ | |
EXTERN(void) jpeg_make_d_derived_tbl | |
JPP((j_decompress_ptr cinfo, boolean isDC, int tblno, | |
d_derived_tbl ** pdtbl)); | |
/* | |
* Fetching the next N bits from the input stream is a time-critical operation | |
* for the Huffman decoders. We implement it with a combination of inline | |
* macros and out-of-line subroutines. Note that N (the number of bits | |
* demanded at one time) never exceeds 15 for JPEG use. | |
* | |
* 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 CHECK_BIT_BUFFER and GET_BITS. When there aren't enough | |
* bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer | |
* as full as possible (not just to the number of bits needed; this | |
* prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). | |
* Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. | |
* On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains | |
* at least the requested number of bits --- dummy zeroes are inserted if | |
* necessary. | |
*/ | |
typedef INT32 bit_buf_type; /* type of bit-extraction buffer */ | |
#define BIT_BUF_SIZE 32 /* size of buffer in bits */ | |
/* If long is > 32 bits on your machine, and shifting/masking longs is | |
* reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE | |
* appropriately should be a win. Unfortunately we can't define the size | |
* with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) | |
* because not all machines measure sizeof in 8-bit bytes. | |
*/ | |
typedef struct { /* Bitreading state saved across MCUs */ | |
bit_buf_type get_buffer; /* current bit-extraction buffer */ | |
int bits_left; /* # of unused bits in it */ | |
} bitread_perm_state; | |
typedef struct { /* Bitreading working state within an MCU */ | |
/* Current data source location */ | |
/* We need a copy, rather than munging the original, in case of suspension */ | |
const JOCTET * next_input_byte; /* => next byte to read from source */ | |
size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ | |
/* Bit input buffer --- note these values are kept in register variables, | |
* not in this struct, inside the inner loops. | |
*/ | |
bit_buf_type get_buffer; /* current bit-extraction buffer */ | |
int bits_left; /* # of unused bits in it */ | |
/* Pointer needed by jpeg_fill_bit_buffer. */ | |
j_decompress_ptr cinfo; /* back link to decompress master record */ | |
} bitread_working_state; | |
/* Macros to declare and load/save bitread local variables. */ | |
#define BITREAD_STATE_VARS \ | |
register bit_buf_type get_buffer; \ | |
register int bits_left; \ | |
bitread_working_state br_state | |
#define BITREAD_LOAD_STATE(cinfop,permstate) \ | |
br_state.cinfo = cinfop; \ | |
br_state.next_input_byte = cinfop->src->next_input_byte; \ | |
br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \ | |
get_buffer = permstate.get_buffer; \ | |
bits_left = permstate.bits_left; | |
#define BITREAD_SAVE_STATE(cinfop,permstate) \ | |
cinfop->src->next_input_byte = br_state.next_input_byte; \ | |
cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \ | |
permstate.get_buffer = get_buffer; \ | |
permstate.bits_left = bits_left | |
/* | |
* These macros provide the in-line portion of bit fetching. | |
* Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer | |
* before using GET_BITS, PEEK_BITS, or DROP_BITS. | |
* The variables get_buffer and bits_left are assumed to be locals, | |
* but the state struct might not be (jpeg_huff_decode needs this). | |
* CHECK_BIT_BUFFER(state,n,action); | |
* Ensure there are N bits in get_buffer; if suspend, take action. | |
* val = GET_BITS(n); | |
* Fetch next N bits. | |
* val = PEEK_BITS(n); | |
* Fetch next N bits without removing them from the buffer. | |
* DROP_BITS(n); | |
* Discard next N bits. | |
* The value N should be a simple variable, not an expression, because it | |
* is evaluated multiple times. | |
*/ | |
#define CHECK_BIT_BUFFER(state,nbits,action) \ | |
{ if (bits_left < (nbits)) { \ | |
if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \ | |
{ action; } \ | |
get_buffer = (state).get_buffer; bits_left = (state).bits_left; } } | |
#define GET_BITS(nbits) \ | |
(((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1)) | |
#define PEEK_BITS(nbits) \ | |
(((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1)) | |
#define DROP_BITS(nbits) \ | |
(bits_left -= (nbits)) | |
/* Load up the bit buffer to a depth of at least nbits */ | |
EXTERN(boolean) jpeg_fill_bit_buffer | |
JPP((bitread_working_state * state, register bit_buf_type get_buffer, | |
register int bits_left, int nbits)); | |
/* | |
* Code for extracting next Huffman-coded symbol from input bit stream. | |
* Again, this is time-critical and we make the main paths be macros. | |
* | |
* We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits | |
* without looping. Usually, more than 95% of the Huffman codes will be 8 | |
* or fewer bits long. The few overlength codes are handled with a loop, | |
* which need not be inline code. | |
* | |
* Notes about the HUFF_DECODE macro: | |
* 1. Near the end of the data segment, we may fail to get enough bits | |
* for a lookahead. In that case, we do it the hard way. | |
* 2. If the lookahead table contains no entry, the next code must be | |
* more than HUFF_LOOKAHEAD bits long. | |
* 3. jpeg_huff_decode returns -1 if forced to suspend. | |
*/ | |
#define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \ | |
{ register int nb, look; \ | |
if (bits_left < HUFF_LOOKAHEAD) { \ | |
if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \ | |
get_buffer = state.get_buffer; bits_left = state.bits_left; \ | |
if (bits_left < HUFF_LOOKAHEAD) { \ | |
nb = 1; goto slowlabel; \ | |
} \ | |
} \ | |
look = PEEK_BITS(HUFF_LOOKAHEAD); \ | |
if ((nb = htbl->look_nbits[look]) != 0) { \ | |
DROP_BITS(nb); \ | |
result = htbl->look_sym[look]; \ | |
} else { \ | |
nb = HUFF_LOOKAHEAD+1; \ | |
slowlabel: \ | |
if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \ | |
{ failaction; } \ | |
get_buffer = state.get_buffer; bits_left = state.bits_left; \ | |
} \ | |
} | |
/* Out-of-line case for Huffman code fetching */ | |
EXTERN(int) jpeg_huff_decode | |
JPP((bitread_working_state * state, register bit_buf_type get_buffer, | |
register int bits_left, d_derived_tbl * htbl, int min_bits)); |