/*
* 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));