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/*
* jdhuff.h * * Copyright (C) 1991-1995, 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 mincode[17]; /* smallest code of length k */ INT32 maxcode[18]; /* largest code of length k (-1 if none) */ /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ int valptr[17]; /* huffval[] index of 1st symbol of length 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, JHUFF_TBL * htbl, 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 do this 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 */ boolean printed_eod; /* flag to suppress multiple warning msgs */} bitread_perm_state;
typedef struct { /* Bitreading working state within an MCU */ /* current data source state */ const JOCTET * next_input_byte; /* => next byte to read from source */ size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ int unread_marker; /* nonzero if we have hit a marker */ /* 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 */ /* pointers needed by jpeg_fill_bit_buffer */ j_decompress_ptr cinfo; /* back link to decompress master record */ boolean * printed_eod_ptr; /* => flag in permanent state */} 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; \ br_state.unread_marker = cinfop->unread_marker; \ get_buffer = permstate.get_buffer; \ bits_left = permstate.bits_left; \ br_state.printed_eod_ptr = & permstate.printed_eod
#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; \ cinfop->unread_marker = br_state.unread_marker; \ 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));
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