1/* 2 * reserved comment block 3 * DO NOT REMOVE OR ALTER! 4 */ 5/* 6 * jdhuff.h 7 * 8 * Copyright (C) 1991-1997, Thomas G. Lane. 9 * This file is part of the Independent JPEG Group's software. 10 * For conditions of distribution and use, see the accompanying README file. 11 * 12 * This file contains declarations for Huffman entropy decoding routines 13 * that are shared between the sequential decoder (jdhuff.c) and the 14 * progressive decoder (jdphuff.c). No other modules need to see these. 15 */ 16 17/* Short forms of external names for systems with brain-damaged linkers. */ 18 19#ifdef NEED_SHORT_EXTERNAL_NAMES 20#define jpeg_make_d_derived_tbl jMkDDerived 21#define jpeg_fill_bit_buffer jFilBitBuf 22#define jpeg_huff_decode jHufDecode 23#endif /* NEED_SHORT_EXTERNAL_NAMES */ 24 25 26/* Derived data constructed for each Huffman table */ 27 28#define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */ 29 30typedef struct { 31 /* Basic tables: (element [0] of each array is unused) */ 32 INT32 maxcode[18]; /* largest code of length k (-1 if none) */ 33 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ 34 INT32 valoffset[17]; /* huffval[] offset for codes of length k */ 35 /* valoffset[k] = huffval[] index of 1st symbol of code length k, less 36 * the smallest code of length k; so given a code of length k, the 37 * corresponding symbol is huffval[code + valoffset[k]] 38 */ 39 40 /* Link to public Huffman table (needed only in jpeg_huff_decode) */ 41 JHUFF_TBL *pub; 42 43 /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of 44 * the input data stream. If the next Huffman code is no more 45 * than HUFF_LOOKAHEAD bits long, we can obtain its length and 46 * the corresponding symbol directly from these tables. 47 */ 48 int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */ 49 UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */ 50} d_derived_tbl; 51 52/* Expand a Huffman table definition into the derived format */ 53EXTERN(void) jpeg_make_d_derived_tbl 54 JPP((j_decompress_ptr cinfo, boolean isDC, int tblno, 55 d_derived_tbl ** pdtbl)); 56 57 58/* 59 * Fetching the next N bits from the input stream is a time-critical operation 60 * for the Huffman decoders. We implement it with a combination of inline 61 * macros and out-of-line subroutines. Note that N (the number of bits 62 * demanded at one time) never exceeds 15 for JPEG use. 63 * 64 * We read source bytes into get_buffer and dole out bits as needed. 65 * If get_buffer already contains enough bits, they are fetched in-line 66 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough 67 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer 68 * as full as possible (not just to the number of bits needed; this 69 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). 70 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. 71 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains 72 * at least the requested number of bits --- dummy zeroes are inserted if 73 * necessary. 74 */ 75 76typedef INT32 bit_buf_type; /* type of bit-extraction buffer */ 77#define BIT_BUF_SIZE 32 /* size of buffer in bits */ 78 79/* If long is > 32 bits on your machine, and shifting/masking longs is 80 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE 81 * appropriately should be a win. Unfortunately we can't define the size 82 * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) 83 * because not all machines measure sizeof in 8-bit bytes. 84 */ 85 86typedef struct { /* Bitreading state saved across MCUs */ 87 bit_buf_type get_buffer; /* current bit-extraction buffer */ 88 int bits_left; /* # of unused bits in it */ 89} bitread_perm_state; 90 91typedef struct { /* Bitreading working state within an MCU */ 92 /* Current data source location */ 93 /* We need a copy, rather than munging the original, in case of suspension */ 94 const JOCTET * next_input_byte; /* => next byte to read from source */ 95 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ 96 /* Bit input buffer --- note these values are kept in register variables, 97 * not in this struct, inside the inner loops. 98 */ 99 bit_buf_type get_buffer; /* current bit-extraction buffer */ 100 int bits_left; /* # of unused bits in it */ 101 /* Pointer needed by jpeg_fill_bit_buffer. */ 102 j_decompress_ptr cinfo; /* back link to decompress master record */ 103} bitread_working_state; 104 105/* Macros to declare and load/save bitread local variables. */ 106#define BITREAD_STATE_VARS \ 107 register bit_buf_type get_buffer; \ 108 register int bits_left; \ 109 bitread_working_state br_state 110 111#define BITREAD_LOAD_STATE(cinfop,permstate) \ 112 br_state.cinfo = cinfop; \ 113 br_state.next_input_byte = cinfop->src->next_input_byte; \ 114 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \ 115 get_buffer = permstate.get_buffer; \ 116 bits_left = permstate.bits_left; 117 118#define BITREAD_SAVE_STATE(cinfop,permstate) \ 119 cinfop->src->next_input_byte = br_state.next_input_byte; \ 120 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \ 121 permstate.get_buffer = get_buffer; \ 122 permstate.bits_left = bits_left 123 124/* 125 * These macros provide the in-line portion of bit fetching. 126 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer 127 * before using GET_BITS, PEEK_BITS, or DROP_BITS. 128 * The variables get_buffer and bits_left are assumed to be locals, 129 * but the state struct might not be (jpeg_huff_decode needs this). 130 * CHECK_BIT_BUFFER(state,n,action); 131 * Ensure there are N bits in get_buffer; if suspend, take action. 132 * val = GET_BITS(n); 133 * Fetch next N bits. 134 * val = PEEK_BITS(n); 135 * Fetch next N bits without removing them from the buffer. 136 * DROP_BITS(n); 137 * Discard next N bits. 138 * The value N should be a simple variable, not an expression, because it 139 * is evaluated multiple times. 140 */ 141 142#define CHECK_BIT_BUFFER(state,nbits,action) \ 143 { if (bits_left < (nbits)) { \ 144 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \ 145 { action; } \ 146 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } } 147 148#define GET_BITS(nbits) \ 149 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1)) 150 151#define PEEK_BITS(nbits) \ 152 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1)) 153 154#define DROP_BITS(nbits) \ 155 (bits_left -= (nbits)) 156 157/* Load up the bit buffer to a depth of at least nbits */ 158EXTERN(boolean) jpeg_fill_bit_buffer 159 JPP((bitread_working_state * state, register bit_buf_type get_buffer, 160 register int bits_left, int nbits)); 161 162 163/* 164 * Code for extracting next Huffman-coded symbol from input bit stream. 165 * Again, this is time-critical and we make the main paths be macros. 166 * 167 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits 168 * without looping. Usually, more than 95% of the Huffman codes will be 8 169 * or fewer bits long. The few overlength codes are handled with a loop, 170 * which need not be inline code. 171 * 172 * Notes about the HUFF_DECODE macro: 173 * 1. Near the end of the data segment, we may fail to get enough bits 174 * for a lookahead. In that case, we do it the hard way. 175 * 2. If the lookahead table contains no entry, the next code must be 176 * more than HUFF_LOOKAHEAD bits long. 177 * 3. jpeg_huff_decode returns -1 if forced to suspend. 178 */ 179 180#define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \ 181{ register int nb, look; \ 182 if (bits_left < HUFF_LOOKAHEAD) { \ 183 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \ 184 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 185 if (bits_left < HUFF_LOOKAHEAD) { \ 186 nb = 1; goto slowlabel; \ 187 } \ 188 } \ 189 look = PEEK_BITS(HUFF_LOOKAHEAD); \ 190 if ((nb = htbl->look_nbits[look]) != 0) { \ 191 DROP_BITS(nb); \ 192 result = htbl->look_sym[look]; \ 193 } else { \ 194 nb = HUFF_LOOKAHEAD+1; \ 195slowlabel: \ 196 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \ 197 { failaction; } \ 198 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 199 } \ 200} 201 202/* Out-of-line case for Huffman code fetching */ 203EXTERN(int) jpeg_huff_decode 204 JPP((bitread_working_state * state, register bit_buf_type get_buffer, 205 register int bits_left, d_derived_tbl * htbl, int min_bits)); 206