jdhuff.h (8221B)
1 /* 2 * jdhuff.h 3 * 4 * Copyright (C) 1991-1995, Thomas G. Lane. 5 * This file is part of the Independent JPEG Group's software. 6 * For conditions of distribution and use, see the accompanying README file. 7 * 8 * This file contains declarations for Huffman entropy decoding routines 9 * that are shared between the sequential decoder (jdhuff.c) and the 10 * progressive decoder (jdphuff.c). No other modules need to see these. 11 */ 12 13 /* Short forms of external names for systems with brain-damaged linkers. */ 14 15 #ifdef NEED_SHORT_EXTERNAL_NAMES 16 #define jpeg_make_d_derived_tbl jMkDDerived 17 #define jpeg_fill_bit_buffer jFilBitBuf 18 #define jpeg_huff_decode jHufDecode 19 #endif /* NEED_SHORT_EXTERNAL_NAMES */ 20 21 22 /* Derived data constructed for each Huffman table */ 23 24 #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */ 25 26 typedef struct { 27 /* Basic tables: (element [0] of each array is unused) */ 28 INT32 mincode[17]; /* smallest code of length k */ 29 INT32 maxcode[18]; /* largest code of length k (-1 if none) */ 30 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ 31 int valptr[17]; /* huffval[] index of 1st symbol of length k */ 32 33 /* Link to public Huffman table (needed only in jpeg_huff_decode) */ 34 JHUFF_TBL *pub; 35 36 /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of 37 * the input data stream. If the next Huffman code is no more 38 * than HUFF_LOOKAHEAD bits long, we can obtain its length and 39 * the corresponding symbol directly from these tables. 40 */ 41 int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */ 42 UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */ 43 } d_derived_tbl; 44 45 /* Expand a Huffman table definition into the derived format */ 46 EXTERN void jpeg_make_d_derived_tbl JPP((j_decompress_ptr cinfo, 47 JHUFF_TBL * htbl, d_derived_tbl ** pdtbl)); 48 49 50 /* 51 * Fetching the next N bits from the input stream is a time-critical operation 52 * for the Huffman decoders. We implement it with a combination of inline 53 * macros and out-of-line subroutines. Note that N (the number of bits 54 * demanded at one time) never exceeds 15 for JPEG use. 55 * 56 * We read source bytes into get_buffer and dole out bits as needed. 57 * If get_buffer already contains enough bits, they are fetched in-line 58 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough 59 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer 60 * as full as possible (not just to the number of bits needed; this 61 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). 62 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. 63 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains 64 * at least the requested number of bits --- dummy zeroes are inserted if 65 * necessary. 66 */ 67 68 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */ 69 #define BIT_BUF_SIZE 32 /* size of buffer in bits */ 70 71 /* If long is > 32 bits on your machine, and shifting/masking longs is 72 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE 73 * appropriately should be a win. Unfortunately we can't do this with 74 * something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) 75 * because not all machines measure sizeof in 8-bit bytes. 76 */ 77 78 typedef struct { /* Bitreading state saved across MCUs */ 79 bit_buf_type get_buffer; /* current bit-extraction buffer */ 80 int bits_left; /* # of unused bits in it */ 81 boolean printed_eod; /* flag to suppress multiple warning msgs */ 82 } bitread_perm_state; 83 84 typedef struct { /* Bitreading working state within an MCU */ 85 /* current data source state */ 86 const JOCTET * next_input_byte; /* => next byte to read from source */ 87 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ 88 int unread_marker; /* nonzero if we have hit a marker */ 89 /* bit input buffer --- note these values are kept in register variables, 90 * not in this struct, inside the inner loops. 91 */ 92 bit_buf_type get_buffer; /* current bit-extraction buffer */ 93 int bits_left; /* # of unused bits in it */ 94 /* pointers needed by jpeg_fill_bit_buffer */ 95 j_decompress_ptr cinfo; /* back link to decompress master record */ 96 boolean * printed_eod_ptr; /* => flag in permanent state */ 97 } bitread_working_state; 98 99 /* Macros to declare and load/save bitread local variables. */ 100 #define BITREAD_STATE_VARS \ 101 register bit_buf_type get_buffer; \ 102 register int bits_left; \ 103 bitread_working_state br_state 104 105 #define BITREAD_LOAD_STATE(cinfop,permstate) \ 106 br_state.cinfo = cinfop; \ 107 br_state.next_input_byte = cinfop->src->next_input_byte; \ 108 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \ 109 br_state.unread_marker = cinfop->unread_marker; \ 110 get_buffer = permstate.get_buffer; \ 111 bits_left = permstate.bits_left; \ 112 br_state.printed_eod_ptr = & permstate.printed_eod 113 114 #define BITREAD_SAVE_STATE(cinfop,permstate) \ 115 cinfop->src->next_input_byte = br_state.next_input_byte; \ 116 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \ 117 cinfop->unread_marker = br_state.unread_marker; \ 118 permstate.get_buffer = get_buffer; \ 119 permstate.bits_left = bits_left 120 121 /* 122 * These macros provide the in-line portion of bit fetching. 123 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer 124 * before using GET_BITS, PEEK_BITS, or DROP_BITS. 125 * The variables get_buffer and bits_left are assumed to be locals, 126 * but the state struct might not be (jpeg_huff_decode needs this). 127 * CHECK_BIT_BUFFER(state,n,action); 128 * Ensure there are N bits in get_buffer; if suspend, take action. 129 * val = GET_BITS(n); 130 * Fetch next N bits. 131 * val = PEEK_BITS(n); 132 * Fetch next N bits without removing them from the buffer. 133 * DROP_BITS(n); 134 * Discard next N bits. 135 * The value N should be a simple variable, not an expression, because it 136 * is evaluated multiple times. 137 */ 138 139 #define CHECK_BIT_BUFFER(state,nbits,action) \ 140 { if (bits_left < (nbits)) { \ 141 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \ 142 { action; } \ 143 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } } 144 145 #define GET_BITS(nbits) \ 146 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1)) 147 148 #define PEEK_BITS(nbits) \ 149 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1)) 150 151 #define DROP_BITS(nbits) \ 152 (bits_left -= (nbits)) 153 154 /* Load up the bit buffer to a depth of at least nbits */ 155 EXTERN boolean jpeg_fill_bit_buffer JPP((bitread_working_state * state, 156 register bit_buf_type get_buffer, register int bits_left, 157 int nbits)); 158 159 160 /* 161 * Code for extracting next Huffman-coded symbol from input bit stream. 162 * Again, this is time-critical and we make the main paths be macros. 163 * 164 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits 165 * without looping. Usually, more than 95% of the Huffman codes will be 8 166 * or fewer bits long. The few overlength codes are handled with a loop, 167 * which need not be inline code. 168 * 169 * Notes about the HUFF_DECODE macro: 170 * 1. Near the end of the data segment, we may fail to get enough bits 171 * for a lookahead. In that case, we do it the hard way. 172 * 2. If the lookahead table contains no entry, the next code must be 173 * more than HUFF_LOOKAHEAD bits long. 174 * 3. jpeg_huff_decode returns -1 if forced to suspend. 175 */ 176 177 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \ 178 { register int nb, look; \ 179 if (bits_left < HUFF_LOOKAHEAD) { \ 180 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \ 181 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 182 if (bits_left < HUFF_LOOKAHEAD) { \ 183 nb = 1; goto slowlabel; \ 184 } \ 185 } \ 186 look = PEEK_BITS(HUFF_LOOKAHEAD); \ 187 if ((nb = htbl->look_nbits[look]) != 0) { \ 188 DROP_BITS(nb); \ 189 result = htbl->look_sym[look]; \ 190 } else { \ 191 nb = HUFF_LOOKAHEAD+1; \ 192 slowlabel: \ 193 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \ 194 { failaction; } \ 195 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 196 } \ 197 } 198 199 /* Out-of-line case for Huffman code fetching */ 200 EXTERN int jpeg_huff_decode JPP((bitread_working_state * state, 201 register bit_buf_type get_buffer, register int bits_left, 202 d_derived_tbl * htbl, int min_bits));