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