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