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