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annotate libs/libjpeg/jdhuff.h @ 0:b2f14e535253

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author John Tsiombikas <nuclear@member.fsf.org>
date Sat, 01 Feb 2014 19:58:19 +0200
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nuclear@0 1 /*
nuclear@0 2 * jdhuff.h
nuclear@0 3 *
nuclear@0 4 * Copyright (C) 1991-1997, Thomas G. Lane.
nuclear@0 5 * This file is part of the Independent JPEG Group's software.
nuclear@0 6 * For conditions of distribution and use, see the accompanying README file.
nuclear@0 7 *
nuclear@0 8 * This file contains declarations for Huffman entropy decoding routines
nuclear@0 9 * that are shared between the sequential decoder (jdhuff.c) and the
nuclear@0 10 * progressive decoder (jdphuff.c). No other modules need to see these.
nuclear@0 11 */
nuclear@0 12
nuclear@0 13 /* Short forms of external names for systems with brain-damaged linkers. */
nuclear@0 14
nuclear@0 15 #ifdef NEED_SHORT_EXTERNAL_NAMES
nuclear@0 16 #define jpeg_make_d_derived_tbl jMkDDerived
nuclear@0 17 #define jpeg_fill_bit_buffer jFilBitBuf
nuclear@0 18 #define jpeg_huff_decode jHufDecode
nuclear@0 19 #endif /* NEED_SHORT_EXTERNAL_NAMES */
nuclear@0 20
nuclear@0 21
nuclear@0 22 /* Derived data constructed for each Huffman table */
nuclear@0 23
nuclear@0 24 #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */
nuclear@0 25
nuclear@0 26 typedef struct {
nuclear@0 27 /* Basic tables: (element [0] of each array is unused) */
nuclear@0 28 INT32 maxcode[18]; /* largest code of length k (-1 if none) */
nuclear@0 29 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
nuclear@0 30 INT32 valoffset[17]; /* huffval[] offset for codes of length k */
nuclear@0 31 /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
nuclear@0 32 * the smallest code of length k; so given a code of length k, the
nuclear@0 33 * corresponding symbol is huffval[code + valoffset[k]]
nuclear@0 34 */
nuclear@0 35
nuclear@0 36 /* Link to public Huffman table (needed only in jpeg_huff_decode) */
nuclear@0 37 JHUFF_TBL *pub;
nuclear@0 38
nuclear@0 39 /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
nuclear@0 40 * the input data stream. If the next Huffman code is no more
nuclear@0 41 * than HUFF_LOOKAHEAD bits long, we can obtain its length and
nuclear@0 42 * the corresponding symbol directly from these tables.
nuclear@0 43 */
nuclear@0 44 int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
nuclear@0 45 UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
nuclear@0 46 } d_derived_tbl;
nuclear@0 47
nuclear@0 48 /* Expand a Huffman table definition into the derived format */
nuclear@0 49 EXTERN(void) jpeg_make_d_derived_tbl
nuclear@0 50 JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
nuclear@0 51 d_derived_tbl ** pdtbl));
nuclear@0 52
nuclear@0 53
nuclear@0 54 /*
nuclear@0 55 * Fetching the next N bits from the input stream is a time-critical operation
nuclear@0 56 * for the Huffman decoders. We implement it with a combination of inline
nuclear@0 57 * macros and out-of-line subroutines. Note that N (the number of bits
nuclear@0 58 * demanded at one time) never exceeds 15 for JPEG use.
nuclear@0 59 *
nuclear@0 60 * We read source bytes into get_buffer and dole out bits as needed.
nuclear@0 61 * If get_buffer already contains enough bits, they are fetched in-line
nuclear@0 62 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough
nuclear@0 63 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
nuclear@0 64 * as full as possible (not just to the number of bits needed; this
nuclear@0 65 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
nuclear@0 66 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
nuclear@0 67 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
nuclear@0 68 * at least the requested number of bits --- dummy zeroes are inserted if
nuclear@0 69 * necessary.
nuclear@0 70 */
nuclear@0 71
nuclear@0 72 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */
nuclear@0 73 #define BIT_BUF_SIZE 32 /* size of buffer in bits */
nuclear@0 74
nuclear@0 75 /* If long is > 32 bits on your machine, and shifting/masking longs is
nuclear@0 76 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
nuclear@0 77 * appropriately should be a win. Unfortunately we can't define the size
nuclear@0 78 * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
nuclear@0 79 * because not all machines measure sizeof in 8-bit bytes.
nuclear@0 80 */
nuclear@0 81
nuclear@0 82 typedef struct { /* Bitreading state saved across MCUs */
nuclear@0 83 bit_buf_type get_buffer; /* current bit-extraction buffer */
nuclear@0 84 int bits_left; /* # of unused bits in it */
nuclear@0 85 } bitread_perm_state;
nuclear@0 86
nuclear@0 87 typedef struct { /* Bitreading working state within an MCU */
nuclear@0 88 /* Current data source location */
nuclear@0 89 /* We need a copy, rather than munging the original, in case of suspension */
nuclear@0 90 const JOCTET * next_input_byte; /* => next byte to read from source */
nuclear@0 91 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */
nuclear@0 92 /* Bit input buffer --- note these values are kept in register variables,
nuclear@0 93 * not in this struct, inside the inner loops.
nuclear@0 94 */
nuclear@0 95 bit_buf_type get_buffer; /* current bit-extraction buffer */
nuclear@0 96 int bits_left; /* # of unused bits in it */
nuclear@0 97 /* Pointer needed by jpeg_fill_bit_buffer. */
nuclear@0 98 j_decompress_ptr cinfo; /* back link to decompress master record */
nuclear@0 99 } bitread_working_state;
nuclear@0 100
nuclear@0 101 /* Macros to declare and load/save bitread local variables. */
nuclear@0 102 #define BITREAD_STATE_VARS \
nuclear@0 103 register bit_buf_type get_buffer; \
nuclear@0 104 register int bits_left; \
nuclear@0 105 bitread_working_state br_state
nuclear@0 106
nuclear@0 107 #define BITREAD_LOAD_STATE(cinfop,permstate) \
nuclear@0 108 br_state.cinfo = cinfop; \
nuclear@0 109 br_state.next_input_byte = cinfop->src->next_input_byte; \
nuclear@0 110 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
nuclear@0 111 get_buffer = permstate.get_buffer; \
nuclear@0 112 bits_left = permstate.bits_left;
nuclear@0 113
nuclear@0 114 #define BITREAD_SAVE_STATE(cinfop,permstate) \
nuclear@0 115 cinfop->src->next_input_byte = br_state.next_input_byte; \
nuclear@0 116 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
nuclear@0 117 permstate.get_buffer = get_buffer; \
nuclear@0 118 permstate.bits_left = bits_left
nuclear@0 119
nuclear@0 120 /*
nuclear@0 121 * These macros provide the in-line portion of bit fetching.
nuclear@0 122 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
nuclear@0 123 * before using GET_BITS, PEEK_BITS, or DROP_BITS.
nuclear@0 124 * The variables get_buffer and bits_left are assumed to be locals,
nuclear@0 125 * but the state struct might not be (jpeg_huff_decode needs this).
nuclear@0 126 * CHECK_BIT_BUFFER(state,n,action);
nuclear@0 127 * Ensure there are N bits in get_buffer; if suspend, take action.
nuclear@0 128 * val = GET_BITS(n);
nuclear@0 129 * Fetch next N bits.
nuclear@0 130 * val = PEEK_BITS(n);
nuclear@0 131 * Fetch next N bits without removing them from the buffer.
nuclear@0 132 * DROP_BITS(n);
nuclear@0 133 * Discard next N bits.
nuclear@0 134 * The value N should be a simple variable, not an expression, because it
nuclear@0 135 * is evaluated multiple times.
nuclear@0 136 */
nuclear@0 137
nuclear@0 138 #define CHECK_BIT_BUFFER(state,nbits,action) \
nuclear@0 139 { if (bits_left < (nbits)) { \
nuclear@0 140 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \
nuclear@0 141 { action; } \
nuclear@0 142 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
nuclear@0 143
nuclear@0 144 #define GET_BITS(nbits) \
nuclear@0 145 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
nuclear@0 146
nuclear@0 147 #define PEEK_BITS(nbits) \
nuclear@0 148 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1))
nuclear@0 149
nuclear@0 150 #define DROP_BITS(nbits) \
nuclear@0 151 (bits_left -= (nbits))
nuclear@0 152
nuclear@0 153 /* Load up the bit buffer to a depth of at least nbits */
nuclear@0 154 EXTERN(boolean) jpeg_fill_bit_buffer
nuclear@0 155 JPP((bitread_working_state * state, register bit_buf_type get_buffer,
nuclear@0 156 register int bits_left, int nbits));
nuclear@0 157
nuclear@0 158
nuclear@0 159 /*
nuclear@0 160 * Code for extracting next Huffman-coded symbol from input bit stream.
nuclear@0 161 * Again, this is time-critical and we make the main paths be macros.
nuclear@0 162 *
nuclear@0 163 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
nuclear@0 164 * without looping. Usually, more than 95% of the Huffman codes will be 8
nuclear@0 165 * or fewer bits long. The few overlength codes are handled with a loop,
nuclear@0 166 * which need not be inline code.
nuclear@0 167 *
nuclear@0 168 * Notes about the HUFF_DECODE macro:
nuclear@0 169 * 1. Near the end of the data segment, we may fail to get enough bits
nuclear@0 170 * for a lookahead. In that case, we do it the hard way.
nuclear@0 171 * 2. If the lookahead table contains no entry, the next code must be
nuclear@0 172 * more than HUFF_LOOKAHEAD bits long.
nuclear@0 173 * 3. jpeg_huff_decode returns -1 if forced to suspend.
nuclear@0 174 */
nuclear@0 175
nuclear@0 176 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
nuclear@0 177 { register int nb, look; \
nuclear@0 178 if (bits_left < HUFF_LOOKAHEAD) { \
nuclear@0 179 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
nuclear@0 180 get_buffer = state.get_buffer; bits_left = state.bits_left; \
nuclear@0 181 if (bits_left < HUFF_LOOKAHEAD) { \
nuclear@0 182 nb = 1; goto slowlabel; \
nuclear@0 183 } \
nuclear@0 184 } \
nuclear@0 185 look = PEEK_BITS(HUFF_LOOKAHEAD); \
nuclear@0 186 if ((nb = htbl->look_nbits[look]) != 0) { \
nuclear@0 187 DROP_BITS(nb); \
nuclear@0 188 result = htbl->look_sym[look]; \
nuclear@0 189 } else { \
nuclear@0 190 nb = HUFF_LOOKAHEAD+1; \
nuclear@0 191 slowlabel: \
nuclear@0 192 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
nuclear@0 193 { failaction; } \
nuclear@0 194 get_buffer = state.get_buffer; bits_left = state.bits_left; \
nuclear@0 195 } \
nuclear@0 196 }
nuclear@0 197
nuclear@0 198 /* Out-of-line case for Huffman code fetching */
nuclear@0 199 EXTERN(int) jpeg_huff_decode
nuclear@0 200 JPP((bitread_working_state * state, register bit_buf_type get_buffer,
nuclear@0 201 register int bits_left, d_derived_tbl * htbl, int min_bits));