compiler/libjpeg/main/jdhuff.h

   1 /*
   2     $Id$
   3 */
   4
   5 /*
   6  * jdhuff.h
   7  *
   8  * Copyright (C) 1991-1998, Thomas G. Lane.
   9  * This file is part of the Independent JPEG Group's software.
  10  * For conditions of distribution and use, see the accompanying README file.
  11  *
  12  * This file contains declarations for Huffman entropy decoding routines
  13  * that are shared between the sequential decoder (jdhuff.c), the
  14  * progressive decoder (jdphuff.c) and the lossless decoder (jdlhuff.c).
  15  * No other modules need to see these.
  16  */
  17
  18 /* Short forms of external names for systems with brain-damaged linkers. */
  19
  20 #ifdef NEED_SHORT_EXTERNAL_NAMES
  21 #define jpeg_make_d_derived_tbl jMkDDerived
  22 #define jpeg_fill_bit_buffer    jFilBitBuf
  23 #define jpeg_huff_decode        jHufDecode
  24 #endif /* NEED_SHORT_EXTERNAL_NAMES */
  25
  26
  27 /* Derived data constructed for each Huffman table */
  28
  29 #define HUFF_LOOKAHEAD  8       /* # of bits of lookahead */
  30
  31 typedef struct {
  32   /* Basic tables: (element [0] of each array is unused) */
  33   INT32 maxcode[18];            /* largest code of length k (-1 if none) */
  34   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
  35   INT32 valoffset[17];          /* huffval[] offset for codes of length k */
  36   /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
  37    * the smallest code of length k; so given a code of length k, the
  38    * corresponding symbol is huffval[code + valoffset[k]]
  39    */
  40
  41   /* Link to public Huffman table (needed only in jpeg_huff_decode) */
  42   JHUFF_TBL *pub;
  43
  44   /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
  45    * the input data stream.  If the next Huffman code is no more
  46    * than HUFF_LOOKAHEAD bits long, we can obtain its length and
  47    * the corresponding symbol directly from these tables.
  48    */
  49   int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
  50   UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
  51 } d_derived_tbl;
  52
  53 /* Expand a Huffman table definition into the derived format */
  54 JEXTERN(void) jpeg_make_d_derived_tbl
  55         JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
  56              d_derived_tbl ** pdtbl));
  57
  58
  59 /*
  60  * Fetching the next N bits from the input stream is a time-critical operation
  61  * for the Huffman decoders.  We implement it with a combination of inline
  62  * macros and out-of-line subroutines.  Note that N (the number of bits
  63  * demanded at one time) never exceeds 15 for JPEG use.
  64  *
  65  * We read source bytes into get_buffer and dole out bits as needed.
  66  * If get_buffer already contains enough bits, they are fetched in-line
  67  * by the macros CHECK_BIT_BUFFER and GET_BITS.  When there aren't enough
  68  * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
  69  * as full as possible (not just to the number of bits needed; this
  70  * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
  71  * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
  72  * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
  73  * at least the requested number of bits --- dummy zeroes are inserted if
  74  * necessary.
  75  */
  76
  77 typedef INT32 bit_buf_type;     /* type of bit-extraction buffer */
  78 #define BIT_BUF_SIZE  32        /* size of buffer in bits */
  79
  80 /* If long is > 32 bits on your machine, and shifting/masking longs is
  81  * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
  82  * appropriately should be a win.  Unfortunately we can't define the size
  83  * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
  84  * because not all machines measure sizeof in 8-bit bytes.
  85  */
  86
  87 typedef struct {                /* Bitreading state saved across MCUs */
  88   bit_buf_type get_buffer;      /* current bit-extraction buffer */
  89   int bits_left;                /* # of unused bits in it */
  90 } bitread_perm_state;
  91
  92 typedef struct {                /* Bitreading working state within an MCU */
  93   /* Current data source location */
  94   /* We need a copy, rather than munging the original, in case of suspension */
  95   const JOCTET * next_input_byte; /* => next byte to read from source */
  96   size_t bytes_in_buffer;       /* # of bytes remaining in source buffer */
  97   /* Bit input buffer --- note these values are kept in register variables,
  98    * not in this struct, inside the inner loops.
  99    */
 100   bit_buf_type get_buffer;      /* current bit-extraction buffer */
 101   int bits_left;                /* # of unused bits in it */
 102   /* Pointer needed by jpeg_fill_bit_buffer. */
 103   j_decompress_ptr cinfo;       /* back link to decompress master record */
 104 } bitread_working_state;
 105
 106 /* Macros to declare and load/save bitread local variables. */
 107 #define BITREAD_STATE_VARS  \
 108         register bit_buf_type get_buffer;  \
 109         register int bits_left;  \
 110         bitread_working_state br_state
 111
 112 #define BITREAD_LOAD_STATE(cinfop,permstate)  \
 113         br_state.cinfo = cinfop; \
 114         br_state.next_input_byte = cinfop->src->next_input_byte; \
 115         br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
 116         get_buffer = permstate.get_buffer; \
 117         bits_left = permstate.bits_left;
 118
 119 #define BITREAD_SAVE_STATE(cinfop,permstate)  \
 120         cinfop->src->next_input_byte = br_state.next_input_byte; \
 121         cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
 122         permstate.get_buffer = get_buffer; \
 123         permstate.bits_left = bits_left
 124
 125 /*
 126  * These macros provide the in-line portion of bit fetching.
 127  * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
 128  * before using GET_BITS, PEEK_BITS, or DROP_BITS.
 129  * The variables get_buffer and bits_left are assumed to be locals,
 130  * but the state struct might not be (jpeg_huff_decode needs this).
 131  *      CHECK_BIT_BUFFER(state,n,action);
 132  *              Ensure there are N bits in get_buffer; if suspend, take action.
 133  *      val = GET_BITS(n);
 134  *              Fetch next N bits.
 135  *      val = PEEK_BITS(n);
 136  *              Fetch next N bits without removing them from the buffer.
 137  *      DROP_BITS(n);
 138  *              Discard next N bits.
 139  * The value N should be a simple variable, not an expression, because it
 140  * is evaluated multiple times.
 141  */
 142
 143 #define CHECK_BIT_BUFFER(state,nbits,action) \
 144         { if (bits_left < (nbits)) {  \
 145             if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits))  \
 146               { action; }  \
 147             get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
 148
 149 #define GET_BITS(nbits) \
 150         (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
 151
 152 #define PEEK_BITS(nbits) \
 153         (((int) (get_buffer >> (bits_left -  (nbits)))) & ((1<<(nbits))-1))
 154
 155 #define DROP_BITS(nbits) \
 156         (bits_left -= (nbits))
 157
 158 /* Load up the bit buffer to a depth of at least nbits */
 159 JEXTERN(boolean) jpeg_fill_bit_buffer
 160         JPP((bitread_working_state * state, register bit_buf_type get_buffer,
 161              register int bits_left, int nbits));
 162
 163
 164 /*
 165  * Code for extracting next Huffman-coded symbol from input bit stream.
 166  * Again, this is time-critical and we make the main paths be macros.
 167  *
 168  * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
 169  * without looping.  Usually, more than 95% of the Huffman codes will be 8
 170  * or fewer bits long.  The few overlength codes are handled with a loop,
 171  * which need not be inline code.
 172  *
 173  * Notes about the HUFF_DECODE macro:
 174  * 1. Near the end of the data segment, we may fail to get enough bits
 175  *    for a lookahead.  In that case, we do it the hard way.
 176  * 2. If the lookahead table contains no entry, the next code must be
 177  *    more than HUFF_LOOKAHEAD bits long.
 178  * 3. jpeg_huff_decode returns -1 if forced to suspend.
 179  */
 180
 181 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
 182 { register int nb, look; \
 183   if (bits_left < HUFF_LOOKAHEAD) { \
 184     if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
 185     get_buffer = state.get_buffer; bits_left = state.bits_left; \
 186     if (bits_left < HUFF_LOOKAHEAD) { \
 187       nb = 1; goto slowlabel; \
 188     } \
 189   } \
 190   look = PEEK_BITS(HUFF_LOOKAHEAD); \
 191   if ((nb = htbl->look_nbits[look]) != 0) { \
 192     DROP_BITS(nb); \
 193     result = htbl->look_sym[look]; \
 194   } else { \
 195     nb = HUFF_LOOKAHEAD+1; \
 196 slowlabel: \
 197     if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
 198         { failaction; } \
 199     get_buffer = state.get_buffer; bits_left = state.bits_left; \
 200   } \
 201 }
 202
 203 /* Out-of-line case for Huffman code fetching */
 204 JEXTERN(int) jpeg_huff_decode
 205         JPP((bitread_working_state * state, register bit_buf_type get_buffer,
 206              register int bits_left, d_derived_tbl * htbl, int min_bits));
 207
 208
 209 /* Common fields between sequential, progressive and lossless Huffman entropy
 210  * decoder master structs.
 211  */
 212
 213 #define huffd_common_fields \
 214   boolean insufficient_data;    /* set TRUE after emmitting warning */ \
 215   /* These fields are loaded into local variables at start of each MCU. \
 216    * In case of suspension, we exit WITHOUT updating them. \
 217    */ \
 218   bitread_perm_state bitstate   /* Bit buffer at start of MCU */
 219
 220 /* Routines that are to be used by any or all of the entropy decoders are
 221  * declared to receive a pointer to this structure.  There are no actual
 222  * instances of huffd_common_struct, only of shuff_entropy_decoder,
 223  * phuff_entropy_decoder and lhuff_entropy_decoder.
 224  */
 225 struct huffd_common_struct {
 226   huffd_common_fields;          /* Fields common to all decoder struct types */
 227   /* Additional fields follow in an actual shuff_entropy_decoder,
 228    * phuff_entropy_decoder or lhuff_entropy_decoder struct.  All four structs
 229    * must agree on these initial fields!  (This would be a lot cleaner in C++.)
 230    */
 231 };
 232
 233 typedef struct huffd_common_struct * huffd_common_ptr;