src/jpeg/jdhuff.h

   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   JPEG_INT32 maxcode[18];               /* largest code of length k (-1 if none) */
  29   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
  30   JPEG_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 JPEG_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));