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jdhuff.h
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00001 /*
00002  * jdhuff.h
00003  *
00004  * Copyright (C) 1991-1997, Thomas G. Lane.
00005  * This file is part of the Independent JPEG Group's software.
00006  * For conditions of distribution and use, see the accompanying README file.
00007  *
00008  * This file contains declarations for Huffman entropy decoding routines
00009  * that are shared between the sequential decoder (jdhuff.c) and the
00010  * progressive decoder (jdphuff.c).  No other modules need to see these.
00011  */
00012 
00013 /* Short forms of external names for systems with brain-damaged linkers. */
00014 
00015 #ifdef NEED_SHORT_EXTERNAL_NAMES
00016 #define jpeg_make_d_derived_tbl    jMkDDerived
00017 #define jpeg_fill_bit_buffer       jFilBitBuf
00018 #define jpeg_huff_decode    jHufDecode
00019 #endif /* NEED_SHORT_EXTERNAL_NAMES */
00020 
00021 
00022 /* Derived data constructed for each Huffman table */
00023 
00024 #define HUFF_LOOKAHEAD      8      /* # of bits of lookahead */
00025 
00026 typedef struct {
00027   /* Basic tables: (element [0] of each array is unused) */
00028   INT32 maxcode[18];        /* largest code of length k (-1 if none) */
00029   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
00030   INT32 valoffset[17];             /* huffval[] offset for codes of length k */
00031   /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
00032    * the smallest code of length k; so given a code of length k, the
00033    * corresponding symbol is huffval[code + valoffset[k]]
00034    */
00035 
00036   /* Link to public Huffman table (needed only in jpeg_huff_decode) */
00037   JHUFF_TBL *pub;
00038 
00039   /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
00040    * the input data stream.  If the next Huffman code is no more
00041    * than HUFF_LOOKAHEAD bits long, we can obtain its length and
00042    * the corresponding symbol directly from these tables.
00043    */
00044   int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
00045   UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
00046 } d_derived_tbl;
00047 
00048 /* Expand a Huffman table definition into the derived format */
00049 EXTERN(void) jpeg_make_d_derived_tbl
00050        JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
00051             d_derived_tbl ** pdtbl));
00052 
00053 
00054 /*
00055  * Fetching the next N bits from the input stream is a time-critical operation
00056  * for the Huffman decoders.  We implement it with a combination of inline
00057  * macros and out-of-line subroutines.  Note that N (the number of bits
00058  * demanded at one time) never exceeds 15 for JPEG use.
00059  *
00060  * We read source bytes into get_buffer and dole out bits as needed.
00061  * If get_buffer already contains enough bits, they are fetched in-line
00062  * by the macros CHECK_BIT_BUFFER and GET_BITS.  When there aren't enough
00063  * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
00064  * as full as possible (not just to the number of bits needed; this
00065  * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
00066  * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
00067  * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
00068  * at least the requested number of bits --- dummy zeroes are inserted if
00069  * necessary.
00070  */
00071 
00072 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */
00073 #define BIT_BUF_SIZE  32    /* size of buffer in bits */
00074 
00075 /* If long is > 32 bits on your machine, and shifting/masking longs is
00076  * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
00077  * appropriately should be a win.  Unfortunately we can't define the size
00078  * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
00079  * because not all machines measure sizeof in 8-bit bytes.
00080  */
00081 
00082 typedef struct {            /* Bitreading state saved across MCUs */
00083   bit_buf_type get_buffer;  /* current bit-extraction buffer */
00084   int bits_left;            /* # of unused bits in it */
00085 } bitread_perm_state;
00086 
00087 typedef struct {            /* Bitreading working state within an MCU */
00088   /* Current data source location */
00089   /* We need a copy, rather than munging the original, in case of suspension */
00090   const JOCTET * next_input_byte; /* => next byte to read from source */
00091   size_t bytes_in_buffer;   /* # of bytes remaining in source buffer */
00092   /* Bit input buffer --- note these values are kept in register variables,
00093    * not in this struct, inside the inner loops.
00094    */
00095   bit_buf_type get_buffer;  /* current bit-extraction buffer */
00096   int bits_left;            /* # of unused bits in it */
00097   /* Pointer needed by jpeg_fill_bit_buffer. */
00098   j_decompress_ptr cinfo;   /* back link to decompress master record */
00099 } bitread_working_state;
00100 
00101 /* Macros to declare and load/save bitread local variables. */
00102 #define BITREAD_STATE_VARS  \
00103        register bit_buf_type get_buffer;  \
00104        register int bits_left;  \
00105        bitread_working_state br_state
00106 
00107 #define BITREAD_LOAD_STATE(cinfop,permstate)  \
00108        br_state.cinfo = cinfop; \
00109        br_state.next_input_byte = cinfop->src->next_input_byte; \
00110        br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
00111        get_buffer = permstate.get_buffer; \
00112        bits_left = permstate.bits_left;
00113 
00114 #define BITREAD_SAVE_STATE(cinfop,permstate)  \
00115        cinfop->src->next_input_byte = br_state.next_input_byte; \
00116        cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
00117        permstate.get_buffer = get_buffer; \
00118        permstate.bits_left = bits_left
00119 
00120 /*
00121  * These macros provide the in-line portion of bit fetching.
00122  * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
00123  * before using GET_BITS, PEEK_BITS, or DROP_BITS.
00124  * The variables get_buffer and bits_left are assumed to be locals,
00125  * but the state struct might not be (jpeg_huff_decode needs this).
00126  *     CHECK_BIT_BUFFER(state,n,action);
00127  *            Ensure there are N bits in get_buffer; if suspend, take action.
00128  *      val = GET_BITS(n);
00129  *            Fetch next N bits.
00130  *      val = PEEK_BITS(n);
00131  *            Fetch next N bits without removing them from the buffer.
00132  *     DROP_BITS(n);
00133  *            Discard next N bits.
00134  * The value N should be a simple variable, not an expression, because it
00135  * is evaluated multiple times.
00136  */
00137 
00138 #define CHECK_BIT_BUFFER(state,nbits,action) \
00139        { if (bits_left < (nbits)) {  \
00140            if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits))  \
00141              { action; }  \
00142            get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
00143 
00144 #define GET_BITS(nbits) \
00145        (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
00146 
00147 #define PEEK_BITS(nbits) \
00148        (((int) (get_buffer >> (bits_left -  (nbits)))) & ((1<<(nbits))-1))
00149 
00150 #define DROP_BITS(nbits) \
00151        (bits_left -= (nbits))
00152 
00153 /* Load up the bit buffer to a depth of at least nbits */
00154 EXTERN(boolean) jpeg_fill_bit_buffer
00155        JPP((bitread_working_state * state, register bit_buf_type get_buffer,
00156             register int bits_left, int nbits));
00157 
00158 
00159 /*
00160  * Code for extracting next Huffman-coded symbol from input bit stream.
00161  * Again, this is time-critical and we make the main paths be macros.
00162  *
00163  * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
00164  * without looping.  Usually, more than 95% of the Huffman codes will be 8
00165  * or fewer bits long.  The few overlength codes are handled with a loop,
00166  * which need not be inline code.
00167  *
00168  * Notes about the HUFF_DECODE macro:
00169  * 1. Near the end of the data segment, we may fail to get enough bits
00170  *    for a lookahead.  In that case, we do it the hard way.
00171  * 2. If the lookahead table contains no entry, the next code must be
00172  *    more than HUFF_LOOKAHEAD bits long.
00173  * 3. jpeg_huff_decode returns -1 if forced to suspend.
00174  */
00175 
00176 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
00177 { register int nb, look; \
00178   if (bits_left < HUFF_LOOKAHEAD) { \
00179     if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
00180     get_buffer = state.get_buffer; bits_left = state.bits_left; \
00181     if (bits_left < HUFF_LOOKAHEAD) { \
00182       nb = 1; goto slowlabel; \
00183     } \
00184   } \
00185   look = PEEK_BITS(HUFF_LOOKAHEAD); \
00186   if ((nb = htbl->look_nbits[look]) != 0) { \
00187     DROP_BITS(nb); \
00188     result = htbl->look_sym[look]; \
00189   } else { \
00190     nb = HUFF_LOOKAHEAD+1; \
00191 slowlabel: \
00192     if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
00193        { failaction; } \
00194     get_buffer = state.get_buffer; bits_left = state.bits_left; \
00195   } \
00196 }
00197 
00198 /* Out-of-line case for Huffman code fetching */
00199 EXTERN(int) jpeg_huff_decode
00200        JPP((bitread_working_state * state, register bit_buf_type get_buffer,
00201             register int bits_left, d_derived_tbl * htbl, int min_bits));