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 
00018 
00019 #ifdef NEED_SHORT_EXTERNAL_NAMES
00020 #define jpeg_make_d_derived_tbl jMkDDerived
00021 #define jpeg_fill_bit_buffer    jFilBitBuf
00022 #define jpeg_huff_decode    jHufDecode
00023 #endif /* NEED_SHORT_EXTERNAL_NAMES */
00024 
00025 
00026 /* Derived data constructed for each Huffman table */
00027 
00028 #define HUFF_LOOKAHEAD  8   /* # of bits of lookahead */
00029 
00030 typedef struct {
00031   /* Basic tables: (element [0] of each array is unused) */
00032   INT32 maxcode[18];        /* largest code of length k (-1 if none) */
00033   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
00034   INT32 valoffset[17];      /* huffval[] offset for codes of length k */
00035   /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
00036    * the smallest code of length k; so given a code of length k, the
00037    * corresponding symbol is huffval[code + valoffset[k]]
00038    */
00039 
00040   /* Link to public Huffman table (needed only in jpeg_huff_decode) */
00041   JHUFF_TBL *pub;
00042 
00043   /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
00044    * the input data stream.  If the next Huffman code is no more
00045    * than HUFF_LOOKAHEAD bits long, we can obtain its length and
00046    * the corresponding symbol directly from these tables.
00047    */
00048   int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
00049   UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
00050 } d_derived_tbl;
00051 
00052 /* Expand a Huffman table definition into the derived format */
00053 EXTERN(void) jpeg_make_d_derived_tbl
00054     JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
00055          d_derived_tbl ** pdtbl));
00056 
00057 
00058 /*
00059  * Fetching the next N bits from the input stream is a time-critical operation
00060  * for the Huffman decoders.  We implement it with a combination of inline
00061  * macros and out-of-line subroutines.  Note that N (the number of bits
00062  * demanded at one time) never exceeds 15 for JPEG use.
00063  *
00064  * We read source bytes into get_buffer and dole out bits as needed.
00065  * If get_buffer already contains enough bits, they are fetched in-line
00066  * by the macros CHECK_BIT_BUFFER and GET_BITS.  When there aren't enough
00067  * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
00068  * as full as possible (not just to the number of bits needed; this
00069  * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
00070  * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
00071  * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
00072  * at least the requested number of bits --- dummy zeroes are inserted if
00073  * necessary.
00074  */
00075 
00076 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */
00077 #define BIT_BUF_SIZE  32    /* size of buffer in bits */
00078 
00079 /* If long is > 32 bits on your machine, and shifting/masking longs is
00080  * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
00081  * appropriately should be a win.  Unfortunately we can't define the size
00082  * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
00083  * because not all machines measure sizeof in 8-bit bytes.
00084  */
00085 
00086 typedef struct {        /* Bitreading state saved across MCUs */
00087   bit_buf_type get_buffer;  /* current bit-extraction buffer */
00088   int bits_left;        /* # of unused bits in it */
00089 } bitread_perm_state;
00090 
00091 typedef struct {        /* Bitreading working state within an MCU */
00092   /* Current data source location */
00093   /* We need a copy, rather than munging the original, in case of suspension */
00094   const JOCTET * next_input_byte; /* => next byte to read from source */
00095   size_t bytes_in_buffer;   /* # of bytes remaining in source buffer */
00096   /* Bit input buffer --- note these values are kept in register variables,
00097    * not in this struct, inside the inner loops.
00098    */
00099   bit_buf_type get_buffer;  /* current bit-extraction buffer */
00100   int bits_left;        /* # of unused bits in it */
00101   /* Pointer needed by jpeg_fill_bit_buffer. */
00102   j_decompress_ptr cinfo;   /* back link to decompress master record */
00103 } bitread_working_state;
00104 
00105 /* Macros to declare and load/save bitread local variables. */
00106 #define BITREAD_STATE_VARS  \
00107     register bit_buf_type get_buffer;  \
00108     register int bits_left;  \
00109     bitread_working_state br_state
00110 
00111 #define BITREAD_LOAD_STATE(cinfop,permstate)  \
00112     br_state.cinfo = cinfop; \
00113     br_state.next_input_byte = cinfop->src->next_input_byte; \
00114     br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
00115     get_buffer = permstate.get_buffer; \
00116     bits_left = permstate.bits_left;
00117 
00118 #define BITREAD_SAVE_STATE(cinfop,permstate)  \
00119     cinfop->src->next_input_byte = br_state.next_input_byte; \
00120     cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
00121     permstate.get_buffer = get_buffer; \
00122     permstate.bits_left = bits_left
00123 
00124 /*
00125  * These macros provide the in-line portion of bit fetching.
00126  * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
00127  * before using GET_BITS, PEEK_BITS, or DROP_BITS.
00128  * The variables get_buffer and bits_left are assumed to be locals,
00129  * but the state struct might not be (jpeg_huff_decode needs this).
00130  *  CHECK_BIT_BUFFER(state,n,action);
00131  *      Ensure there are N bits in get_buffer; if suspend, take action.
00132  *      val = GET_BITS(n);
00133  *      Fetch next N bits.
00134  *      val = PEEK_BITS(n);
00135  *      Fetch next N bits without removing them from the buffer.
00136  *  DROP_BITS(n);
00137  *      Discard next N bits.
00138  * The value N should be a simple variable, not an expression, because it
00139  * is evaluated multiple times.
00140  */
00141 
00142 #define CHECK_BIT_BUFFER(state,nbits,action) \
00143     { if (bits_left < (nbits)) {  \
00144         if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits))  \
00145           { action; }  \
00146         get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
00147 
00148 #define GET_BITS(nbits) \
00149     (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
00150 
00151 #define PEEK_BITS(nbits) \
00152     (((int) (get_buffer >> (bits_left -  (nbits)))) & ((1<<(nbits))-1))
00153 
00154 #define DROP_BITS(nbits) \
00155     (bits_left -= (nbits))
00156 
00157 /* Load up the bit buffer to a depth of at least nbits */
00158 EXTERN(boolean) jpeg_fill_bit_buffer
00159     JPP((bitread_working_state * state, register bit_buf_type get_buffer,
00160          register int bits_left, int nbits));
00161 
00162 
00163 /*
00164  * Code for extracting next Huffman-coded symbol from input bit stream.
00165  * Again, this is time-critical and we make the main paths be macros.
00166  *
00167  * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
00168  * without looping.  Usually, more than 95% of the Huffman codes will be 8
00169  * or fewer bits long.  The few overlength codes are handled with a loop,
00170  * which need not be inline code.
00171  *
00172  * Notes about the HUFF_DECODE macro:
00173  * 1. Near the end of the data segment, we may fail to get enough bits
00174  *    for a lookahead.  In that case, we do it the hard way.
00175  * 2. If the lookahead table contains no entry, the next code must be
00176  *    more than HUFF_LOOKAHEAD bits long.
00177  * 3. jpeg_huff_decode returns -1 if forced to suspend.
00178  */
00179 
00180 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
00181 { register int nb, look; \
00182   if (bits_left < HUFF_LOOKAHEAD) { \
00183     if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
00184     get_buffer = state.get_buffer; bits_left = state.bits_left; \
00185     if (bits_left < HUFF_LOOKAHEAD) { \
00186       nb = 1; goto slowlabel; \
00187     } \
00188   } \
00189   look = PEEK_BITS(HUFF_LOOKAHEAD); \
00190   if ((nb = htbl->look_nbits[look]) != 0) { \
00191     DROP_BITS(nb); \
00192     result = htbl->look_sym[look]; \
00193   } else { \
00194     nb = HUFF_LOOKAHEAD+1; \
00195 slowlabel: \
00196     if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
00197     { failaction; } \
00198     get_buffer = state.get_buffer; bits_left = state.bits_left; \
00199   } \
00200 }
00201 
00202 /* Out-of-line case for Huffman code fetching */
00203 EXTERN(int) jpeg_huff_decode
00204     JPP((bitread_working_state * state, register bit_buf_type get_buffer,
00205          register int bits_left, d_derived_tbl * htbl, int min_bits));

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