dcmjpeg/libijg12/jdhuff12.h

00001 /*
00002  * jdhuff.h
00003  *
00004  * Copyright (C) 1991-1998, 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), the
00010  * progressive decoder (jdphuff.c) and the lossless decoder (jdlhuff.c).
00011  * No other modules need to see these.
00012  */
00013 
00014 /* Short forms of external names for systems with brain-damaged linkers. */
00015 
00016 #ifdef NEED_SHORT_EXTERNAL_NAMES
00017 #define jpeg_make_d_derived_tbl     jpeg12_make_d_derived_tbl
00018 #define jpeg_fill_bit_buffer        jpeg12_fill_bit_buffer
00019 #define jpeg_huff_decode        jpeg12_huff_decode
00020 #endif /* NEED_SHORT_EXTERNAL_NAMES */
00021 
00022 
00023 /* Derived data constructed for each Huffman table */
00024 
00025 #define HUFF_LOOKAHEAD  8   /* # of bits of lookahead */
00026 
00027 typedef struct {
00028   /* Basic tables: (element [0] of each array is unused) */
00029   IJG_INT32 maxcode[18];        /* largest code of length k (-1 if none) */
00030   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
00031   IJG_INT32 valoffset[17];      /* huffval[] offset for codes of length k */
00032   /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
00033    * the smallest code of length k; so given a code of length k, the
00034    * corresponding symbol is huffval[code + valoffset[k]]
00035    */
00036 
00037   /* Link to public Huffman table (needed only in jpeg_huff_decode) */
00038   JHUFF_TBL *pub;
00039 
00040   /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
00041    * the input data stream.  If the next Huffman code is no more
00042    * than HUFF_LOOKAHEAD bits long, we can obtain its length and
00043    * the corresponding symbol directly from these tables.
00044    */
00045   int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
00046   UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
00047 } d_derived_tbl;
00048 
00049 /* Expand a Huffman table definition into the derived format */
00050 EXTERN(void) jpeg_make_d_derived_tbl
00051     JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
00052          d_derived_tbl ** pdtbl));
00053 
00054 
00055 /*
00056  * Fetching the next N bits from the input stream is a time-critical operation
00057  * for the Huffman decoders.  We implement it with a combination of inline
00058  * macros and out-of-line subroutines.  Note that N (the number of bits
00059  * demanded at one time) never exceeds 15 for JPEG use.
00060  *
00061  * We read source bytes into get_buffer and dole out bits as needed.
00062  * If get_buffer already contains enough bits, they are fetched in-line
00063  * by the macros CHECK_BIT_BUFFER and GET_BITS.  When there aren't enough
00064  * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
00065  * as full as possible (not just to the number of bits needed; this
00066  * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
00067  * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
00068  * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
00069  * at least the requested number of bits --- dummy zeroes are inserted if
00070  * necessary.
00071  */
00072 
00073 typedef IJG_INT32 bit_buf_type; /* type of bit-extraction buffer */
00074 #define BIT_BUF_SIZE  32    /* size of buffer in bits */
00075 
00076 /* If long is > 32 bits on your machine, and shifting/masking longs is
00077  * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
00078  * appropriately should be a win.  Unfortunately we can't define the size
00079  * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
00080  * because not all machines measure sizeof in 8-bit bytes.
00081  */
00082 
00083 typedef struct {        /* Bitreading state saved across MCUs */
00084   bit_buf_type get_buffer;  /* current bit-extraction buffer */
00085   int bits_left;        /* # of unused bits in it */
00086 } bitread_perm_state;
00087 
00088 typedef struct {        /* Bitreading working state within an MCU */
00089   /* Current data source location */
00090   /* We need a copy, rather than munging the original, in case of suspension */
00091   const JOCTET * next_input_byte; /* => next byte to read from source */
00092   size_t bytes_in_buffer;   /* # of bytes remaining in source buffer */
00093   /* Bit input buffer --- note these values are kept in register variables,
00094    * not in this struct, inside the inner loops.
00095    */
00096   bit_buf_type get_buffer;  /* current bit-extraction buffer */
00097   int bits_left;        /* # of unused bits in it */
00098   /* Pointer needed by jpeg_fill_bit_buffer. */
00099   j_decompress_ptr cinfo;   /* back link to decompress master record */
00100 } bitread_working_state;
00101 
00102 /* Macros to declare and load/save bitread local variables. */
00103 #define BITREAD_STATE_VARS  \
00104     register bit_buf_type get_buffer;  \
00105     register int bits_left;  \
00106     bitread_working_state br_state
00107 
00108 #define BITREAD_LOAD_STATE(cinfop,permstate)  \
00109     br_state.cinfo = cinfop; \
00110     br_state.next_input_byte = cinfop->src->next_input_byte; \
00111     br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
00112     get_buffer = permstate.get_buffer; \
00113     bits_left = permstate.bits_left;
00114 
00115 #define BITREAD_SAVE_STATE(cinfop,permstate)  \
00116     cinfop->src->next_input_byte = br_state.next_input_byte; \
00117     cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
00118     permstate.get_buffer = get_buffer; \
00119     permstate.bits_left = bits_left
00120 
00121 /*
00122  * These macros provide the in-line portion of bit fetching.
00123  * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
00124  * before using GET_BITS, PEEK_BITS, or DROP_BITS.
00125  * The variables get_buffer and bits_left are assumed to be locals,
00126  * but the state struct might not be (jpeg_huff_decode needs this).
00127  *  CHECK_BIT_BUFFER(state,n,action);
00128  *      Ensure there are N bits in get_buffer; if suspend, take action.
00129  *      val = GET_BITS(n);
00130  *      Fetch next N bits.
00131  *      val = PEEK_BITS(n);
00132  *      Fetch next N bits without removing them from the buffer.
00133  *  DROP_BITS(n);
00134  *      Discard next N bits.
00135  * The value N should be a simple variable, not an expression, because it
00136  * is evaluated multiple times.
00137  */
00138 
00139 #define CHECK_BIT_BUFFER(state,nbits,action) \
00140     { if (bits_left < (nbits)) {  \
00141         if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits))  \
00142           { action; }  \
00143         get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
00144 
00145 #define GET_BITS(nbits) \
00146     (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
00147 
00148 #define PEEK_BITS(nbits) \
00149     (((int) (get_buffer >> (bits_left -  (nbits)))) & ((1<<(nbits))-1))
00150 
00151 #define DROP_BITS(nbits) \
00152     (bits_left -= (nbits))
00153 
00154 /* Load up the bit buffer to a depth of at least nbits */
00155 EXTERN(boolean) jpeg_fill_bit_buffer
00156     JPP((bitread_working_state * state, register bit_buf_type get_buffer,
00157          register int bits_left, int nbits));
00158 
00159 
00160 /*
00161  * Code for extracting next Huffman-coded symbol from input bit stream.
00162  * Again, this is time-critical and we make the main paths be macros.
00163  *
00164  * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
00165  * without looping.  Usually, more than 950f the Huffman codes will be 8
00166  * or fewer bits long.  The few overlength codes are handled with a loop,
00167  * which need not be inline code.
00168  *
00169  * Notes about the HUFF_DECODE macro:
00170  * 1. Near the end of the data segment, we may fail to get enough bits
00171  *    for a lookahead.  In that case, we do it the hard way.
00172  * 2. If the lookahead table contains no entry, the next code must be
00173  *    more than HUFF_LOOKAHEAD bits long.
00174  * 3. jpeg_huff_decode returns -1 if forced to suspend.
00175  */
00176 
00177 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
00178 { register int nb, look; \
00179   if (bits_left < HUFF_LOOKAHEAD) { \
00180     if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
00181     get_buffer = state.get_buffer; bits_left = state.bits_left; \
00182     if (bits_left < HUFF_LOOKAHEAD) { \
00183       nb = 1; goto slowlabel; \
00184     } \
00185   } \
00186   look = PEEK_BITS(HUFF_LOOKAHEAD); \
00187   if ((nb = htbl->look_nbits[look]) != 0) { \
00188     DROP_BITS(nb); \
00189     result = htbl->look_sym[look]; \
00190   } else { \
00191     nb = HUFF_LOOKAHEAD+1; \
00192 slowlabel: \
00193     if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
00194     { failaction; } \
00195     get_buffer = state.get_buffer; bits_left = state.bits_left; \
00196   } \
00197 }
00198 
00199 /* Out-of-line case for Huffman code fetching */
00200 EXTERN(int) jpeg_huff_decode
00201     JPP((bitread_working_state * state, register bit_buf_type get_buffer,
00202          register int bits_left, d_derived_tbl * htbl, int min_bits));
00203 
00204 
00205 /* Common fields between sequential, progressive and lossless Huffman entropy
00206  * decoder master structs.
00207  */
00208 
00209 #define huffd_common_fields \
00210   boolean insufficient_data;    /* set TRUE after emmitting warning */ \
00211   /* These fields are loaded into local variables at start of each MCU. \
00212    * In case of suspension, we exit WITHOUT updating them. \
00213    */ \
00214   bitread_perm_state bitstate   /* Bit buffer at start of MCU */
00215 
00216 /* Routines that are to be used by any or all of the entropy decoders are
00217  * declared to receive a pointer to this structure.  There are no actual
00218  * instances of huffd_common_struct, only of shuff_entropy_decoder,
00219  * phuff_entropy_decoder and lhuff_entropy_decoder.
00220  */
00221 struct huffd_common_struct {
00222   huffd_common_fields;      /* Fields common to all decoder struct types */
00223   /* Additional fields follow in an actual shuff_entropy_decoder,
00224    * phuff_entropy_decoder or lhuff_entropy_decoder struct.  All four structs
00225    * must agree on these initial fields!  (This would be a lot cleaner in C++.)
00226    */
00227 };
00228 
00229 typedef struct huffd_common_struct * huffd_common_ptr;


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