src/3rdparty/libjpeg/jdhuff.h
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     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   INT32 maxcode[18];		/* largest code of length k (-1 if none) */
       
    29   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
       
    30   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 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));