Home | History | Annotate | Download | only in libjpeg
      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));
    202