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src/Source/LibJPEG/jdhuff.c  view on Meta::CPAN

#define CHECK_BIT_BUFFER(state,nbits,action) \
	{ if (bits_left < (nbits)) {  \
	    if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits))  \
	      { action; }  \
	    get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }

#define GET_BITS(nbits) \
	(((int) (get_buffer >> (bits_left -= (nbits)))) & BIT_MASK(nbits))

#define PEEK_BITS(nbits) \
	(((int) (get_buffer >> (bits_left -  (nbits)))) & BIT_MASK(nbits))

#define DROP_BITS(nbits) \
	(bits_left -= (nbits))


/*
 * Code for extracting next Huffman-coded symbol from input bit stream.
 * Again, this is time-critical and we make the main paths be macros.
 *
 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
 * without looping.  Usually, more than 95% of the Huffman codes will be 8
 * or fewer bits long.  The few overlength codes are handled with a loop,
 * which need not be inline code.
 *
 * Notes about the HUFF_DECODE macro:
 * 1. Near the end of the data segment, we may fail to get enough bits
 *    for a lookahead.  In that case, we do it the hard way.
 * 2. If the lookahead table contains no entry, the next code must be
 *    more than HUFF_LOOKAHEAD bits long.
 * 3. jpeg_huff_decode returns -1 if forced to suspend.
 */

#define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
{ register int nb, look; \
  if (bits_left < HUFF_LOOKAHEAD) { \
    if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
    get_buffer = state.get_buffer; bits_left = state.bits_left; \
    if (bits_left < HUFF_LOOKAHEAD) { \
      nb = 1; goto slowlabel; \
    } \
  } \
  look = PEEK_BITS(HUFF_LOOKAHEAD); \
  if ((nb = htbl->look_nbits[look]) != 0) { \
    DROP_BITS(nb); \
    result = htbl->look_sym[look]; \
  } else { \
    nb = HUFF_LOOKAHEAD+1; \
slowlabel: \
    if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
	{ failaction; } \
    get_buffer = state.get_buffer; bits_left = state.bits_left; \
  } \
}


/*
 * Expanded entropy decoder object for Huffman decoding.
 *
 * The savable_state subrecord contains fields that change within an MCU,
 * but must not be updated permanently until we complete the MCU.
 */

typedef struct {
  unsigned int EOBRUN;			/* remaining EOBs in EOBRUN */
  int last_dc_val[MAX_COMPS_IN_SCAN];	/* last DC coef for each component */
} savable_state;

/* This macro is to work around compilers with missing or broken
 * structure assignment.  You'll need to fix this code if you have
 * such a compiler and you change MAX_COMPS_IN_SCAN.
 */

#ifndef NO_STRUCT_ASSIGN
#define ASSIGN_STATE(dest,src)  ((dest) = (src))
#else
#if MAX_COMPS_IN_SCAN == 4
#define ASSIGN_STATE(dest,src)  \
	((dest).EOBRUN = (src).EOBRUN, \
	 (dest).last_dc_val[0] = (src).last_dc_val[0], \
	 (dest).last_dc_val[1] = (src).last_dc_val[1], \
	 (dest).last_dc_val[2] = (src).last_dc_val[2], \
	 (dest).last_dc_val[3] = (src).last_dc_val[3])
#endif
#endif


typedef struct {
  struct jpeg_entropy_decoder pub; /* public fields */

  /* These fields are loaded into local variables at start of each MCU.
   * In case of suspension, we exit WITHOUT updating them.
   */
  bitread_perm_state bitstate;	/* Bit buffer at start of MCU */
  savable_state saved;		/* Other state at start of MCU */

  /* These fields are NOT loaded into local working state. */
  boolean insufficient_data;	/* set TRUE after emitting warning */
  unsigned int restarts_to_go;	/* MCUs left in this restart interval */

  /* Following two fields used only in progressive mode */

  /* Pointers to derived tables (these workspaces have image lifespan) */
  d_derived_tbl * derived_tbls[NUM_HUFF_TBLS];

  d_derived_tbl * ac_derived_tbl; /* active table during an AC scan */

  /* Following fields used only in sequential mode */

  /* Pointers to derived tables (these workspaces have image lifespan) */
  d_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
  d_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];

  /* Precalculated info set up by start_pass for use in decode_mcu: */

  /* Pointers to derived tables to be used for each block within an MCU */
  d_derived_tbl * dc_cur_tbls[D_MAX_BLOCKS_IN_MCU];
  d_derived_tbl * ac_cur_tbls[D_MAX_BLOCKS_IN_MCU];
  /* Whether we care about the DC and AC coefficient values for each block */
  int coef_limit[D_MAX_BLOCKS_IN_MCU];
} huff_entropy_decoder;