text: /*
   * jdhuff.h
   *
   * Copyright (C) 1991-1998, Thomas G. Lane.
   * This file is part of the Independent JPEG Group's software.
   * For conditions of distribution and use, see the accompanying README file.
   *
   * This file contains declarations for Huffman entropy decoding routines
   * that are shared between the sequential decoder (jdhuff.c), the
   * progressive decoder (jdphuff.c) and the lossless decoder (jdlhuff.c).
   * No other modules need to see these.
   */
  
  /* Short forms of external names for systems with brain-damaged linkers. */
  
  #ifdef NEED_SHORT_EXTERNAL_NAMES
  #define jpeg_make_d_derived_tbl		jpeg8_make_d_derived_tbl
  #define jpeg_fill_bit_buffer		jpeg8_fill_bit_buffer
  #define jpeg_huff_decode		jpeg8_huff_decode
  #endif /* NEED_SHORT_EXTERNAL_NAMES */
  
  
  /* Derived data constructed for each Huffman table */
  
  #define HUFF_LOOKAHEAD	8	/* # of bits of lookahead */
  
  typedef struct {
    /* Basic tables: (element [0] of each array is unused) */
    IJG_INT32 maxcode[18];		/* largest code of length k (-1 if none) */
    /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
    IJG_INT32 valoffset[17];		/* huffval[] offset for codes of length k */
    /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
     * the smallest code of length k; so given a code of length k, the
     * corresponding symbol is huffval[code + valoffset[k]]
     */
  
    /* Link to public Huffman table (needed only in jpeg_huff_decode) */
    JHUFF_TBL *pub;
  
    /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
     * the input data stream.  If the next Huffman code is no more
     * than HUFF_LOOKAHEAD bits long, we can obtain its length and
     * the corresponding symbol directly from these tables.
     */
    int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
    UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
  } d_derived_tbl;
  
  /* Expand a Huffman table definition into the derived format */
  EXTERN(void) jpeg_make_d_derived_tbl
  	JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
  	     d_derived_tbl ** pdtbl));
  
  
  /*
   * Fetching the next N bits from the input stream is a time-critical operation
   * for the Huffman decoders.  We implement it with a combination of inline
   * macros and out-of-line subroutines.  Note that N (the number of bits
   * demanded at one time) never exceeds 15 for JPEG use.
   *
   * We read source bytes into get_buffer and dole out bits as needed.
   * If get_buffer already contains enough bits, they are fetched in-line
   * by the macros CHECK_BIT_BUFFER and GET_BITS.  When there aren't enough
   * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
   * as full as possible (not just to the number of bits needed; this
   * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
   * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
   * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
   * at least the requested number of bits --- dummy zeroes are inserted if
   * necessary.
   */
  
  typedef IJG_INT32 bit_buf_type;	/* type of bit-extraction buffer */
  #define BIT_BUF_SIZE  32	/* size of buffer in bits */
  
  /* If long is > 32 bits on your machine, and shifting/masking longs is
   * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
   * appropriately should be a win.  Unfortunately we can't define the size
   * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
   * because not all machines measure sizeof in 8-bit bytes.
   */
  
  typedef struct {		/* Bitreading state saved across MCUs */
    bit_buf_type get_buffer;	/* current bit-extraction buffer */
    int bits_left;		/* # of unused bits in it */
  } bitread_perm_state;
  
  typedef struct {		/* Bitreading working state within an MCU */
    /* Current data source location */
    /* We need a copy, rather than munging the original, in case of suspension */
    const JOCTET * next_input_byte; /* => next byte to read from source */
    size_t bytes_in_buffer;	/* # of bytes remaining in source buffer */
    /* Bit input buffer --- note these values are kept in register variables,
     * not in this struct, inside the inner loops.
     */
    bit_buf_type get_buffer;	/* current bit-extraction buffer */
    int bits_left;		/* # of unused bits in it */
    /* Pointer needed by jpeg_fill_bit_buffer. */
    j_decompress_ptr cinfo;	/* back link to decompress master record */
  } bitread_working_state;
  
  /* Macros to declare and load/save bitread local variables. */
  #define BITREAD_STATE_VARS  \
  	register bit_buf_type get_buffer;  \
  	register int bits_left;  \
  	bitread_working_state br_state
  
  #define BITREAD_LOAD_STATE(cinfop,permstate)  \
  	br_state.cinfo = cinfop; \
  	br_state.next_input_byte = cinfop->src->next_input_byte; \
  	br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
  	get_buffer = permstate.get_buffer; \
  	bits_left = permstate.bits_left;
  
  #define BITREAD_SAVE_STATE(cinfop,permstate)  \
  	cinfop->src->next_input_byte = br_state.next_input_byte; \
  	cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
  	permstate.get_buffer = get_buffer; \
  	permstate.bits_left = bits_left
  
  /*
   * These macros provide the in-line portion of bit fetching.
   * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
   * before using GET_BITS, PEEK_BITS, or DROP_BITS.
   * The variables get_buffer and bits_left are assumed to be locals,
   * but the state struct might not be (jpeg_huff_decode needs this).
   *	CHECK_BIT_BUFFER(state,n,action);
   *		Ensure there are N bits in get_buffer; if suspend, take action.
   *      val = GET_BITS(n);
   *		Fetch next N bits.
   *      val = PEEK_BITS(n);
   *		Fetch next N bits without removing them from the buffer.
   *	DROP_BITS(n);
   *		Discard next N bits.
   * The value N should be a simple variable, not an expression, because it
   * is evaluated multiple times.
   */
  
  #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)))) & ((1<<(nbits))-1))
  
  #define PEEK_BITS(nbits) \
  	(((int) (get_buffer >> (bits_left -  (nbits)))) & ((1<<(nbits))-1))
  
  #define DROP_BITS(nbits) \
  	(bits_left -= (nbits))
  
  /* Load up the bit buffer to a depth of at least nbits */
  EXTERN(boolean) jpeg_fill_bit_buffer
  	JPP((bitread_working_state * state, register bit_buf_type get_buffer,
  	     register int bits_left, int 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; \
    } \
  }
  
  /* Out-of-line case for Huffman code fetching */
  EXTERN(int) jpeg_huff_decode
  	JPP((bitread_working_state * state, register bit_buf_type get_buffer,
  	     register int bits_left, d_derived_tbl * htbl, int min_bits));
  
  
  /* Common fields between sequential, progressive and lossless Huffman entropy
   * decoder master structs.
   */
  
  #define huffd_common_fields \
    boolean insufficient_data;	/* set TRUE after emmitting warning */ \
    /* 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 */
  
  /* Routines that are to be used by any or all of the entropy decoders are
   * declared to receive a pointer to this structure.  There are no actual
   * instances of huffd_common_struct, only of shuff_entropy_decoder,
   * phuff_entropy_decoder and lhuff_entropy_decoder.
   */
  struct huffd_common_struct {
    huffd_common_fields;		/* Fields common to all decoder struct types */
    /* Additional fields follow in an actual shuff_entropy_decoder,
     * phuff_entropy_decoder or lhuff_entropy_decoder struct.  All four structs
     * must agree on these initial fields!  (This would be a lot cleaner in C++.)
     */
  };
  
  typedef struct huffd_common_struct * huffd_common_ptr;
  