    /*
     * jdhuff.h
     *
     * Copyright (C) 1991-1997, 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) and the
     * progressive decoder (jdphuff.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	jMkDDerived
    #define jpeg_fill_bit_buffer	jFilBitBuf
    #define jpeg_huff_decode	jHufDecode
    #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) */
      long maxcode[18];		/* largest code of length k (-1 if none) */
      /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
      long 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 */
      unsigned char 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, int 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 long 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(int) 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));