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/*

 * 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) */

  INT32 maxcode[18];            /* largest code of length k (-1 if none) */

  /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */

  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 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));