jdhuff.h

Go to the documentation of this file.

/*
 * 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_t maxcode[18];      /* largest code of length k (-1 if none) */
  /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
  int32_t 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_t 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_t 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));