jdhuff.c

/*
 * jdhuff.c
 *
 * Copyright (C) 1991, 1992, 1993, 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 Huffman entropy decoding routines.
 * These routines are invoked via the methods entropy_decode
 * and entropy_decode_init/term.
 */

#include "jinclude.h"


/* Static variables to avoid passing 'round extra parameters */

static decompress_info_ptr dcinfo;

static INT32 get_buffer;	/* current bit-extraction buffer */
static int bits_left;		/* # of unused bits in it */
static boolean printed_eod;	/* flag to suppress multiple end-of-data msgs */

LOCAL void
fix_huff_tbl (HUFF_TBL * htbl)
/* Compute derived values for a Huffman table */
{
  int p, i, l, si;
  int lookbits, ctr;
  char huffsize[257];
  UINT16 huffcode[257];
  UINT16 code;
  
  /* Figure C.1: make table of Huffman code length for each symbol */
  /* Note that this is in code-length order. */

  p = 0;
  for (l = 1; l <= 16; l++) {
    for (i = 1; i <= (int) htbl->bits[l]; i++)
      huffsize[p++] = (char) l;
  }
  huffsize[p] = 0;
  
  /* Figure C.2: generate the codes themselves */
  /* Note that this is in code-length order. */
  
  code = 0;
  si = huffsize[0];
  p = 0;
  while (huffsize[p]) {
    while (((int) huffsize[p]) == si) {
      huffcode[p++] = code;
      code++;
    }
    code <<= 1;
    si++;
  }

  /* Figure F.15: generate decoding tables for bit-sequential decoding */

  p = 0;
  for (l = 1; l <= 16; l++) {
    if (htbl->bits[l]) {
      htbl->priv.dec.valptr[l] = p; /* huffval[] index of 1st symbol of code length l */
      htbl->priv.dec.mincode[l] = huffcode[p]; /* minimum code of length l */
      p += htbl->bits[l];
      htbl->priv.dec.maxcode[l] = huffcode[p-1]; /* maximum code of length l */
    } else {
      htbl->priv.dec.maxcode[l] = -1; /* -1 if no codes of this length */
    }
  }
  htbl->priv.dec.maxcode[17] = 0xFFFFFL; /* ensures huff_DECODE terminates */

  /* Compute lookahead tables to speed up decoding.
   * First we set all the table entries to 0, indicating "too long";
   * then we iterate through the Huffman codes that are short enough and
   * fill in all the entries that correspond to bit sequences starting
   * with that code.
   */

  MEMZERO(htbl->priv.dec.look_nbits, SIZEOF(htbl->priv.dec.look_nbits));

  p = 0;
  for (l = 1; l <= HUFF_LOOKAHEAD; l++) {
    for (i = 1; i <= (int) htbl->bits[l]; i++, p++) {
      /* l = current code's length, p = its index in huffcode[] & huffval[]. */
      /* Generate left-justified code followed by all possible bit sequences */
      lookbits = huffcode[p] << (HUFF_LOOKAHEAD-l);
      for (ctr = 1 << (HUFF_LOOKAHEAD-l); ctr > 0; ctr--) {
	htbl->priv.dec.look_nbits[lookbits] = l;
	htbl->priv.dec.look_sym[lookbits] = htbl->huffval[p];
	lookbits++;
      }
    }
  }
}


/*
 * Code for extracting the next N bits from the input stream.
 * (N never exceeds 15 for JPEG data.)
 * This needs to go as fast as possible!
 *
 * 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, fill_bit_buffer is called; it will attempt to fill get_buffer to
 * the "high water mark" (not just to the number of bits needed; this reduces
 * the function-call overhead cost of entering fill_bit_buffer).
 * On return, fill_bit_buffer guarantees that get_buffer contains at least
 * the requested number of bits --- dummy zeroes are inserted if necessary.
 *
 * On most machines MIN_GET_BITS should be 25 to allow the full 32-bit width
 * of get_buffer to be used.  (On machines with wider words, an even larger
 * buffer could be used.)  However, on some machines 32-bit shifts are
 * relatively slow and take time proportional to the number of places shifted.
 * (This is true with most PC compilers, for instance.)  In this case it may
 * be a win to set MIN_GET_BITS to the minimum value of 15.  This reduces the
 * average shift distance at the cost of more calls to fill_bit_buffer.
 */

#ifdef SLOW_SHIFT_32
#define MIN_GET_BITS  15	/* minimum allowable value */
#else
#define MIN_GET_BITS  25	/* max value for 32-bit get_buffer */
#endif


LOCAL void
fill_bit_buffer (int nbits)
/* Load up the bit buffer to a depth of at least nbits */
{
  /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
  /* (It is assumed that no request will be for more than that many bits.) */
  while (bits_left < MIN_GET_BITS) {
    register int c = JGETC(dcinfo);
    
    /* If it's 0xFF, check and discard stuffed zero byte */
    if (c == 0xFF) {
      int c2 = JGETC(dcinfo);
      if (c2 != 0) {
	/* Oops, it's actually a marker indicating end of compressed data. */
	/* Better put it back for use later */
	JUNGETC(c2,dcinfo);
	JUNGETC(c,dcinfo);
	/* There should be enough bits still left in the data segment; */
	/* if so, just break out of the while loop. */
	if (bits_left >= nbits)
	  break;
	/* Uh-oh.  Report corrupted data to user and stuff zeroes into
	 * the data stream, so that we can produce some kind of image.
	 * Note that this will be repeated for each byte demanded for the
	 * rest of the segment; this is a bit slow but not unreasonably so.
	 * The main thing is to avoid getting a zillion warnings, hence
	 * we use a flag to ensure that only one warning appears.
	 */
	if (! printed_eod) {
	  WARNMS(dcinfo->emethods, "Corrupt JPEG data: premature end of data segment");
	  printed_eod = TRUE;
	}
	c = 0;			/* insert a zero byte into bit buffer */
      }
    }

    /* OK, load c into get_buffer */
    get_buffer = (get_buffer << 8) | c;
    bits_left += 8;
  }
}


/*
 * These macros provide the in-line portion of bit fetching.
 * Correct usage is:
 *	check_bit_buffer(n);		ensure there are N bits in get_buffer
 *      val = get_bits(n);		fetch N bits
 * The value n should be a simple variable, not an expression, because it
 * is evaluated multiple times.
 * peek_bits() fetches next N bits without removing them from the buffer.
 */

#define check_bit_buffer(nbits) \
	{ if (bits_left < (nbits))  fill_bit_buffer(nbits); }

#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))


/*
 * Routines to extract next Huffman-coded symbol from input bit stream.
 * 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.
 * The primary case is made a macro for speed reasons; the secondary
 * routine slow_DECODE is rarely entered and need not be inline code.
 *
 * Notes about the huff_DECODE macro:
 * 1. The first if-test is coded to call fill_bit_buffer only when necessary.
 * 2. If the lookahead succeeds, we need only decrement bits_left to remove
 *    the proper number of bits from get_buffer.
 * 3. If the lookahead table contains no entry, the next code must be
 *    more than HUFF_LOOKAHEAD bits long.
 * 4. 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.
 */

#define huff_DECODE(htbl,result) \
{ register int nb, look;					\
  if (bits_left >= HUFF_LOOKAHEAD ||				\
      (fill_bit_buffer(0), bits_left >= HUFF_LOOKAHEAD)) {	\
    look = peek_bits(HUFF_LOOKAHEAD);				\
    if ((nb = htbl->priv.dec.look_nbits[look]) != 0) {		\
      bits_left -= nb;						\
      result = htbl->priv.dec.look_sym[look];			\
    } else							\
      result = slow_DECODE(htbl, HUFF_LOOKAHEAD+1);		\
  } else							\
    result = slow_DECODE(htbl, 1);				\
}

  
LOCAL int
slow_DECODE (HUFF_TBL * htbl, int min_bits)
{
  register int l = min_bits;
  register INT32 code;

  /* huff_DECODE has determined that the code is at least min_bits */
  /* bits long, so fetch that many bits in one swoop. */

  check_bit_buffer(l);
  code = get_bits(l);

  /* Collect the rest of the Huffman code one bit at a time. */
  /* This is per Figure F.16 in the JPEG spec. */

  while (code > htbl->priv.dec.maxcode[l]) {
    code <<= 1;
    check_bit_buffer(1);
    code |= get_bits(1);
    l++;
  }

  /* With garbage input we may reach the sentinel value l = 17. */

  if (l > 16) {
    WARNMS(dcinfo->emethods, "Corrupt JPEG data: bad Huffman code");
    return 0;			/* fake a zero as the safest result */
  }

  return htbl->huffval[ htbl->priv.dec.valptr[l] +
		        ((int) (code - htbl->priv.dec.mincode[l])) ];
}


/* Figure F.12: extend sign bit.
 * On some machines, a shift and add will be faster than a table lookup.
 */

#ifdef AVOID_TABLES

#define huff_EXTEND(x,s)  ((x) < (1<<((s)-1)) ? (x) + (((-1)<<(s)) + 1) : (x))

#else

#define huff_EXTEND(x,s)  ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))

static const int extend_test[16] =   /* entry n is 2**(n-1) */
  { 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,
    0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };

static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
  { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
    ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
    ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
    ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };

#endif /* AVOID_TABLES */


/*
 * Initialize for a Huffman-compressed scan.
 * This is invoked after reading the SOS marker.
 */

METHODDEF void
decoder_init (decompress_info_ptr cinfo)
{
  short ci;
  jpeg_component_info * compptr;

  /* Initialize static variables */
  dcinfo = cinfo;
  bits_left = 0;
  printed_eod = FALSE;

  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
    compptr = cinfo->cur_comp_info[ci];
    /* Make sure requested tables are present */
    if (cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no] == NULL ||
	cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no] == NULL)
      ERREXIT(cinfo->emethods, "Use of undefined Huffman table");
    /* Compute derived values for Huffman tables */
    /* We may do this more than once for same table, but it's not a big deal */
    fix_huff_tbl(cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no]);
    fix_huff_tbl(cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no]);
    /* Initialize DC predictions to 0 */
    cinfo->last_dc_val[ci] = 0;
  }

  /* Initialize restart stuff */
  cinfo->restarts_to_go = cinfo->restart_interval;
  cinfo->next_restart_num = 0;
}


/*
 * Check for a restart marker & resynchronize decoder.
 */

LOCAL void
process_restart (decompress_info_ptr cinfo)
{
  int c, nbytes;
  short ci;

  /* Throw away any unused bits remaining in bit buffer */
  nbytes = bits_left / 8;	/* count any full bytes loaded into buffer */
  bits_left = 0;
  printed_eod = FALSE;		/* next segment can get another warning */

  /* Scan for next JPEG marker */
  do {
    do {			/* skip any non-FF bytes */
      nbytes++;
      c = JGETC(cinfo);
    } while (c != 0xFF);
    do {			/* skip any duplicate FFs */
      /* we don't increment nbytes here since extra FFs are legal */
      c = JGETC(cinfo);
    } while (c == 0xFF);
  } while (c == 0);		/* repeat if it was a stuffed FF/00 */

  if (nbytes != 1)
    WARNMS2(cinfo->emethods,
	    "Corrupt JPEG data: %d extraneous bytes before marker 0x%02x",
	    nbytes-1, c);

  if (c != (RST0 + cinfo->next_restart_num)) {
    /* Uh-oh, the restart markers have been messed up too. */
    /* Let the file-format module try to figure out how to resync. */
    (*cinfo->methods->resync_to_restart) (cinfo, c);
  } else
    TRACEMS1(cinfo->emethods, 2, "RST%d", cinfo->next_restart_num);

  /* Re-initialize DC predictions to 0 */
  for (ci = 0; ci < cinfo->comps_in_scan; ci++)
    cinfo->last_dc_val[ci] = 0;

  /* Update restart state */
  cinfo->restarts_to_go = cinfo->restart_interval;
  cinfo->next_restart_num = (cinfo->next_restart_num + 1) & 7;
}


/* ZAG[i] is the natural-order position of the i'th element of zigzag order.
 * If the incoming data is corrupted, decode_mcu could attempt to
 * reference values beyond the end of the array.  To avoid a wild store,
 * we put some extra zeroes after the real entries.
 */

static const short ZAG[DCTSIZE2+16] = {
  0,  1,  8, 16,  9,  2,  3, 10,
 17, 24, 32, 25, 18, 11,  4,  5,
 12, 19, 26, 33, 40, 48, 41, 34,
 27, 20, 13,  6,  7, 14, 21, 28,
 35, 42, 49, 56, 57, 50, 43, 36,
 29, 22, 15, 23, 30, 37, 44, 51,
 58, 59, 52, 45, 38, 31, 39, 46,
 53, 60, 61, 54, 47, 55, 62, 63,
  0,  0,  0,  0,  0,  0,  0,  0, /* extra entries in case k>63 below */
  0,  0,  0,  0,  0,  0,  0,  0
};


/*
 * Decode and return one MCU's worth of Huffman-compressed coefficients.
 * This routine also handles quantization descaling and zigzag reordering
 * of coefficient values.
 *
 * The i'th block of the MCU is stored into the block pointed to by
 * MCU_data[i].  WE ASSUME THIS AREA HAS BEEN ZEROED BY THE CALLER.
 * (Wholesale zeroing is usually a little faster than retail...)
 */

METHODDEF void
decode_mcu (decompress_info_ptr cinfo, JBLOCKROW *MCU_data)
{
  register int s, k, r;
  short blkn, ci;
  register JBLOCKROW block;
  register QUANT_TBL_PTR quanttbl;
  HUFF_TBL *dctbl;
  HUFF_TBL *actbl;
  jpeg_component_info * compptr;

  /* Account for restart interval, process restart marker if needed */
  if (cinfo->restart_interval) {
    if (cinfo->restarts_to_go == 0)
      process_restart(cinfo);
    cinfo->restarts_to_go--;
  }

  /* Outer loop handles each block in the MCU */

  for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
    block = MCU_data[blkn];
    ci = cinfo->MCU_membership[blkn];
    compptr = cinfo->cur_comp_info[ci];
    quanttbl = cinfo->quant_tbl_ptrs[compptr->quant_tbl_no];
    actbl = cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no];
    dctbl = cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no];

    /* Decode a single block's worth of coefficients */

    /* Section F.2.2.1: decode the DC coefficient difference */
    huff_DECODE(dctbl, s);
    if (s) {
      check_bit_buffer(s);
      r = get_bits(s);
      s = huff_EXTEND(r, s);
    }

    /* Convert DC difference to actual value, update last_dc_val */
    s += cinfo->last_dc_val[ci];
    cinfo->last_dc_val[ci] = (JCOEF) s;
    /* Descale and output the DC coefficient (assumes ZAG[0] = 0) */
    (*block)[0] = (JCOEF) (((JCOEF) s) * quanttbl[0]);
    
    /* Section F.2.2.2: decode the AC coefficients */
    /* Since zero values are skipped, output area must be zeroed beforehand */
    for (k = 1; k < DCTSIZE2; k++) {
      huff_DECODE(actbl, s);
      
      r = s >> 4;
      s &= 15;
      
      if (s) {
	k += r;
	check_bit_buffer(s);
	r = get_bits(s);
	s = huff_EXTEND(r, s);
	/* Descale coefficient and output in natural (dezigzagged) order */
	(*block)[ZAG[k]] = (JCOEF) (((JCOEF) s) * quanttbl[k]);
      } else {
	if (r != 15)
	  break;
	k += 15;
      }
    }
  }
}


/*
 * Finish up at the end of a Huffman-compressed scan.
 */

METHODDEF void
decoder_term (decompress_info_ptr cinfo)
{
  /* No work needed */
}


/*
 * The method selection routine for Huffman entropy decoding.
 */

GLOBAL void
jseldhuffman (decompress_info_ptr cinfo)
{
  if (! cinfo->arith_code) {
    cinfo->methods->entropy_decode_init = decoder_init;
    cinfo->methods->entropy_decode = decode_mcu;
    cinfo->methods->entropy_decode_term = decoder_term;
  }
}