jdhuff.c

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	/* 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 code will be repeated for each byte demanded
	 * for the rest of the segment.  We use a nonvolatile flag to ensure
	 * that only one warning message appears.
	 */
	if (! *(state->printed_eod_ptr)) {
	  WARNMS(state->cinfo, JWRN_HIT_MARKER);
	  *(state->printed_eod_ptr) = 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;
  }

  /* Unload the local registers */
  state->next_input_byte = next_input_byte;
  state->bytes_in_buffer = bytes_in_buffer;
  state->get_buffer = get_buffer;
  state->bits_left = bits_left;

  return TRUE;
}


/*
 * Out-of-line code for Huffman code decoding.
 * See jdhuff.h for info about usage.
 */

GLOBAL int
jpeg_huff_decode (bitread_working_state * state,
		  register bit_buf_type get_buffer, register int bits_left,
		  d_derived_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(*state, l, return -1);
  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->maxcode[l]) {
    code <<= 1;
    CHECK_BIT_BUFFER(*state, 1, return -1);
    code |= GET_BITS(1);
    l++;
  }

  /* Unload the local registers */
  state->get_buffer = get_buffer;
  state->bits_left = bits_left;

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

  if (l > 16) {
    WARNMS(state->cinfo, JWRN_HUFF_BAD_CODE);
    return 0;			/* fake a zero as the safest result */
  }

  return htbl->pub->huffval[ htbl->valptr[l] +
			    ((int) (code - htbl->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 */


/*
 * Check for a restart marker & resynchronize decoder.
 * Returns FALSE if must suspend.
 */

LOCAL boolean
process_restart (j_decompress_ptr cinfo)
{
  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  int ci;

  /* Throw away any unused bits remaining in bit buffer; */
  /* include any full bytes in next_marker's count of discarded bytes */
  cinfo->marker->discarded_bytes += entropy->bitstate.bits_left / 8;
  entropy->bitstate.bits_left = 0;

  /* Advance past the RSTn marker */
  if (! (*cinfo->marker->read_restart_marker) (cinfo))
    return FALSE;

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

  /* Reset restart counter */
  entropy->restarts_to_go = cinfo->restart_interval;

  /* Next segment can get another out-of-data warning */
  entropy->bitstate.printed_eod = FALSE;

  return TRUE;
}


/*
 * Decode and return one MCU's worth of Huffman-compressed coefficients.
 * The coefficients are reordered from zigzag order into natural array order,
 * but are not dequantized.
 *
 * 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...)
 *
 * Returns FALSE if data source requested suspension.  In that case no
 * changes have been made to permanent state.  (Exception: some output
 * coefficients may already have been assigned.  This is harmless for
 * this module, since we'll just re-assign them on the next call.)
 */
OVERLAY_SEGMENT("code_jeg")
boolean
decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
{
  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  register int s, k, r;
  int blkn, ci;
  JBLOCKROW block;
  BITREAD_STATE_VARS;
  savable_state state;
  d_derived_tbl * dctbl;
  d_derived_tbl * actbl;
  jpeg_component_info * compptr;

  /* Process restart marker if needed; may have to suspend */
  if (cinfo->restart_interval) {
    if (entropy->restarts_to_go == 0)
      if (! process_restart(cinfo))
	return FALSE;
  }

  /* Load up working state */
  BITREAD_LOAD_STATE(cinfo,entropy->bitstate);
  ASSIGN_STATE(state, entropy->saved);

  /* 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];
    dctbl = entropy->dc_derived_tbls[compptr->dc_tbl_no];
    actbl = entropy->ac_derived_tbls[compptr->ac_tbl_no];

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

    /* Section F.2.2.1: decode the DC coefficient difference */
    HUFF_DECODE(s, br_state, dctbl, return FALSE, label1);
    if (s) {
      CHECK_BIT_BUFFER(br_state, s, return FALSE);
      r = GET_BITS(s);
      s = HUFF_EXTEND(r, s);
    }

    /* Shortcut if component's values are not interesting */
    if (! compptr->component_needed)
      goto skip_ACs;

    /* Convert DC difference to actual value, update last_dc_val */
    s += state.last_dc_val[ci];
    state.last_dc_val[ci] = s;
    /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
    (*block)[0] = (JCOEF) s;

    /* Do we need to decode the AC coefficients for this component? */
    if (compptr->DCT_scaled_size > 1) {

      /* Section F.2.2.2: decode the AC coefficients */
      /* Since zeroes are skipped, output area must be cleared beforehand */
      for (k = 1; k < DCTSIZE2; k++) {
	HUFF_DECODE(s, br_state, actbl, return FALSE, label2);
      
	r = s >> 4;
	s &= 15;
      
	if (s) {
	  k += r;
	  CHECK_BIT_BUFFER(br_state, s, return FALSE);
	  r = GET_BITS(s);
	  s = HUFF_EXTEND(r, s);
	  /* Output coefficient in natural (dezigzagged) order.
	   * Note: the extra entries in jpeg_natural_order[] will save us
	   * if k >= DCTSIZE2, which could happen if the data is corrupted.
	   */
	  (*block)[jpeg_natural_order[k]] = (JCOEF) s;
	} else {
	  if (r != 15)
	    break;
	  k += 15;
	}
      }

    } else {
skip_ACs:

      /* Section F.2.2.2: decode the AC coefficients */
      /* In this path we just discard the values */
      for (k = 1; k < DCTSIZE2; k++) {
	HUFF_DECODE(s, br_state, actbl, return FALSE, label3);
      
	r = s >> 4;
	s &= 15;
      
	if (s) {
	  k += r;
	  CHECK_BIT_BUFFER(br_state, s, return FALSE);
	  DROP_BITS(s);
	} else {
	  if (r != 15)
	    break;
	  k += 15;
	}
      }

    }
  }

  /* Completed MCU, so update state */
  BITREAD_SAVE_STATE(cinfo,entropy->bitstate);
  ASSIGN_STATE(entropy->saved, state);

  /* Account for restart interval (no-op if not using restarts) */
  entropy->restarts_to_go--;

  return TRUE;
}


/*
 * Module initialization routine for Huffman entropy decoding.
 */

GLOBAL void
jinit_huff_decoder (j_decompress_ptr cinfo)
{
  huff_entropy_ptr entropy;
  int i;

  entropy = (huff_entropy_ptr)
    (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
				SIZEOF(huff_entropy_decoder));
  cinfo->entropy = (struct jpeg_entropy_decoder *) entropy;
  entropy->pub.start_pass = start_pass_huff_decoder;
  entropy->pub.decode_mcu = decode_mcu;

  /* Mark tables unallocated */
  for (i = 0; i < NUM_HUFF_TBLS; i++) {
    entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
  }
}