jdhuff.cpp

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					* Fine point: it might appear that we should save the marker into
					* bitread working state, not straight into permanent state.  But
					* once we have hit a marker, we cannot need to suspend within the
					* current MCU, because we will read no more bytes from the data
					* source.  So it is OK to update permanent state right away.
					*/
					cinfo->unread_marker = c;
					/* See if we need to insert some fake zero bits. */
					goto no_more_bytes;
				}
			}

			/* OK, load c into get_buffer */
			get_buffer = (get_buffer << 8) | c;
			bits_left += 8;
		} /* end while */
	} 
	else 
	{
		no_more_bytes:
		/* We get here if we've read the marker that terminates the compressed
		* data segment.  There should be enough bits in the buffer register
		* to satisfy the request; if so, no problem.
		*/
		if (nbits > bits_left) 
		{
			/* Uh-oh.  Report corrupted data to user and stuff zeroes into
			* the data stream, so that we can produce some kind of image.
			* We use a nonvolatile flag to ensure that only one warning message
			* appears per data segment.
			*/
			if (! cinfo->entropy->insufficient_data) 
			{
				cinfo->WARNMS(JWRN_HIT_MARKER);
				cinfo->entropy->insufficient_data = TRUE;
			}
			/* Fill the buffer with zero bits */
			get_buffer <<= MIN_GET_BITS - bits_left;
			bits_left = MIN_GET_BITS;
		}
	}

	/* Unload the local registers */
	m_next_input_byte = next_input_byte;
	m_bytes_in_buffer = bytes_in_buffer;
	m_get_buffer = get_buffer;
	m_bits_left = bits_left;

	return TRUE;
}


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

int bitread_working_state::jpeg_huff_decode (bit_buf_type get_buffer, int bits_left, d_derived_tbl * htbl, int min_bits)
{
	register int i = min_bits;
	register long code;

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

/*	if ( bits_left>16 )
	{
		code = GET_BITS(i);

		// 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[i]) 
		{
			code = (code << 1) | GET_BITS(1);
			i++;
		}

	}
	else */
	{
		if ( ! CHECK_BIT_BUFFER(i, bits_left, get_buffer) )
			return -1;
	
		code = GET_BITS(i);

		/* 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[i]) 
		{
			code <<= 1;
		
			if ( ! CHECK_BIT_BUFFER(1, bits_left, get_buffer ) )
				return -1;
		
			code |= GET_BITS(1);
			i++;
		}
	}

	/* Unload the local registers */
	m_get_buffer = get_buffer;
	m_bits_left  = bits_left;

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

	if (i > 16) 
	{
		m_cinfo->WARNMS(JWRN_HUFF_BAD_CODE);
		return 0;			/* fake a zero as the safest result */
	}

	return htbl->pub->huffval[ (int) (code + htbl->valoffset[i]) ];
}


/*
 * 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;

  /* Reset out-of-data flag, unless read_restart_marker left us smack up
   * against a marker.  In that case we will end up treating the next data
   * segment as empty, and we can avoid producing bogus output pixels by
   * leaving the flag set.
   */
  if (cinfo->unread_marker == 0)
    entropy->pub.insufficient_data = 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.)
 */

boolean decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
{
  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  int blkn;
  BITREAD_STATE_VARS;
  savable_state state;

  /* 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;
  }

	// If we've run out of data, just leave the MCU set to zeroes.
	// This way, we return uniform gray for the remainder of the segment.
	
	if (! entropy->pub.insufficient_data) 
	{
	    /* 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++) 
		{
			JBLOCKROW block = MCU_data[blkn];
			d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];
			d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];
			register int s, k, r;

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

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

			if (entropy->dc_needed[blkn]) 
			{
				/* Convert DC difference to actual value, update last_dc_val */
				int ci = cinfo->MCU_membership[blkn];
				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;
			}

			if (entropy->ac_needed[blkn]) 
			{

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

	entropy = (huff_entropy_ptr)
		cinfo->mem->alloc_small(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 (int i = 0; i < NUM_HUFF_TBLS; i++) 
	{
		entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
	}
}