jdhuff.c

/*
 * jdhuff.c
 *
 * Copyright (C) 1991, 1992, 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;
  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++;
  }

  /* We don't bother to fill in the encoding tables ehufco[] and ehufsi[], */
  /* since they are not used for decoding. */

  /* Figure F.15: generate decoding tables */

  p = 0;
  for (l = 1; l <= 16; l++) {
    if (htbl->bits[l]) {
      htbl->valptr[l] = p;	/* huffval[] index of 1st sym of code len l */
      htbl->mincode[l] = huffcode[p]; /* minimum code of length l */
      p += htbl->bits[l];
      htbl->maxcode[l] = huffcode[p-1];	/* maximum code of length l */
    } else {
      htbl->maxcode[l] = -1;
    }
  }
  htbl->maxcode[17] = 0xFFFFFL;	/* ensures huff_DECODE terminates */
}


/*
 * 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 get_bits() and get_bit().  When there aren't enough bits,
 * fill_bit_buffer is called; it will attempt to fill get_buffer to the
 * "high water mark", then extract the desired number of bits.  The idea,
 * of course, is to minimize the function-call overhead cost of entering
 * fill_bit_buffer.
 * 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

static const int bmask[16] =	/* bmask[n] is mask for n rightmost bits */
  { 0, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF,
    0x01FF, 0x03FF, 0x07FF, 0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF };


LOCAL int
fill_bit_buffer (int nbits)
/* Load up the bit buffer and do get_bits(nbits) */
{
  /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
  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 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:
	 */
	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;
  }

  /* Having filled get_buffer, extract desired bits (this simplifies macros) */
  bits_left -= nbits;
  return ((int) (get_buffer >> bits_left)) & bmask[nbits];
}


/* Macros to make things go at some speed! */
/* NB: parameter to get_bits should be simple variable, not expression */

#define get_bits(nbits) \
	(bits_left >= (nbits) ? \
	 ((int) (get_buffer >> (bits_left -= (nbits)))) & bmask[nbits] : \
	 fill_bit_buffer(nbits))

#define get_bit() \
	(bits_left ? \
	 ((int) (get_buffer >> (--bits_left))) & 1 : \
	 fill_bit_buffer(1))


/* Figure F.16: extract next coded symbol from input stream */
  
INLINE
LOCAL int
huff_DECODE (HUFF_TBL * htbl)
{
  register int l;
  register INT32 code;
  
  code = get_bit();
  l = 1;
  while (code > htbl->maxcode[l]) {
    code = (code << 1) | get_bit();
    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->valptr[l] + ((int) (code - htbl->mincode[l])) ];
}


/* Figure F.12: extend sign bit */

#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 };


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

METHODDEF void
huff_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, huff_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
huff_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 */
    s = huff_DECODE(dctbl);
    if (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++) {
      r = huff_DECODE(actbl);
      
      s = r & 15;
      r = r >> 4;
      
      if (s) {
	k += r;
	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
huff_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 = huff_decoder_init;
    cinfo->methods->entropy_decode = huff_decode_mcu;
    cinfo->methods->entropy_decode_term = huff_decoder_term;
  }
}