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      *st = (sv & 0x80) ^ nm;	/* Estimate_after_MPS */
    }
  }

  return sv >> 7;
}


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

LOCAL(void)
process_restart (j_decompress_ptr cinfo)
{
  arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
  int ci;
  jpeg_component_info * compptr;

  /* Advance past the RSTn marker */
  if (! (*cinfo->marker->read_restart_marker) (cinfo))
    ERREXIT(cinfo, JERR_CANT_SUSPEND);

  /* Re-initialize statistics areas */
  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
    compptr = cinfo->cur_comp_info[ci];
    if (! cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
      MEMZERO(entropy->dc_stats[compptr->dc_tbl_no], DC_STAT_BINS);
      /* Reset DC predictions to 0 */
      entropy->last_dc_val[ci] = 0;
      entropy->dc_context[ci] = 0;
    }
    if ((! cinfo->progressive_mode && cinfo->lim_Se) ||
	(cinfo->progressive_mode && cinfo->Ss)) {
      MEMZERO(entropy->ac_stats[compptr->ac_tbl_no], AC_STAT_BINS);
    }
  }

  /* Reset arithmetic decoding variables */
  entropy->c = 0;
  entropy->a = 0;
  entropy->ct = -16;	/* force reading 2 initial bytes to fill C */

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


/*
 * Arithmetic MCU decoding.
 * Each of these routines decodes and returns one MCU's worth of
 * arithmetic-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 IS INITIALLY ZEROED BY THE CALLER.
 */

/*
 * MCU decoding for DC initial scan (either spectral selection,
 * or first pass of successive approximation).
 */

METHODDEF(boolean)
decode_mcu_DC_first (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
{
  arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
  JBLOCKROW block;
  unsigned char *st;
  int blkn, ci, tbl, sign;
  int v, m;

  /* Process restart marker if needed */
  if (cinfo->restart_interval) {
    if (entropy->restarts_to_go == 0)
      process_restart(cinfo);
    entropy->restarts_to_go--;
  }

  if (entropy->ct == -1) return TRUE;	/* if error do nothing */

  /* 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];
    tbl = cinfo->cur_comp_info[ci]->dc_tbl_no;

    /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */

    /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
    st = entropy->dc_stats[tbl] + entropy->dc_context[ci];

    /* Figure F.19: Decode_DC_DIFF */
    if (arith_decode(cinfo, st) == 0)
      entropy->dc_context[ci] = 0;
    else {
      /* Figure F.21: Decoding nonzero value v */
      /* Figure F.22: Decoding the sign of v */
      sign = arith_decode(cinfo, st + 1);
      st += 2; st += sign;
      /* Figure F.23: Decoding the magnitude category of v */
      if ((m = arith_decode(cinfo, st)) != 0) {
	st = entropy->dc_stats[tbl] + 20;	/* Table F.4: X1 = 20 */
	while (arith_decode(cinfo, st)) {
	  if ((m <<= 1) == 0x8000) {
	    WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
	    entropy->ct = -1;			/* magnitude overflow */
	    return TRUE;
	  }
	  st += 1;
	}
      }
      /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
      if (m < (int) ((1L << cinfo->arith_dc_L[tbl]) >> 1))
	entropy->dc_context[ci] = 0;		   /* zero diff category */
      else if (m > (int) ((1L << cinfo->arith_dc_U[tbl]) >> 1))
	entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
      else
	entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
      v = m;
      /* Figure F.24: Decoding the magnitude bit pattern of v */
      st += 14;
      while (m >>= 1)
	if (arith_decode(cinfo, st)) v |= m;
      v += 1; if (sign) v = -v;
      entropy->last_dc_val[ci] += v;
    }

    /* Scale and output the DC coefficient (assumes jpeg_natural_order[0]=0) */
    (*block)[0] = (JCOEF) (entropy->last_dc_val[ci] << cinfo->Al);
  }

  return TRUE;
}


/*
 * MCU decoding for AC initial scan (either spectral selection,
 * or first pass of successive approximation).
 */

METHODDEF(boolean)
decode_mcu_AC_first (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
{
  arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
  JBLOCKROW block;
  unsigned char *st;
  int tbl, sign, k;
  int v, m;
  const int * natural_order;

  /* Process restart marker if needed */
  if (cinfo->restart_interval) {
    if (entropy->restarts_to_go == 0)
      process_restart(cinfo);
    entropy->restarts_to_go--;
  }

  if (entropy->ct == -1) return TRUE;	/* if error do nothing */

  natural_order = cinfo->natural_order;

  /* There is always only one block per MCU */
  block = MCU_data[0];
  tbl = cinfo->cur_comp_info[0]->ac_tbl_no;

  /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */

  /* Figure F.20: Decode_AC_coefficients */
  k = cinfo->Ss - 1;
  do {
    st = entropy->ac_stats[tbl] + 3 * k;
    if (arith_decode(cinfo, st)) break;		/* EOB flag */
    for (;;) {
      k++;
      if (arith_decode(cinfo, st + 1)) break;
      st += 3;
      if (k >= cinfo->Se) {
	WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
	entropy->ct = -1;			/* spectral overflow */
	return TRUE;
      }
    }
    /* Figure F.21: Decoding nonzero value v */
    /* Figure F.22: Decoding the sign of v */
    sign = arith_decode(cinfo, entropy->fixed_bin);
    st += 2;
    /* Figure F.23: Decoding the magnitude category of v */
    if ((m = arith_decode(cinfo, st)) != 0) {
      if (arith_decode(cinfo, st)) {
	m <<= 1;
	st = entropy->ac_stats[tbl] +
	     (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
	while (arith_decode(cinfo, st)) {
	  if ((m <<= 1) == 0x8000) {
	    WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
	    entropy->ct = -1;			/* magnitude overflow */
	    return TRUE;