text: /*
   * jdarith.c
   *
   * Developed 1997-2019 by Guido Vollbeding.
   * 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 portable arithmetic entropy decoding routines for JPEG
   * (implementing the ISO/IEC IS 10918-1 and CCITT Recommendation ITU-T T.81).
   *
   * Both sequential and progressive modes are supported in this single module.
   *
   * Suspension is not currently supported in this module.
   */
  
  #define JPEG_INTERNALS
  #include "jinclude.h"
  #include "jpeglib.h"
  
  
  /* Expanded entropy decoder object for arithmetic decoding. */
  
  typedef struct {
    struct jpeg_entropy_decoder pub; /* public fields */
  
    INT32 c;       /* C register, base of coding interval + input bit buffer */
    INT32 a;               /* A register, normalized size of coding interval */
    int ct;     /* bit shift counter, # of bits left in bit buffer part of C */
                                                           /* init: ct = -16 */
                                                           /* run: ct = 0..7 */
                                                           /* error: ct = -1 */
    int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
    int dc_context[MAX_COMPS_IN_SCAN]; /* context index for DC conditioning */
  
    unsigned int restarts_to_go;	/* MCUs left in this restart interval */
  
    /* Pointers to statistics areas (these workspaces have image lifespan) */
    unsigned char * dc_stats[NUM_ARITH_TBLS];
    unsigned char * ac_stats[NUM_ARITH_TBLS];
  
    /* Statistics bin for coding with fixed probability 0.5 */
    unsigned char fixed_bin[4];
  } arith_entropy_decoder;
  
  typedef arith_entropy_decoder * arith_entropy_ptr;
  
  /* The following two definitions specify the allocation chunk size
   * for the statistics area.
   * According to sections F.1.4.4.1.3 and F.1.4.4.2, we need at least
   * 49 statistics bins for DC, and 245 statistics bins for AC coding.
   *
   * We use a compact representation with 1 byte per statistics bin,
   * thus the numbers directly represent byte sizes.
   * This 1 byte per statistics bin contains the meaning of the MPS
   * (more probable symbol) in the highest bit (mask 0x80), and the
   * index into the probability estimation state machine table
   * in the lower bits (mask 0x7F).
   */
  
  #define DC_STAT_BINS 64
  #define AC_STAT_BINS 256
  
  
  LOCAL(int)
  get_byte (j_decompress_ptr cinfo)
  /* Read next input byte; we do not support suspension in this module. */
  {
    struct jpeg_source_mgr * src = cinfo->src;
  
    if (src->bytes_in_buffer == 0)
      if (! (*src->fill_input_buffer) (cinfo))
        ERREXIT(cinfo, JERR_CANT_SUSPEND);
    src->bytes_in_buffer--;
    return GETJOCTET(*src->next_input_byte++);
  }
  
  
  /*
   * The core arithmetic decoding routine (common in JPEG and JBIG).
   * This needs to go as fast as possible.
   * Machine-dependent optimization facilities
   * are not utilized in this portable implementation.
   * However, this code should be fairly efficient and
   * may be a good base for further optimizations anyway.
   *
   * Return value is 0 or 1 (binary decision).
   *
   * Note: I've changed the handling of the code base & bit
   * buffer register C compared to other implementations
   * based on the standards layout & procedures.
   * While it also contains both the actual base of the
   * coding interval (16 bits) and the next-bits buffer,
   * the cut-point between these two parts is floating
   * (instead of fixed) with the bit shift counter CT.
   * Thus, we also need only one (variable instead of
   * fixed size) shift for the LPS/MPS decision, and
   * we can do away with any renormalization update
   * of C (except for new data insertion, of course).
   *
   * I've also introduced a new scheme for accessing
   * the probability estimation state machine table,
   * derived from Markus Kuhn's JBIG implementation.
   */
  
  LOCAL(int)
  arith_decode (j_decompress_ptr cinfo, unsigned char *st)
  {
    register arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
    register unsigned char nl, nm;
    register INT32 qe, temp;
    register int sv, data;
  
    /* Renormalization & data input per section D.2.6 */
    while (e->a < 0x8000L) {
      if (--e->ct < 0) {
        /* Need to fetch next data byte */
        if (cinfo->unread_marker)
  	data = 0;		/* stuff zero data */
        else {
  	data = get_byte(cinfo);	/* read next input byte */
  	if (data == 0xFF) {	/* zero stuff or marker code */
  	  do data = get_byte(cinfo);
  	  while (data == 0xFF);	/* swallow extra 0xFF bytes */
  	  if (data == 0)
  	    data = 0xFF;	/* discard stuffed zero byte */
  	  else {
  	    /* Note: Different from the Huffman decoder, hitting
  	     * a marker while processing the compressed data
  	     * segment is legal in arithmetic coding.
  	     * The convention is to supply zero data
  	     * then until decoding is complete.
  	     */
  	    cinfo->unread_marker = data;
  	    data = 0;
  	  }
  	}
        }
        e->c = (e->c << 8) | data; /* insert data into C register */
        if ((e->ct += 8) < 0)	 /* update bit shift counter */
  	/* Need more initial bytes */
  	if (++e->ct == 0)
  	  /* Got 2 initial bytes -> re-init A and exit loop */
  	  e->a = 0x8000L; /* => e->a = 0x10000L after loop exit */
      }
      e->a <<= 1;
    }
  
    /* Fetch values from our compact representation of Table D.3(D.2):
     * Qe values and probability estimation state machine
     */
    sv = *st;
    qe = jpeg_aritab[sv & 0x7F];	/* => Qe_Value */
    nl = qe & 0xFF; qe >>= 8;	/* Next_Index_LPS + Switch_MPS */
    nm = qe & 0xFF; qe >>= 8;	/* Next_Index_MPS */
  
    /* Decode & estimation procedures per sections D.2.4 & D.2.5 */
    temp = e->a - qe;
    e->a = temp;
    temp <<= e->ct;
    if (e->c >= temp) {
      e->c -= temp;
      /* Conditional LPS (less probable symbol) exchange */
      if (e->a < qe) {
        e->a = qe;
        *st = (sv & 0x80) ^ nm;	/* Estimate_after_MPS */
      } else {
        e->a = qe;
        *st = (sv & 0x80) ^ nl;	/* Estimate_after_LPS */
        sv ^= 0x80;		/* Exchange LPS/MPS */
      }
    } else if (e->a < 0x8000L) {
      /* Conditional MPS (more probable symbol) exchange */
      if (e->a < qe) {
        *st = (sv & 0x80) ^ nl;	/* Estimate_after_LPS */
        sv ^= 0x80;		/* Exchange LPS/MPS */
      } else {
        *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) == (int) 0x8000U) {
  	    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) == (int) 0x8000U) {
  	    WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
  	    entropy->ct = -1;			/* magnitude overflow */
  	    return TRUE;
  	  }
  	  st += 1;
  	}
        }
      }
      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;
      /* Scale and output coefficient in natural (dezigzagged) order */
      (*block)[natural_order[k]] = (JCOEF) (v << cinfo->Al);
    } while (k < cinfo->Se);
  
    return TRUE;
  }
  
  
  /*
   * MCU decoding for DC successive approximation refinement scan.
   * Note: we assume such scans can be multi-component,
   * although the spec is not very clear on the point.
   */
  
  METHODDEF(boolean)
  decode_mcu_DC_refine (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
  {
    arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
    unsigned char *st;
    JCOEF p1;
    int blkn;
  
    /* Process restart marker if needed */
    if (cinfo->restart_interval) {
      if (entropy->restarts_to_go == 0)
        process_restart(cinfo);
      entropy->restarts_to_go--;
    }
  
    st = entropy->fixed_bin;	/* use fixed probability estimation */
    p1 = 1 << cinfo->Al;		/* 1 in the bit position being coded */
  
    /* Outer loop handles each block in the MCU */
  
    for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
      /* Encoded data is simply the next bit of the two's-complement DC value */
      if (arith_decode(cinfo, st))
        MCU_data[blkn][0][0] |= p1;
    }
  
    return TRUE;
  }
  
  
  /*
   * MCU decoding for AC successive approximation refinement scan.
   */
  
  METHODDEF(boolean)
  decode_mcu_AC_refine (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
  {
    arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
    JBLOCKROW block;
    JCOEFPTR thiscoef;
    unsigned char *st;
    int tbl, k, kex;
    JCOEF p1, m1;
    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;
  
    p1 = 1 << cinfo->Al;		/* 1 in the bit position being coded */
    m1 = -p1;			/* -1 in the bit position being coded */
  
    /* Establish EOBx (previous stage end-of-block) index */
    kex = cinfo->Se;
    do {
      if ((*block)[natural_order[kex]]) break;
    } while (--kex);
  
    k = cinfo->Ss - 1;
    do {
      st = entropy->ac_stats[tbl] + 3 * k;
      if (k >= kex)
        if (arith_decode(cinfo, st)) break;	/* EOB flag */
      for (;;) {
        thiscoef = *block + natural_order[++k];
        if (*thiscoef) {				/* previously nonzero coef */
  	if (arith_decode(cinfo, st + 2)) {
  	  if (*thiscoef < 0)
  	    *thiscoef += m1;
  	  else
  	    *thiscoef += p1;
  	}
  	break;
        }
        if (arith_decode(cinfo, st + 1)) {	/* newly nonzero coef */
  	if (arith_decode(cinfo, entropy->fixed_bin))
  	  *thiscoef = m1;
  	else
  	  *thiscoef = p1;
  	break;
        }
        st += 3;
        if (k >= cinfo->Se) {
  	WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
  	entropy->ct = -1;			/* spectral overflow */
  	return TRUE;
        }
      }
    } while (k < cinfo->Se);
  
    return TRUE;
  }
  
  
  /*
   * Decode one MCU's worth of arithmetic-compressed coefficients.
   */
  
  METHODDEF(boolean)
  decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
  {
    arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
    jpeg_component_info * compptr;
    JBLOCKROW block;
    unsigned char *st;
    int blkn, ci, 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;
  
    /* 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];
  
      /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
  
      tbl = compptr->dc_tbl_no;
  
      /* 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) == (int) 0x8000U) {
  	    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;
      }
  
      (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
  
      /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
  
      if (cinfo->lim_Se == 0) continue;
      tbl = compptr->ac_tbl_no;
      k = 0;
  
      /* Figure F.20: Decode_AC_coefficients */
      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->lim_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) == (int) 0x8000U) {
  	      WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
  	      entropy->ct = -1;			/* magnitude overflow */
  	      return TRUE;
  	    }
  	    st += 1;
  	  }
  	}
        }
        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;
        (*block)[natural_order[k]] = (JCOEF) v;
      } while (k < cinfo->lim_Se);
    }
  
    return TRUE;
  }
  
  
  /*
   * Initialize for an arithmetic-compressed scan.
   */
  
  METHODDEF(void)
  start_pass (j_decompress_ptr cinfo)
  {
    arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
    int ci, tbl;
    jpeg_component_info * compptr;
  
    if (cinfo->progressive_mode) {
      /* Validate progressive scan parameters */
      if (cinfo->Ss == 0) {
        if (cinfo->Se != 0)
  	goto bad;
      } else {
        /* need not check Ss/Se < 0 since they came from unsigned bytes */
        if (cinfo->Se < cinfo->Ss || cinfo->Se > cinfo->lim_Se)
  	goto bad;
        /* AC scans may have only one component */
        if (cinfo->comps_in_scan != 1)
  	goto bad;
      }
      if (cinfo->Ah != 0) {
        /* Successive approximation refinement scan: must have Al = Ah-1. */
        if (cinfo->Ah-1 != cinfo->Al)
  	goto bad;
      }
      if (cinfo->Al > 13) {	/* need not check for < 0 */
        bad:
        ERREXIT4(cinfo, JERR_BAD_PROGRESSION,
  	       cinfo->Ss, cinfo->Se, cinfo->Ah, cinfo->Al);
      }
      /* Update progression status, and verify that scan order is legal.
       * Note that inter-scan inconsistencies are treated as warnings
       * not fatal errors ... not clear if this is right way to behave.
       */
      for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
        int coefi, cindex = cinfo->cur_comp_info[ci]->component_index;
        int *coef_bit_ptr = & cinfo->coef_bits[cindex][0];
        if (cinfo->Ss && coef_bit_ptr[0] < 0) /* AC without prior DC scan */
  	WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, 0);
        for (coefi = cinfo->Ss; coefi <= cinfo->Se; coefi++) {
  	int expected = (coef_bit_ptr[coefi] < 0) ? 0 : coef_bit_ptr[coefi];
  	if (cinfo->Ah != expected)
  	  WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, coefi);
  	coef_bit_ptr[coefi] = cinfo->Al;
        }
      }
      /* Select MCU decoding routine */
      if (cinfo->Ah == 0) {
        if (cinfo->Ss == 0)
  	entropy->pub.decode_mcu = decode_mcu_DC_first;
        else
  	entropy->pub.decode_mcu = decode_mcu_AC_first;
      } else {
        if (cinfo->Ss == 0)
  	entropy->pub.decode_mcu = decode_mcu_DC_refine;
        else
  	entropy->pub.decode_mcu = decode_mcu_AC_refine;
      }
    } else {
      /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
       * This ought to be an error condition, but we make it a warning.
       */
      if (cinfo->Ss != 0 || cinfo->Ah != 0 || cinfo->Al != 0 ||
  	(cinfo->Se < DCTSIZE2 && cinfo->Se != cinfo->lim_Se))
        WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
      /* Select MCU decoding routine */
      entropy->pub.decode_mcu = decode_mcu;
    }
  
    /* Allocate & initialize requested 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)) {
        tbl = compptr->dc_tbl_no;
        if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
  	ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
        if (entropy->dc_stats[tbl] == NULL)
  	entropy->dc_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
  	  ((j_common_ptr) cinfo, JPOOL_IMAGE, DC_STAT_BINS);
        MEMZERO(entropy->dc_stats[tbl], DC_STAT_BINS);
        /* Initialize 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)) {
        tbl = compptr->ac_tbl_no;
        if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
  	ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
        if (entropy->ac_stats[tbl] == NULL)
  	entropy->ac_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
  	  ((j_common_ptr) cinfo, JPOOL_IMAGE, AC_STAT_BINS);
        MEMZERO(entropy->ac_stats[tbl], AC_STAT_BINS);
      }
    }
  
    /* Initialize arithmetic decoding variables */
    entropy->c = 0;
    entropy->a = 0;
    entropy->ct = -16;	/* force reading 2 initial bytes to fill C */
  
    /* Initialize restart counter */
    entropy->restarts_to_go = cinfo->restart_interval;
  }
  
  
  /*
   * Finish up at the end of an arithmetic-compressed scan.
   */
  
  METHODDEF(void)
  finish_pass (j_decompress_ptr cinfo)
  {
    /* no work necessary here */
    (void)cinfo;
  }
  
  
  /*
   * Module initialization routine for arithmetic entropy decoding.
   */
  
  GLOBAL(void)
  jinit_arith_decoder (j_decompress_ptr cinfo)
  {
    arith_entropy_ptr entropy;
    int i;
  
    entropy = (arith_entropy_ptr) (*cinfo->mem->alloc_small)
      ((j_common_ptr) cinfo, JPOOL_IMAGE, SIZEOF(arith_entropy_decoder));
    cinfo->entropy = &entropy->pub;
    entropy->pub.start_pass = start_pass;
    entropy->pub.finish_pass = finish_pass;
  
    /* Mark tables unallocated */
    for (i = 0; i < NUM_ARITH_TBLS; i++) {
      entropy->dc_stats[i] = NULL;
      entropy->ac_stats[i] = NULL;
    }
  
    /* Initialize index for fixed probability estimation */
    entropy->fixed_bin[0] = 113;
  
    if (cinfo->progressive_mode) {
      /* Create progression status table */
      int *coef_bit_ptr, ci;
      cinfo->coef_bits = (int (*)[DCTSIZE2]) (*cinfo->mem->alloc_small)
        ((j_common_ptr) cinfo, JPOOL_IMAGE,
         cinfo->num_components * DCTSIZE2 * SIZEOF(int));
      coef_bit_ptr = & cinfo->coef_bits[0][0];
      for (ci = 0; ci < cinfo->num_components; ci++) 
        for (i = 0; i < DCTSIZE2; i++)
  	*coef_bit_ptr++ = -1;
    }
  }
  