/*

 * jdarith.c

 *

 * Developed 1997-2012 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 get 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) == 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;

	  }

	  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.

 */

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;

  int p1, 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;

  int 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 = (-1) << cinfo->Al;	/* -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) == 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;

    }

    (*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) == 0x8000) {

	      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;

}

/*

 * 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;

  /* 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;

  }

}