/*
 * jdarith.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Developed 1997-2015 by Guido Vollbeding.
 * libjpeg-turbo Modifications:
 * Copyright (C) 2015, D. R. Commander.
 * For conditions of distribution and use, see the accompanying README.ijg
 * 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 */

	JLONG c;       /* C register, base of coding interval + input bit buffer */
	JLONG 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 JLONG 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.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->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) LEFT_SHIFT(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;

	/* 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 */

	/* 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 */
	for (k = cinfo->Ss; k <= cinfo->Se; k++) {
		st = entropy->ac_stats[tbl] + 3 * (k - 1);
		if (arith_decode(cinfo, st)) break;         /* EOB flag */
		while (arith_decode(cinfo, st + 1) == 0) {
			st += 3;
			k++;
			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)[jpeg_natural_order[k]] = (JCOEF) (v << cinfo->Al);
	}

	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;

	/* 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 */

	/* 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 */
	for (kex = cinfo->Se; kex > 0; kex--)
		if ((*block)[jpeg_natural_order[kex]]) break;

	for (k = cinfo->Ss; k <= cinfo->Se; k++) {
		st = entropy->ac_stats[tbl] + 3 * (k - 1);
		if (k > kex)
			if (arith_decode(cinfo, st)) break;       /* EOB flag */
		for (;;) {
			thiscoef = *block + jpeg_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;
			k++;
			if (k > cinfo->Se) {
				WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
				entropy->ct = -1;                       /* spectral overflow */
				return TRUE;
			}
		}
	}

	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;

	/* 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 ? MCU_data[blkn] : NULL;
		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;
		}

		if (block)
			(*block)[0] = (JCOEF) entropy->last_dc_val[ci];

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

		tbl = compptr->ac_tbl_no;

		/* Figure F.20: Decode_AC_coefficients */
		for (k = 1; k <= DCTSIZE2 - 1; k++) {
			st = entropy->ac_stats[tbl] + 3 * (k - 1);
			if (arith_decode(cinfo, st)) break;       /* EOB flag */
			while (arith_decode(cinfo, st + 1) == 0) {
				st += 3;
				k++;
				if (k > DCTSIZE2 - 1) {
					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;
			if (block)
				(*block)[jpeg_natural_order[k]] = (JCOEF) v;
		}
	}

	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 > DCTSIZE2 - 1)
				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 != DCTSIZE2 - 1))
			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->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 = (struct jpeg_entropy_decoder *) entropy;
	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;
	}
}
