jcarith.c

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/*
 * jcarith.c
 *
 * Copyright (C) 1997, Guido Vollbeding <[e-mail]>.
 * This file is NOT part of the Independent JPEG Group's software
 * for legal reasons.
 * See the accompanying README file for conditions of distribution and use.
 *
 * This file contains portable arithmetic entropy encoding 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 encoder object for arithmetic encoding. */

typedef struct {
  struct jpeg_entropy_encoder pub; /* public fields */

  INT32 c; /* C register, base of coding interval, layout as in sec. D.1.3 */
  INT32 a;               /* A register, normalized size of coding interval */
  INT32 sc;        /* counter for stacked 0xFF values which might overflow */
  INT32 zc;          /* counter for pending 0x00 output values which might *
                          * be discarded at the end ("Pacman" termination) */
  int ct;  /* bit shift counter, determines when next byte will be written */
  int buffer;                /* buffer for most recent output byte != 0xFF */

  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 */
  int next_restart_num;		/* next restart number to write (0-7) */

  /* Pointers to statistics areas (these workspaces have image lifespan) */
  unsigned char * dc_stats[NUM_ARITH_TBLS];
  unsigned char * ac_stats[NUM_ARITH_TBLS];
} arith_entropy_encoder;

typedef arith_entropy_encoder * 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.
 * Note that we use one additional AC bin for codings with fixed
 * probability (0.5), thus the minimum number for AC is 246.
 *
 * 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

/* NOTE: Uncomment the following #define if you want to use the
 * given formula for calculating the AC conditioning parameter Kx
 * for spectral selection progressive coding in section G.1.3.2
 * of the spec (Kx = Kmin + SRL (8 + Se - Kmin) 4).
 * Although the spec and P&M authors claim that this "has proven
 * to give good results for 8 bit precision samples", I'm not
 * convinced yet that this is really beneficial.
 * Early tests gave only very marginal compression enhancements
 * (a few - around 5 or so - bytes even for very large files),
 * which would turn out rather negative if we'd suppress the
 * DAC (Define Arithmetic Conditioning) marker segments for
 * the default parameters in the future.
 * Note that currently the marker writing module emits 12-byte
 * DAC segments for a full-component scan in a color image.
 * This is not worth worrying about IMHO. However, since the
 * spec defines the default values to be used if the tables
 * are omitted (unlike Huffman tables, which are required
 * anyway), one might optimize this behaviour in the future,
 * and then it would be disadvantageous to use custom tables if
 * they don't provide sufficient gain to exceed the DAC size.
 *
 * On the other hand, I'd consider it as a reasonable result
 * that the conditioning has no significant influence on the
 * compression performance. This means that the basic
 * statistical model is already rather stable.
 *
 * Thus, at the moment, we use the default conditioning values
 * anyway, and do not use the custom formula.
 *
#define CALCULATE_SPECTRAL_CONDITIONING
 */

/* IRIGHT_SHIFT is like RIGHT_SHIFT, but works on int rather than INT32.
 * We assume that int right shift is unsigned if INT32 right shift is,
 * which should be safe.
 */

#ifdef RIGHT_SHIFT_IS_UNSIGNED
#define ISHIFT_TEMPS	int ishift_temp;
#define IRIGHT_SHIFT(x,shft)  \
	((ishift_temp = (x)) < 0 ? \
	 (ishift_temp >> (shft)) | ((~0) << (16-(shft))) : \
	 (ishift_temp >> (shft)))
#else
#define ISHIFT_TEMPS
#define IRIGHT_SHIFT(x,shft)	((x) >> (shft))
#endif


LOCAL(void)
emit_byte (int val, j_compress_ptr cinfo)
/* Write next output byte; we do not support suspension in this module. */
{
  struct jpeg_destination_mgr * dest = cinfo->dest;

  *dest->next_output_byte++ = (JOCTET) val;
  if (--dest->free_in_buffer == 0)
    if (! (*dest->empty_output_buffer) (cinfo))
      ERREXIT(cinfo, JERR_CANT_SUSPEND);
}


/*
 * Finish up at the end of an arithmetic-compressed scan.
 */

METHODDEF(void)
finish_pass (j_compress_ptr cinfo)
{
  arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
  INT32 temp;

  /* Section D.1.8: Termination of encoding */

  /* Find the e->c in the coding interval with the largest
   * number of trailing zero bits */
  if ((temp = (e->a - 1 + e->c) & 0xFFFF0000L) < e->c)
    e->c = temp + 0x8000L;
  else
    e->c = temp;
  /* Send remaining bytes to output */
  e->c <<= e->ct;
  if (e->c & 0xF8000000L) {
    /* One final overflow has to be handled */
    if (e->buffer >= 0) {
      if (e->zc)
	do emit_byte(0x00, cinfo);
	while (--e->zc);
      emit_byte(e->buffer + 1, cinfo);
      if (e->buffer + 1 == 0xFF)
	emit_byte(0x00, cinfo);
    }
    e->zc += e->sc;  /* carry-over converts stacked 0xFF bytes to 0x00 */
    e->sc = 0;
  } else {
    if (e->buffer == 0)
      ++e->zc;
    else if (e->buffer >= 0) {
      if (e->zc)
	do emit_byte(0x00, cinfo);
	while (--e->zc);
      emit_byte(e->buffer, cinfo);
    }
    if (e->sc) {
      if (e->zc)
	do emit_byte(0x00, cinfo);
	while (--e->zc);
      do {
	emit_byte(0xFF, cinfo);
	emit_byte(0x00, cinfo);
      } while (--e->sc);
    }
  }
  /* Output final bytes only if they are not 0x00 */
  if (e->c & 0x7FFF800L) {
    if (e->zc)  /* output final pending zero bytes */
      do emit_byte(0x00, cinfo);
      while (--e->zc);
    emit_byte((e->c >> 19) & 0xFF, cinfo);
    if (((e->c >> 19) & 0xFF) == 0xFF)
      emit_byte(0x00, cinfo);
    if (e->c & 0x7F800L) {
      emit_byte((e->c >> 11) & 0xFF, cinfo);
      if (((e->c >> 11) & 0xFF) == 0xFF)
	emit_byte(0x00, cinfo);
    }
  }
}


/*
 * The core arithmetic encoding 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.
 *
 * Parameter 'val' to be encoded may be 0 or 1 (binary decision).
 *
 * Note: I've added full "Pacman" termination support to the
 * byte output routines, which is equivalent to the optional
 * Discard_final_zeros procedure (Figure D.15) in the spec.
 * Thus, we always produce the shortest possible output
 * stream compliant to the spec (no trailing zero bytes,
 * except for FF stuffing).
 *
 * I've also introduced a new scheme for accessing
 * the probability estimation state machine table,
 * derived from Markus Kuhn's JBIG implementation.
 */

LOCAL(void)
arith_encode (j_compress_ptr cinfo, unsigned char *st, int val) 
{
  extern const INT32 jaritab[];
  register arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
  register unsigned char nl, nm;
  register INT32 qe, temp;
  register int sv;

  /* Fetch values from our compact representation of Table D.2:
   * Qe values and probability estimation state machine
   */
  sv = *st;
  qe = jaritab[sv & 0x7F];	/* => Qe_Value */
  nl = qe & 0xFF; qe >>= 8;	/* Next_Index_LPS + Switch_MPS */
  nm = qe & 0xFF; qe >>= 8;	/* Next_Index_MPS */

  /* Encode & estimation procedures per sections D.1.4 & D.1.5 */
  e->a -= qe;
  if (val != (sv >> 7)) {
    /* Encode the less probable symbol */
    if (e->a >= qe) {
      /* If the interval size (qe) for the less probable symbol (LPS)
       * is larger than the interval size for the MPS, then exchange
       * the two symbols for coding efficiency, otherwise code the LPS
       * as usual: */
      e->c += e->a;
      e->a = qe;
    }
    *st = (sv & 0x80) ^ nl;	/* Estimate_after_LPS */
  } else {
    /* Encode the more probable symbol */
    if (e->a >= 0x8000L)
      return;  /* A >= 0x8000 -> ready, no renormalization required */
    if (e->a < qe) {
      /* If the interval size (qe) for the less probable symbol (LPS)
       * is larger than the interval size for the MPS, then exchange
       * the two symbols for coding efficiency: */
      e->c += e->a;
      e->a = qe;
    }
    *st = (sv & 0x80) ^ nm;	/* Estimate_after_MPS */
  }

  /* Renormalization & data output per section D.1.6 */
  do {
    e->a <<= 1;
    e->c <<= 1;
    if (--e->ct == 0) {
      /* Another byte is ready for output */
      temp = e->c >> 19;
      if (temp > 0xFF) {
	/* Handle overflow over all stacked 0xFF bytes */
	if (e->buffer >= 0) {
	  if (e->zc)
	    do emit_byte(0x00, cinfo);
	    while (--e->zc);
	  emit_byte(e->buffer + 1, cinfo);
	  if (e->buffer + 1 == 0xFF)
	    emit_byte(0x00, cinfo);
	}
	e->zc += e->sc;  /* carry-over converts stacked 0xFF bytes to 0x00 */
	e->sc = 0;
	/* Note: The 3 spacer bits in the C register guarantee
	 * that the new buffer byte can't be 0xFF here
	 * (see page 160 in the P&M JPEG book). */
	e->buffer = temp & 0xFF;  /* new output byte, might overflow later */
      } else if (temp == 0xFF) {
	++e->sc;  /* stack 0xFF byte (which might overflow later) */
      } else {
	/* Output all stacked 0xFF bytes, they will not overflow any more */
	if (e->buffer == 0)
	  ++e->zc;
	else if (e->buffer >= 0) {
	  if (e->zc)
	    do emit_byte(0x00, cinfo);
	    while (--e->zc);
	  emit_byte(e->buffer, cinfo);
	}
	if (e->sc) {
	  if (e->zc)
	    do emit_byte(0x00, cinfo);
	    while (--e->zc);
	  do {
	    emit_byte(0xFF, cinfo);
	    emit_byte(0x00, cinfo);
	  } while (--e->sc);
	}
	e->buffer = temp & 0xFF;  /* new output byte (can still overflow) */
      }
      e->c &= 0x7FFFFL;
      e->ct += 8;
    }
  } while (e->a < 0x8000L);
}


/*
 * Emit a restart marker & resynchronize predictions.
 */

LOCAL(void)
emit_restart (j_compress_ptr cinfo, int restart_num)
{
  arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
  int ci;
  jpeg_component_info * compptr;

  finish_pass(cinfo);

  emit_byte(0xFF, cinfo);
  emit_byte(JPEG_RST0 + restart_num, cinfo);

  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
    compptr = cinfo->cur_comp_info[ci];
    /* Re-initialize statistics areas */
    if (cinfo->progressive_mode == 0 || (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 == 0 || cinfo->Ss) {
      MEMZERO(entropy->ac_stats[compptr->ac_tbl_no], AC_STAT_BINS);
    }
  }

  /* Reset arithmetic encoding variables */
  entropy->c = 0;
  entropy->a = 0x10000L;
  entropy->sc = 0;
  entropy->zc = 0;
  entropy->ct = 11;
  entropy->buffer = -1;  /* empty */
}


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

METHODDEF(boolean)
encode_mcu_DC_first (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
{
  arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
  JBLOCKROW block;
  unsigned char *st;
  int blkn, ci, tbl;
  int v, v2, m;
  ISHIFT_TEMPS

  /* Emit restart marker if needed */
  if (cinfo->restart_interval) {
    if (entropy->restarts_to_go == 0) {
      emit_restart(cinfo, entropy->next_restart_num);
      entropy->restarts_to_go = cinfo->restart_interval;
      entropy->next_restart_num++;
      entropy->next_restart_num &= 7;
    }
    entropy->restarts_to_go--;
  }

  /* Encode the MCU data blocks */
  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;

    /* Compute the DC value after the required point transform by Al.
     * This is simply an arithmetic right shift.
     */
    m = IRIGHT_SHIFT((int) ((*block)[0]), cinfo->Al);

    /* Sections F.1.4.1 & F.1.4.4.1: Encoding 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.4: Encode_DC_DIFF */
    if ((v = m - entropy->last_dc_val[ci]) == 0) {
      arith_encode(cinfo, st, 0);
      entropy->dc_context[ci] = 0;	/* zero diff category */
    } else {
      entropy->last_dc_val[ci] = m;
      arith_encode(cinfo, st, 1);
      /* Figure F.6: Encoding nonzero value v */
      /* Figure F.7: Encoding the sign of v */
      if (v > 0) {
	arith_encode(cinfo, st + 1, 0);	/* Table F.4: SS = S0 + 1 */
	st += 2;			/* Table F.4: SP = S0 + 2 */
	entropy->dc_context[ci] = 4;	/* small positive diff category */
      } else {
	v = -v;
	arith_encode(cinfo, st + 1, 1);	/* Table F.4: SS = S0 + 1 */
	st += 3;			/* Table F.4: SN = S0 + 3 */
	entropy->dc_context[ci] = 8;	/* small negative diff category */
      }
      /* Figure F.8: Encoding the magnitude category of v */
      m = 0;
      if (v -= 1) {
	arith_encode(cinfo, st, 1);
	m = 1;
	v2 = v;
	st = entropy->dc_stats[tbl] + 20; /* Table F.4: X1 = 20 */
	while (v2 >>= 1) {
	  arith_encode(cinfo, st, 1);
	  m <<= 1;
	  st += 1;
	}
      }
      arith_encode(cinfo, st, 0);
      /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
      if (m < (int) (((INT32) 1 << cinfo->arith_dc_L[tbl]) >> 1))
	entropy->dc_context[ci] = 0;	/* zero diff category */
      else if (m > (int) (((INT32) 1 << cinfo->arith_dc_U[tbl]) >> 1))
	entropy->dc_context[ci] += 8;	/* large diff category */
      /* Figure F.9: Encoding the magnitude bit pattern of v */
      st += 14;
      while (m >>= 1)
	arith_encode(cinfo, st, (m & v) ? 1 : 0);
    }
  }

  return TRUE;
}


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

METHODDEF(boolean)
encode_mcu_AC_first (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
{
  arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
  JBLOCKROW block;
  unsigned char *st;
  int tbl, k, ke;
  int v, v2, m;

  /* Emit restart marker if needed */
  if (cinfo->restart_interval) {
    if (entropy->restarts_to_go == 0) {