/************************************************************************ *SAC  ************************************************************************//********************************************************************* * * SAC Encoder Module * Algorithm as specified in H263 (Annex E) *         (c) BT Labs 1995 * * Author: Pat Mulroy <pmulroy@visual.bt.co.uk> * *********************************************************************/#include <stdio.h>#include "sim.h"#define   q1    16384#define   q2    32768#define   q3    49152#define   top   65535static long   low=0, high=top, opposite_bits=0, length=0, zerorun=0;/********************************************************************* * *      Name:           AR_Encode * *      Description:    Encodes a symbol using syntax based arithmetic *        coding. Algorithm specified in H.263 (Annex E). * *      Input:          Array holding cumulative frequency data. *        Index into specific cumulative frequency array. *                      Static data for encoding endpoints. * *      Returns:        Number of bits used while encoding symbol. * *      Side Effects:   Modifies low, high, length and opposite_bits *        variables. * *      Author:         pmulroy@visual.bt.co.uk * *********************************************************************/int AR_Encode(int index, int cumul_freq[ ]){  int bitcount=0;  if (index<0)     return -1; /* Escape Code */  length = high - low + 1;  high = low - 1 + (length * cumul_freq[index]) / cumul_freq[0];  low += (length * cumul_freq[index+1]) / cumul_freq[0];  for ( ; ; ) {    if (high < q2) {      bitcount+=bit_opp_bits(0);    }    else if (low >= q2) {      bitcount+=bit_opp_bits(1);	      low -= q2;       high -= q2;    }    else if (low >= q1 && high < q3) {      opposite_bits += 1;       low -= q1;       high -= q1;    }    else break;     low *= 2;     high = 2*high+1;  }  return bitcount;}int bit_opp_bits(int bit) /* Output a bit and the following opposite bits */              {                                     int bitcount=0;  bitcount = bit_in_psc_layer(bit);  while(opposite_bits > 0){    bitcount += bit_in_psc_layer(!bit);    opposite_bits--;  }  return bitcount;}/********************************************************************* * *      Name:           encoder_flush * *      Description:    Completes arithmetic coding stream before any *        fixed length codes are transmitted. * *      Input:          None * *      Returns:        Number of bits used. * *      Side Effects:   Resets low, high, zerorun and opposite_bits  *        variables. * *      Author:         pmulroy@visual.bt.co.uk * *********************************************************************/int encoder_flush(){  int bitcount = 0;  if (trace)    fprintf(tf, "encoder_flush:\n");  opposite_bits++;  if (low < q1) {    bitcount+=bit_opp_bits(0);  }  else {    bitcount+=bit_opp_bits(1);  }  low = 0;   high = top;  zerorun=0;  return bitcount;}/********************************************************************* * *      Name:           bit_in_psc_layer * *      Description:    Inserts a bit into output bitstream and avoids *        picture start code emulation by stuffing a one *        bit. * *      Input:          Bit to be output. * *      Returns:        Nothing * *      Side Effects:   Updates zerorun variable. * *      Author:         pmulroy@visual.bt.co.uk * *********************************************************************/int bit_in_psc_layer(int bit){  void putbits (int, int);  int bitcount = 0;  if (zerorun > 13) {    if (trace)      fprintf(tf, "PSC emulation ... Bit stuffed.\n");    putbits (1, 1);    bitcount++;    zerorun = 0;  }  putbits (1, bit);  bitcount++;  if (bit)    zerorun = 0;  else    zerorun++;  return bitcount;}/********************************************************************* * *      Name:           indexfn * *      Description:    Translates between symbol value and symbol *        index. * *      Input:          Symbol value, index table, max number of *        values. * *      Returns:        Index into cumulative frequency tables or *        escape code. * *      Side Effects:   none * *      Author:         pmulroy@visual.bt.co.uk * *********************************************************************/int indexfn(int value, int table[], int max){  int n=0;  while(1) {    if (table[n++]==value) return n-1;    if (n>max) return -1;  }}