/************************************************************************ * *  sac.c, part of tmndecode (H.263 decoder) *  Copyright (C) 1995, 1996  Telenor R&D, Norway * *  Contacts: *  Robert Danielsen                  <Robert.Danielsen@nta.no> * *  Telenor Research and Development  http://www.nta.no/brukere/DVC/ *  P.O.Box 83                        tel.:   +47 63 84 84 00 *  N-2007 Kjeller, Norway            fax.:   +47 63 81 00 76 * *  Copyright (C) 1997  University of BC, Canada *  Modified by: Michael Gallant <mikeg@ee.ubc.ca> *               Guy Cote <guyc@ee.ubc.ca> *               Berna Erol <bernae@ee.ubc.ca> * *  Contacts: *  Michael Gallant                   <mikeg@ee.ubc.ca> * *  UBC Image Processing Laboratory   http://www.ee.ubc.ca/image *  2356 Main Mall                    tel.: +1 604 822 4051 *  Vancouver BC Canada V6T1Z4        fax.: +1 604 822 5949 * ************************************************************************//* Disclaimer of Warranty *  * These software programs are available to the user without any license fee * or royalty on an "as is" basis. The University of British Columbia * disclaims any and all warranties, whether express, implied, or * statuary, including any implied warranties or merchantability or of * fitness for a particular purpose.  In no event shall the * copyright-holder be liable for any incidental, punitive, or * consequential damages of any kind whatsoever arising from the use of * these programs. *  * This disclaimer of warranty extends to the user of these programs and * user's customers, employees, agents, transferees, successors, and * assigns. *  * The University of British Columbia does not represent or warrant that the * programs furnished hereunder are free of infringement of any * third-party patents. *  * Commercial implementations of H.263, including shareware, are subject to * royalty fees to patent holders.  Many of these patents are general * enough such that they are unavoidable regardless of implementation * design. *  *//********************************************************************* *        SAC Decoder Module *        Algorithm as Specified in H26P Annex -E *              (c) 1995 BT Labs * *	Author:	Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/#include <stdio.h>#include <string.h>#include "config.h"#include "tmndec.h"#include "global.h"#define   q1    16384#define   q2    32768#define   q3    49152#define   top   65535/* local prototypes */void bit_out_psc_layer ();/********************************************************************* *        SAC Decoder Algorithm as Specified in H26P Annex -E * *        Name:        decode_a_symbol * *	Description:	Decodes an Aritmetically Encoded Symbol * *	Input:        array holding cumulative freq. data *        also uses static data for decoding endpoints *        and code_value variable * *	Returns:	Index to relevant symbol model * *	Side Effects:	Modifies low, high, length, cum and code_value * *	Author:        Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/static long low, high, code_value, bit, length, sacindex, cum, zerorun = 0;int decode_a_symbol (int cumul_freq[]){  length = high - low + 1;  cum = (-1 + (code_value - low + 1) * cumul_freq[0]) / length;  for (sacindex = 1; cumul_freq[sacindex] > cum; sacindex++);  high = low - 1 + (length * cumul_freq[sacindex - 1]) / cumul_freq[0];  low += (length * cumul_freq[sacindex]) / cumul_freq[0];  for (;;)  {    if (high < q2);    else if (low >= q2)    {      code_value -= q2;      low -= q2;      high -= q2;    } else if (low >= q1 && high < q3)    {      code_value -= q1;      low -= q1;      high -= q1;    } else    {      break;    }    low *= 2;    high = 2 * high + 1;    bit_out_psc_layer ();    code_value = 2 * code_value + bit;  }  return (sacindex - 1);}/********************************************************************* * *        Name:        decoder_reset * *	Description:	Fills Decoder FIFO after a fixed word length *        string has been detected. * *	Input:        None * *	Returns:	Nothing * *	Side Effects:	Fills Arithmetic Decoder FIFO * *	Author:        Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/void decoder_reset (){  int i;  zerorun = 0;                  /* clear consecutive zero's counter */  code_value = 0;  low = 0;  high = top;  for (i = 1; i <= 16; i++)  {    bit_out_psc_layer ();    code_value = 2 * code_value + bit;  }  if (trace)    fprintf (trace_file, "\nArithmetic Decoder Reset \n");}/********************************************************************* * *        Name:        bit_out_psc_layer * *	Description:	Gets a bit from the Encoded Stream, Checks for *        and removes any PSC emulation prevention bits *        inserted at the decoder, provides 'zeros' to the *        Arithmetic Decoder FIFO to allow it to finish *        data prior to the next PSC. (Garbage bits) * *	Input:        None * *	Returns:	Nothing * *	Side Effects:	Gets a bit from the Input Data Stream * *	Author:        Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/void bit_out_psc_layer (){  if (showbits (17) != 1)  {                             /* check for startcode in Arithmetic                                 * Decoder FIFO */    bit = getbits (1);    if (zerorun > 13)    {                           /* if number of consecutive zeros = 14 */      if (!bit)      {        if (trace)          fprintf (trace_file, "PSC/GBSC, Header Data, or Encoded Stream Error \n");        zerorun = 1;      } else      {                         /* if there is a 'stuffing bit present */        if (trace)          fprintf (trace_file, "Removing Startcode Emulation Prevention bit \n");        bit = getbits (1);      /* overwrite the last bit */        zerorun = !bit;         /* zerorun=1 if bit is a '0' */      }    } else    {                           /* if consecutive zero's not exceeded 14 */      if (!bit)        zerorun++;      else        zerorun = 0;    }  }   /* end of if !(showbits(17)) */   else  {    bit = 0;    if (trace)      fprintf (trace_file, "Startcode Found:Finishing Arithmetic Decoding using 'Garbage bits'\n");  }  /* printf("lastbit = %ld bit = %ld zerorun = %ld \n", lastbit, bit,   * zerorun); lastbit = bit; */  /* latent diagnostics */}