// ------------------------------------------------------------------------
// File:   simulation313.c
//
// Simulation of a repetition code over a binary symmetric channel
//
// ------------------------------------------------------------------------
// This program is complementary material for the book:
//
// R.H. Morelos-Zaragoza, The Art of Error Correcting Coding, Wiley, 2002.
//
// ISBN 0471 49581 6
//
// This and other programs are available at http://the-art-of-ecc.com
//
// You may use this program for academic and personal purposes only. 
// If this program is used to perform simulations whose results are 
// published in a journal or book, please refer to the book above.
//
// The use of this program in a commercial product requires explicit
// written permission from the author. The author is not responsible or 
// liable for damage or loss that may be caused by the use of this program. 
//
// Copyright (c) 2002. Robert H. Morelos-Zaragoza. All rights reserved.
// ------------------------------------------------------------------------

#include <stdio.h>
#include <math.h>
#include <limits.h>

#define MAXRAND LONG_MAX      // Maximum value of random()
#define N 3                   // Code length
#define TWOTTN 3              // Equal to 2^{N-1}-1

main(int argc, char *argv[])
{

  long seed;
  double init_p, final_p, inc_p;
  double sim, num_sim;

  double p;
  int codeword[N], received[N], decoded[N];

  double error;
  int i, info, estimate, onescount;

  FILE *fp;
  char name[80];

  if (argc != 7)
    {
      printf("Usage: %s seed init_p final_p inc_p num_sim output_file\n",
              argv[0]);
      exit(0);
    }

  sscanf(argv[1],"%ld", &seed);
  sscanf(argv[2],"%lf", &init_p);
  sscanf(argv[3],"%lf", &final_p);
  sscanf(argv[4],"%lf", &inc_p);
  sscanf(argv[5],"%lf", &num_sim);
  sscanf(argv[6],"%s",  &name);

  fp = fopen(name,"w");
  srandom(seed);

  p = init_p;

  while (p < final_p)
    {
    error = 0.0;
    sim = 0.0;

    while (sim < num_sim)
      {

      // SOURCE: Generate random symbols
      info = (random()>>10) % 2;

      // ENCODER: Encode information
      if (info)
        {
        for (i=0; i<N; i++)
        codeword[i] = 1;
        }
      else
        {
        for (i=0; i<N; i++)
        codeword[i] = 0;
        }

      // CHANNEL: Pass through BSC with parameter p
      for (i=0; i<N; i++)
        received[i] = codeword[i] ^ bsc(p);

      // DECODER: Majority logic decoding is easiest
      onescount = 0;
      for (i=0; i<N; i++)
        if (received[i]) onescount++;

      if (onescount > N/2) 
        {
        estimate = 1;
        for (i=0; i<N; i++)
          decoded[i] = 1;
        }
      else
        {
        estimate = 0;
        for (i=0; i<N; i++)
          decoded[i] = 0;
        }

      if (estimate != info) error += 1.0;

      sim += 1.0;

      }

    fprintf(fp, "%e %e\n", p, (double)error/num_sim);
    printf("%e %e\n", p, (double)error/num_sim);

    fflush(stdout); fflush(fp);

    p += inc_p;

    }

}

int bsc(double p)
{
  if ( ((double)(random())/MAXRAND) <= p )
    return(1);
  else
    return(0);
}