/* Simple program to computer Erlang B formula */
/* Usage: cc -o formula formula.c -lm          */
/*        ./formula lambda duration capacity   */
/*                                             */
/* Original version by Jean Cao, May 2003      */
/* Modified by Carey Williamson, December 2003 */

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

double factorial(int f)
  {
    if( f == 1 )
	return( f * 1.0 );
    else return( (double) (factorial(f-1) * f) ); 
  }

double sigma(float Y, int M)
  {
    if ( M == 0 )
	return 1;
    else return( sigma(Y, M-1) + pow(Y, M)/factorial(M) );
  }

int main(int argc, char *argv[])
  {
    float lambda, h, Y, R, V;
    int M;
    float lost;

    if (argc != 4)
      {
	fprintf(stderr, "Usage: formula ArrivalRate HoldingTime NumChannels\n");
	exit(1);
      }
  
    /* Determine arrival rate, mean holding time, and num channels */
    lambda = atof(argv[1]);
    h     = atof(argv[2]);
    M     = atoi(argv[3]);

    /* Compute load in Erlangs */
    Y     = lambda * h;

    /* Compute ErlangB loss probability */
    lost = (pow(Y, M)/factorial(M))/sigma(Y, M);
    printf("ErlangB(%f, %i) = %f\n", Y, M, lost);

    return(0);
  }