/* Simulation model for a slotted-time switch with finite buffer. */
/* The switch has N input links, and only one output link.        */
/* There is a finite buffer available at the output link.         */
/* Configuration parameters are specified as interactive input.   */
/* The simulation models the packet arrival, queueing, and loss.  */
/* Written by Carey Williamson for CPSC 441     March 24, 2018    */

#include <stdio.h>

#define MAX_INPUTS 4
#define MAXINT 2147483647.0

/* #define DEBUG 1 */
/* #define RAND_DEBUG 1*/

int random();

int main()
  {
    int inpkt[MAX_INPUTS], pktssent[MAX_INPUTS], pktslost[MAX_INPUTS];
    int totpktssent, totpktslost;
    int outputq, maxqsize;
    int i, j, n, steps;
    int pktsthistime;
    float lambda;
    float prob;

    /* prompt user for configuration parameters */
    printf("Number of input links? ");
    scanf("%d", &n);
    if( n > MAX_INPUTS )
      {
	printf("Too many input links!!! Resetting to %d...\n", MAX_INPUTS);
	n = MAX_INPUTS;
      }

    printf("Maximum buffer size at output link? ");
    scanf("%d", &maxqsize);

    printf("Traffic load level? (lambda) ");
    scanf("%f", &lambda);

    printf("Simulation run length (time slots)? ");
    scanf("%d", &steps);

    /* Initialization */
    for( j = 0; j < n; j++ )
      {
	pktssent[j] = 0;
	pktslost[j] = 0;
	inpkt[j] = 0;
      }
    totpktssent = 0;
    totpktslost = 0;
    outputq = 0;
    pktsthistime=0;
    prob = 0;

    /* seed the pseudo-random number generator */
    srandom(time(NULL));


    /* Main loop: repeat once for each time slot being simulated */
    for( i = 0; i < steps; i++ )
      {
#ifdef DEBUG
	printf("Top of loop: output queue size at time step %d is %d\n", i, outputq);
#endif

	/* Generate the new packet arrivals in time step i */
	pktsthistime = 0;
	for( j = 0; j < n; j++ )
	  {
	    /* Generate a packet (at random) from each input link */
	    prob = random() / MAXINT;
#ifdef RAND_DEBUG
	    printf("random value for pkt generation is %f\n", prob);
#endif
	    if( prob <= lambda )
	      inpkt[j] = 1;
	    else inpkt[j] = 0;

	    if( inpkt[j] > 0 )
	      {
		pktsthistime++;
		pktssent[j]++;
		totpktssent++;
	      }
	  }
#ifdef DEBUG
	printf("There are %d new packets arriving in slot %d\n", pktsthistime, i);
#endif

	/* Move each new packet onto the output queue, if possible */
	for( j = 0; j < n; j++ )
	  {
	    if( inpkt[j] == 0 )
	      continue;

	    /* Check to see if it fits in queue or not */
	    if( outputq < maxqsize )
	      outputq++;
	    else
	      {
		pktslost[j]++;
		totpktslost++;
	      }
	  }
#ifdef DEBUG
	printf("Output queue size for step %d: %d\n", i, outputq);
	printf("Cumulative losses so far, by input link:\n");
	for( j = 0; j < n; j++ )
	  printf("%2d ", pktslost[j]);
	printf("\n");
#endif

	/* Transmit a packet on the output link (if queue is non-empty) */
	if( outputq > 0 )
	  outputq--;

#ifdef DEBUG
	printf("Bottom of loop: output queue size at time step %d is %d\n", i, outputq);
#endif
      }

    /* Print simulation results */
    printf("\n\nSimulation run length: %d time steps\n", steps);
    printf("Total packets generated: %d\n", totpktssent);
    printf("      By input link: ");
    for( j = 0; j < n; j++ )
      printf("%2d ", pktssent[j]);
    printf("\n");
    printf("Total packets lost: %d (%10.8f)\n",
	   totpktslost, (float) 1.0 * totpktslost/totpktssent);
    printf("      By input link: ");
    for( j = 0; j < n; j++ )
      printf("%2d ", pktslost[j]);
    printf("\n");
  }