#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <time.h>

//  Written by Chris.McDonald@uwa.edu.au

//  RETURNS A PROBABILITY IN THE RANGE [0.0 .. 1.0]
double PROB(void)
{
    return ((double)rand()/(double)RAND_MAX);
}

//  SIMULATE SLOTTED-ALOHA COMMUNICATION, RETURN UTILISATION OF THE MEDIUM
double SlottedAloha(int nslots, int nnodes, double ProbNew, double ProbRetry)
{
#define FOREACH_TIME_SLOT    for(int s=0 ; s<nslots ; ++s)
#define FOREACH_NODE         for(int n=0 ; n<nnodes ; ++n)

//  COUNT THE NUMBER OF SUCCESSFUL TRANSMISSIONS ACROSS ALL SLOTS
    int nsuccesses  = 0;

//  REMEMBER WHETHER EACH NODE HAS A PENDING TRANSMISSION
    bool pending[nnodes];
    FOREACH_NODE {
        pending[n]  = false;
    }

//  ITERATE THROUGH TIME
    FOREACH_TIME_SLOT {
        int     nattempting     = 0;
        int     successful_node;

//  DETERMINE WHAT EACH NODE DOES IN CURRENT TIMESLOT
        FOREACH_NODE {

//  DOES THE NODE HAVE A NEW TRANSMISSION?
            if(!pending[n] && PROB() < ProbNew) {
                pending[n]      = true;
                successful_node = n;
                ++nattempting;
            }
//  DOES THE NODE HAVE A RE-TRANSMISSION?
            else if(pending[n] && PROB() < ProbRetry) {
                successful_node = n;
                ++nattempting;
            }
        }

//  IFF JUST ONE NODE WAS SUCCESSFUL...
        if(nattempting == 1) {
            ++nsuccesses;
            pending[successful_node]    = false;    // frame sent
        }
    }

//  RETURN UTILISATION OF THE MEDIUM
    return (double)nsuccesses / nslots;

#undef FOREACH_TIME_SLOT
#undef FOREACH_NODE
}


int main(int argc, char *argv[])
{
//  DEFAULT ATTRIBUTES FOR OUR SIMULATION
    int    nslots       = 10000;
    int    nnodes       = 5;
    double ProbNew      = 0.1;
    double ProbRetry    = 0.1;

//  ACCEPT COMMAND-LINE ARGUMENTS TO MODIFY THE DEFAULT ATTRIBUTES
    if(argc == 5) {
        nslots          = atoi(argv[1]);
        nnodes          = atoi(argv[2]);
        ProbNew         = atof(argv[3]);
        ProbRetry       = atof(argv[4]);
    }

//  CHECK THE CHOSEN ATTRIBUTES
    if(nslots<1 || nnodes<1 ||
       ProbNew<0.0 || ProbNew>1.0 || ProbRetry<0.0 || ProbRetry>1.0) {
        fprintf(stderr, "Usage: %s nslots nnodes prob-new prob-Tx\n", argv[0]);
        exit(EXIT_FAILURE);
    }

//  SEED RANDOM NUMBER SEQUENCE
    srand((long)time(NULL));

//  SIMULATE THE COMMUNICATION, PRINT ITS UTILISATION
    printf("%.2f\n", 100.0 * SlottedAloha(nslots, nnodes, ProbNew, ProbRetry));

    exit(EXIT_SUCCESS);
}

//  vim: ts=8 sw=4