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

unsigned short summera(unsigned short x, unsigned short y){
    if ((int)x + (int)y > 65535){
        return (x + y) + 1;
    }
	return x + y;
}

unsigned short sum(unsigned short x, unsigned short y){
	return x + y;
}


int main()
{
//    Used by the random number generator. It makes sure that the random numbers
//    generated are different when the program is run more than once.
    time_t t;
    srand((unsigned) time(&t));

    const int nbrWords = 4; //The number of 16-bit words in a package
    const int nbrFaults = 2; //The number of faults in each packet

//    Used when a bit is changed in a data word, position[i] = 1 in bit number i, 0 in all other bits
    unsigned short position[16];
    position[0] = 1;
    for (int i = 1; i < 16; i++){
        position[i] = position[i - 1]*2;
    }

    int numberThrown = 0; //Counts the number of packets that are regarded as correct by the receiver

    for (int k = 0; k < 100000; k++){

        unsigned short packet[nbrWords + 1]; //The packet that will be generated. After that a number of bits
                                             //are changed and then it is seen if the checksum still is FFF

        //The data of the packet is given a random content
        for (int i = 0; i < nbrWords; i++){
            packet[i] = (rand() % 0x100)*0x100 + (rand() % 0x100);
        }

        //The checksum is calculated and added to the packet
        unsigned short checksum = packet[0];
        for (int i = 1; i < nbrWords; i++){
            checksum = summera(checksum, packet[i]);
        }
        packet[nbrWords] = 0xFFFF - checksum;


        //nabFaults bits are changed, the result is put in the packet badPacket
        int nbrChanged = 0;
        unsigned short used[nbrWords + 1];
        for (int i = 0; i <= nbrWords; i++){
            used[i] = 0x0000;
        }
        unsigned short badPacket[nbrWords + 1];
        for (int n = 0; n <= nbrWords; n++){
            badPacket[n] = packet[n];
        }
        while (nbrChanged < nbrFaults){
            int row = rand() % (nbrWords + 1);
            int pos = rand() % 16;
            if (!(used[row]&position[pos])){
                badPacket[row] = (badPacket[row]^position[pos]);
                used[row] = used[row] + position[pos];
                nbrChanged++;
            }
        }

        //It is checked if badPacket will be accepted by the receiver. If so, numberFaults is increased.
        unsigned short test2 = badPacket[0];
        for (int i = 1; i <= nbrWords; i++){
            test2 = summera(test2, badPacket[i]);
        }
        if (test2 != 0xFFFF){
            numberThrown++;
        }

    }

    printf("Number thrown: %i", numberThrown);

    return 0;
}