CODEWITHSUNDEEP
c++random
tim
tim

Reputation: 319

generating a random dice roll using LFSR

I have code that generates a random number using a Left Feedback Shift Register that I got off the internet:

#define POLY_MASK_32 0xB4BCD35C
#define POLY_MASK_31 0x7A5BC2E3

#include <iostream>

typedef unsigned int uint;

uint lfsr32, lfsr31;

int shift_lfsr(uint *lfsr,  uint polymonial_mask)
{
    int feedback;

    feedback = *lfsr & 1;
    *lfsr >>= 1;
    if (feedback == 1)
        *lfsr ^= polymonial_mask;
    return *lfsr;
}

void init_lfsrs(void)
{
    lfsr32 = 0xABCDE; //seed values
    lfsr31 = 0x23456789;
}

int get_random(void)
{
    /*this random number generator shifts the 32-bit LFSR twice before XORing
      it with the 31-bit LFSR. the bottom 16 bits are used for the random number*/
    shift_lfsr(&lfsr32, POLY_MASK_32);
    return(shift_lfsr(&lfsr32, POLY_MASK_32) ^ shift_lfsr(&lfsr31, POLY_MASK_31));
}

void main(void)
{
    int random_value[10];

    init_lfsrs();
    for(int i = 0; i < 10; i++)
    {
        random_value[i] = get_random();
        std::cout << random_value[i] << std::endl;
    }
}

I dont fully unerstand whats going on here but I know that it will produce a 32-bit number in a non repeating sequence. I think it then takes the first 16-bits to be the number displayed.

What I am wanting to do is produce a number between 1-6 from this...anyone able to help out?

Edit I was also going to change the 2 seed values to an srand(time) rand() number so that it doesn't start the same every time. is this right and how would i go about that?

Upvotes: 0

Views: 3348

Answers (1)

Muckle_ewe
Muckle_ewe

Reputation: 1123

To produce a random number between 1 and 6, do this

 int myRandomNumber = 1 + (random_value[i] % 6);

Why does this work?

random_value[i] % 6

will produce a value between 0 and 5, so we add 1 to it to get a value between 1 and 6.

Read up on the modulus operator

http://www.cprogramming.com/tutorial/modulus.html

In general, to produce a random number in some closed set [a, b], you would do

int myRandomNumber = a + ( random_value[i] % (b - a + 1) )

Upvotes: 1

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