Question

I need to generate random integers within a maximum. Since performance is critical, I decided to use a XORShift generator instead of Java’s Random class.

long seed = System.nanoTime();
seed ^= (seed << 21);
seed ^= (seed >>> 35);
seed ^= (seed << 4);

This implementation _(source) gives me a long integer, but what I really want is an integer between 0 and a maximum.

public int random(int max){ /*...*/}

What it is the most efficient way to implement this method?

Answer 1

I had some fun with your code and came up with this:

public class XORShiftRandom {

private long last;
private long inc;

public XORShiftRandom() {
    this(System.currentTimeMillis());
}

public XORShiftRandom(long seed) {
    this.last = seed | 1;
    inc = seed;
}

public int nextInt(int max) {
    last ^= (last << 21);
    last ^= (last >>> 35);
    last ^= (last << 4);
    inc += 123456789123456789L;
    int out = (int) ((last+inc) % max);     
    return (out < 0) ? -out : out;
}

}

I did a simple test and it is about Four times as fast as the java.util.Random

If you are intrested in how it works you can read this paper:

Disclamer:

The code above is designed to be used for research only, and not as a
replacement to the stock Random or SecureRandom.