A Pseudo Random Generator in Java to Shuffle an Array/List

Sometimes, we need to shuffle an array or list in Java - but we want to get the same "random" output for the same input. The following is a Pseudo Random Generator that is implemented in Java that shuffles an array or list using a seed.

The RANDOM_GENERATOR_NUMBER is a prime number (e.g. 999_999_000_001) and we use it to do some bit shifting and XOR. Then, we generate a random index to swap.

Prime numbers when factorization are using integers only 1 and itself. For example, prime number 999999000001 can only be represented as the product of 1 and 999999000001.

package helloacm.com;
 
import java.util.Arrays;
import java.util.List;
 
public class RandomGenerator<T> {
    private static long RANDOM_GENERATOR_NUMBER = 999_999_000_001L;
    public List<T> shuffle(List<T> list, long seed) {
        seed = seed << 32;
        final var sz = list.size();
        for (int i = 0; i < sz; i++) {
            long v = seed + i * RANDOM_GENERATOR_NUMBER;
            v = v ^ (v >> 12);
            v = v ^ (v << 25);
            v = v ^ (v >> 27);
            v = v * RANDOM_GENERATOR_NUMBER;
            int index = (int) (i + v % (sz - i));
            if (index >= 0 && index < sz) {
                T tmp = list.get(index);
                list.set(index, list.get(i));
                list.set(i, tmp);
            }
        }
        return list;
    }
}

For example, if you run the following program to shuffle an array of 7 numbers (1 to 7):

package helloacm.com
 
public class Main {
    public static void main(String[] args) {
        List<Integer> data = Arrays.asList(new Integer[] {1, 2, 3, 4, 5, 6, 7});
        RandomGenerator<Integer> rand = new RandomGenerator<>();
        rand.shuffle(data, 1234);
        System.out.println(data);;
    }
}

It will always print:

[7, 5, 3, 4, 2, 6, 1]

Reposted to Blog

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