programming-examples/java/Data_Structures/CycleDetection.java

import java.util.*;
import java.util.function.IntFunction;

public class CycleDetection {

	public static int[] floyd(IntFunction<Integer> f, int x0) {
		int tortoise = f.apply(x0);
		int hare = f.apply(f.apply(x0));

		while (tortoise != hare) {
			tortoise = f.apply(tortoise);
			hare = f.apply(f.apply(hare));
		}

		int startPos = 0;
		tortoise = x0;
		while (tortoise != hare) {
			tortoise = f.apply(tortoise);
			hare = f.apply(hare);
			++startPos;
		}

		int length = 1;
		hare = f.apply(tortoise);
		while (tortoise != hare) {
			hare = f.apply(hare);
			++length;
		}

		return new int[]{startPos, length};
	}

	public static int[] brent(IntFunction<Integer> f, int x0) {
		int power = 1;
		int length = 1;

		int tortoise = x0;
		int hare = f.apply(x0);

		while (tortoise != hare) {
			if (power == length) {
				tortoise = hare;
				power *= 2;
				length = 0;
			}
			hare = f.apply(hare);
			++length;
		}

		int startPos = 0;
		tortoise = x0;
		hare = x0;
		for (int i = 0; i < length; i++)
			hare = f.apply(hare);

		while (tortoise != hare) {
			tortoise = f.apply(tortoise);
			hare = f.apply(hare);
			++startPos;
		}

		return new int[]{startPos, length};
	}

	// random test
	public static void main(String[] args) {
		Random rnd = new Random(1);
		for (int step = 0; step < 100_000; step++) {
			int a = rnd.nextInt(100);
			int b = rnd.nextInt(100);
			int c = rnd.nextInt(100) + 1;
			IntFunction<Integer> f = x -> (a * x * x + b) % c;
			int[] floyd = floyd(f, 0);
			int[] brent = brent(f, 0);
			if (!Arrays.equals(floyd, brent))
				throw new RuntimeException();
		}
		System.out.println(floyd(x -> (41 * x + 1) % 1000_000, 0)[1]);
	}
}