Repeating Pattern in a sequence

Question

I am making an encrypting program in java and I have a little problem.
I need to know how I can find repeating patterns in a sequences of numbers:
For Ex:
{1,2,3,1,2,3}
repeating pattern is 1,2,3 and the pattern occurs twice
{1,2,3,2,3}
no repeating pattern
{5,0,0,5,0,0}
500 is the repeating pattern and occurs twice
I hope you get the idea. What the best way to do this efficiently? I think there is an algorithm already designed for this.. but not sure.
Appreciate any help,
John

Answer 1

This little program can detect a period in a list of ints
[not optimal but functional :^]:
public class PeriodDetector {
    public static boolean isCyclic(int list[], int period) {
        for (int i=period; i<list.length; i++) {
            if (list[i-period]!=list[i ]) {
                return false;
        return true;
    public static int detectPeriod(int list[]) {
        for (int period=1; period<=list.length/2; period++) {
            if (list[0]==list[period] && list.length%period==0) {
                if (isCyclic(list, period)) {
                    return period;
        return -1;
    public static void main(String argv[]) {
        int list[] = new int[argv.length];
        for (int l=0; l<list.length; l++) {
            list[l] = Integer.parseInt(argv[l]);
        System.out.println("Period..." + detectPeriod(list));