SlidingWindowAverage.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 *
 * @author Maikel Chandika ([email protected])
 *
 * Java Array Window Sliding Average.
 * With O(n) time complexity.
 *
 */
public class SlidingWindowAverage {

    public static void main(String[] args) {
        int[] input = {2, 3, 4, 5}; // -> [2.5, 3.5, 4.5]
        System.out.println("before: " + Arrays.toString(input));
        System.out.println("after : "+ Arrays.toString(calc(input, 2)));
        System.out.println();

        int[] input2 = {2, 3, 4, 5, 8, 9, 7}; // -> [2.5, 3.5, 4.5, 6.5, 8.5, 8]
        System.out.println("before: " + Arrays.toString(input2));
        System.out.println("after : "+ Arrays.toString(calc(input2, 2)));
        System.out.println();

        int[] input3 = {2, 5, 7}; // -> [3.5, 6]
        System.out.println("before: " + Arrays.toString(input3));
        System.out.println("after : "+ Arrays.toString(calc(input3, 2)));
    }

    static Double[] calc(int[] arr, int window) {
        List<Double> r = new ArrayList<>();
        int n = 1;
        double sum = 0.0d, w = window, start = (double) arr[0];
        for (int i = 0; i < arr.length; i++) {
            double d = arr[i];
            if (n < window) {
                sum += d;
                n++;
            } else {
                sum = sum + d;
                r.add(sum / w);
                sum = sum - start;
                start = arr[i - (window - 2) > 0 ? i - (window - 2) : 0];
                n = window < 3 ? window : window - 1;
            }
        }
        return r.toArray(new Double[r.size()]);
    }
}