using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

    class Program {

        static void Main(string[] args) {
            var source = Enumerable.Range(0, 10000000);
            var sw = Stopwatch.StartNew();
            var r = Windowed(source, 3).Count();
            sw.Stop();
            Console.WriteLine(r);
            Console.WriteLine(sw.Elapsed);
        }

        static IEnumerable<T[]> Windowed<T>(IEnumerable<T> source, int windowSize) {
            if (source == null)
                throw new ArgumentNullException("source");
            if (windowSize <= 0)
                throw new ArgumentOutOfRangeException("windowSize");
            var arr = new T[windowSize];
            int r = (windowSize - 1);
            int i = 0;
            foreach (var item in source) {
                arr[i] = item;
                i = (i + 1) % windowSize;
                if (r == 0) {
                    var arrR = new T[windowSize];
                    for (int j = 0; j < windowSize; j++)
                        arrR[j] = arr[(i + j) % windowSize];
                    yield return arrR;
                } else {
                    r--;
                }
            }
        }
    }