using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Linq;

​

public static class Program

{

    public static void Main()

    {

        IEnumerable<int> source = Iterator();

        // Enumerate the source, to prove that each enumeration restarts the iterator

        Enumerate("Source.Take(5)", source.Take(5));

        Enumerate("Source", source);

        Enumerate("Source.TakeLast(5)", source.TakeLast(5));

        Console.Write($"Source.Sum(): "); Console.WriteLine(source.Sum());

        Console.WriteLine();

​

        var memoised = source.MemoiseNotSynchronized();

        // Now enumerate the memoised, to prove that the iterator is started once

        Enumerate("Memoised.Take(5)", memoised.Take(5));

        Enumerate("Memoised", memoised);

        Enumerate("Memoised.TakeLast(5)", memoised.TakeLast(5));

        Console.Write($"Memoised.Sum(): "); Console.WriteLine(memoised.Sum());

        Console.WriteLine();

​

        static IEnumerable<int> Iterator()

        {

            Console.Write("Starting! ");

            try { foreach (var i in Enumerable.Range(1, 10)) yield return i; }

            finally { Console.Write("Finished! "); }

        }

​

        static void Enumerate(string title, IEnumerable<int> sequence)

        {

            Console.Write($"{title}: ");

            foreach (var i in sequence) Console.Write($"{i}, ");

            Console.WriteLine();

        }

    }

​

    public static IEnumerable<T> MemoiseNotSynchronized<T>(this IEnumerable<T> source)

    {

        // Argument validation omitted

        IEnumerator<T> enumerator = null;

        List<T> buffer = null;

        return Implementation();

​

        IEnumerable<T> Implementation()

        {

            if (buffer != null && enumerator == null)

            {

                // The source has been fully enumerated

                foreach (var item in buffer) yield return item;

                yield break;

            }

​

            enumerator ??= source.GetEnumerator();

            buffer ??= new();

            for (int i = 0; ; i = checked(i + 1))

            {

                if (i < buffer.Count)

                {

                    yield return buffer[i];

                }

                else if (enumerator.MoveNext())

                {

                    Debug.Assert(buffer.Count == i);

                    var current = enumerator.Current;

                    buffer.Add(current);

                    yield return current;

                }

                else

                {

                    enumerator.Dispose(); enumerator = null;

                    yield break;

                }

            }

        }

    }

}