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

namespace SkipWhilePerf
{
    public static class Program
    {
        private static readonly Stopwatch s_watch = new Stopwatch();

        static void Start() { s_watch.Restart(); }
        static void Stop() { s_watch.Stop(); Console.WriteLine(s_watch.Elapsed); }
        static void Restart() { Stop(); Start(); }

        public static void Main()
        {
            int[] array = Enumerable.Range(1, 1000000).ToArray();

            // old
            Start();

            foreach (var foo in array.OldSkipWhile(i => i < 500000));

            Restart();

            foreach (var foo in array.OldSkipWhile((i, index) => i < 500000 && index < 500000));

            // new
            Restart();

            foreach (var bar in array.NewSkipWhile(i => i < 500000));

            Restart();

            foreach (var bar in array.NewSkipWhile((i, index) => i < 500000 && index < 500000));

            Stop();
        }

        public static IEnumerable<T> OldSkipWhile<T>(this IEnumerable<T> src, Func<T, bool> func)
        {
            bool yielding = false;
            foreach (T element in src)
            {
                if (!yielding && !func(element)) yielding = true;
                if (yielding) yield return element;
            }
        }

        public static IEnumerable<T> OldSkipWhile<T>(this IEnumerable<T> src, Func<T, int, bool> func)
        {
            int index = -1;
            bool yielding = false;
            foreach (T element in src)
            {
                checked { index++; }
                if (!yielding && !func(element, index)) yielding = true;
                if (yielding) yield return element;
            }
        }

        public static IEnumerable<T> NewSkipWhile<T>(this IEnumerable<T> src, Func<T, bool> func)
        {
            using (IEnumerator<T> e = src.GetEnumerator())
            {
                do
                    if (!e.MoveNext()) yield break;
                while (func(e.Current));
                do
                    yield return e.Current;
                while (e.MoveNext());
            }
        }

        public static IEnumerable<T> NewSkipWhile<T>(this IEnumerable<T> src, Func<T, int, bool> func)
        {
            int index = -1;
            using (IEnumerator<T> e = src.GetEnumerator())
            {
                do
                {
                    if (!e.MoveNext()) yield break;
                    checked { index++; }
                } while (func(e.Current, index));
                do
                    yield return e.Current;
                while (e.MoveNext());
            }
        }
    }
}