PracticeSeminar

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

namespace ConsoleApplication1
{
    internal class Program
    {
        public static void Main()
        {
            Console.WriteLine("- Exponential -");
            foreach (var item in new Exponential().Take(4))
            {
                Console.WriteLine(item);
            }
            Console.WriteLine("ElementGenerationsCount: " + Exponential.ElementGenerationsCount);

            Console.WriteLine("- Exponential with changes -");
            foreach (var item in Test( new Exponential().Take(4)))
            {
                Console.WriteLine(item);
            }
            Console.WriteLine("ElementGenerationsCount: " + Exponential.ElementGenerationsCount);
        }

        static IEnumerable<int> Test(IEnumerable<int> sequence)
        {
            // генерация новых элеменов не происходит
            var enumerator = sequence.GetEnumerator();
            while (enumerator.MoveNext())
            {
                var item = enumerator.Current;
                item += 1; // изменяем значение последовательности
                yield return item;
            }
        }
    }

    public class Exponential : IEnumerable<int>
    {
        public Exponential() { ElementGenerationsCount = 0; }

        public static int ElementGenerationsCount { get; private set; }

        public IEnumerator<int> GetEnumerator()
        {
            var value = 1;
            while (true)
            {
                ElementGenerationsCount++;
                yield return value;
                value *= 2;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

    public class Fibonacci : IEnumerable<int>
    {
        public IEnumerator<int> GetEnumerator()
        {
            var currentValue = 1;
            var previousValue = 1;
            yield return currentValue;
            yield return previousValue;
            while (true)
            {
                var newValue = currentValue + previousValue;
                previousValue = currentValue;
                currentValue = newValue;
                yield return currentValue;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}