length of Collatz series from n to 1

collatz.cs

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

namespace collatz {
    class Program {
        static void Main(string[] args) {
            IEnumerable<T> iterate<T>(T seed, Func<T, T> step) {
                while (true) {
                    yield return seed;
                    seed = step(seed);
                }
            }

            Func<int, int> nextCollatz = n =>
                n % 2 == 0 ? n / 2 : n * 3 + 1;

            Console.WriteLine("(はじまり, 1になるまでの長さ)");
            Console.WriteLine(string.Join(
                "\n",
                Enumerable.Range(2, 100)
                .Select(n => iterate(n, nextCollatz))
                .Select(list => list.TakeWhile(x => x != 1))
                .Select(list => (list, list.Count()))
                .OrderByDescending(x => x.Item2)
                .Take(20)
                .Select(x => $"({x.Item1.First()}, {x.Item2})")));
        }
    }
}