using System;

using System.Collections.Generic;

using System.Linq;

​

namespace Function_Kata_Happy_Numbers

{

    public class Program

    {

        public static void Main(string[] args)

        {

            var isHappyNumber = HappyNumberCalculator.CheckIfHappy(19);

            Console.WriteLine(isHappyNumber);

            Console.WriteLine(HappyNumberCalculator.Check(21));

        }

​

    }

​

   class HappyNumberCalculator

    {

       const int MAX_TRIALS = 100;

​

       public static bool Check(int number) {

           return Check2(number, MAX_TRIALS);

       }

       private static bool Check(int number, int trialsLeft) {

           if (number == 1) return true;

           if (trialsLeft == 0) return false;

           var digits = Digits_of_number(number);

           var squares = Square_digits(digits);

           var sum = squares.Sum();

           return Check(sum, trialsLeft-1);

       }

       private static IEnumerable<int> Digits_of_number(int number) => number.ToString().ToCharArray().Select(d => int.Parse(d.ToString()));

       private static IEnumerable<int> Square_digits(IEnumerable<int> digits) => digits.Select(d => d*d);

       private static bool Check2(int number, int trialsLeft) {

           return Is_happy(number, trialsLeft, () => {

                    var digits = Digits_of_number(number);

                    var squares = Square_digits(digits);

                    var sum = squares.Sum();

                    return Check2(sum, trialsLeft-1);

           });

       }

       private static bool Is_happy(int number, int trialsLeft, Func<bool> yetUndecided) {

           if (number == 1) return true;

           if (trialsLeft == 0) return false;

           return yetUndecided();

       }

        public static bool CheckIfHappy(int number)

        {

            var sumOfDigitSquares = 0;

            var tries = 0;

​

            while (sumOfDigitSquares != 1)

            {

                int[] digits = SplitNumberIntoDigits(number);

                sumOfDigitSquares = SumDigitSquares(digits);

​

​

                if (sumOfDigitSquares == 1)

                {

                    return true;

                }

                else

                {

                    tries += 1;

                    if (tries == 100000) return false;

                    number = sumOfDigitSquares;

                }

            }

​

            return false;

        }

​

​

​

        static int SumDigitSquares(IEnumerable<int> digits)

        {

            var result = 0;

            foreach (var digit in digits)

            {

                result += digit * digit;

            }

            return result;

        }

​

        static int[] SplitNumberIntoDigits(int num)

        {

            List<int> listOfInts = new List<int>();

            while (num > 0)

            {

                listOfInts.Add(num % 10);

                num = num / 10;

            }

            listOfInts.Reverse();

            return listOfInts.ToArray();

        }

    }

}

​

​