SillyDigitSumFun.cs

using System;
using System.Collections.Generic;
using System.Linq;
using DotNetXtensions;
using static System.Console;

namespace Play1
{
	public class Prog
	{
		static void Main()
		{
			var sillyDigitSum = new SillyDigitSumFun();

			int[] hits = sillyDigitSum.GetHits(0, int.MaxValue);

			$@"Hits Count: {hits.Length}
{hits.JoinToString(n => n.ToString("n0"), "\r\n")}".Print();

			ReadLine();
		}
	}

	public class SillyDigitSumFun
	{
		public readonly int[] DigitsToItselfArray;

		public SillyDigitSumFun()
		{
			var arr = DigitsToItselfArray = new int[10];
			for (int num = 0; num < arr.Length; num++)
				arr[num] = num == 0 ? 0 : (int)Math.Pow(num, num);
		}

		public int[] GetHits(int min, int max)
		{
			if (min < 0 || min >= max) throw new ArgumentOutOfRangeException();

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

			for (int num = min; num < max; num++) {

				if (num % 10_000_000 == 0)
					WriteLine($"... on number {num.ToString("n0")}");

				List<int> digits = SplitNumberIntoDigits(num);

				if (IsHit(num, digits)) {
					hits.Add(num);
					WriteLine($"HIT! - {num.ToString("n0")}");
				}
			}

			return hits.ToArray();
		}

		public bool IsHit(int num, List<int> digits)
		{
			if (digits == null) throw new ArgumentNullException();

			int sum = 0;

			for (int i = 0; i < digits.Count; i++) {

				int digit = digits[i];

				int digitToItself = DigitsToItselfArray[digit]; // we've cached these once, much more performant;

				sum += digitToItself;

				// above in 1 line, but harder to grasp: sum += DigitsToItselfArray[digits[i]];

				if (sum > num)
					return false;
			}

			return sum == num;
		}

		/// <summary>
		/// Modified from source: https://stackoverflow.com/a/4808815/264031.
		/// I'm guessing this is a lot more performant than the to string, split the string, and back route.
		/// </summary>
		public static List<int> SplitNumberIntoDigits(int num)
		{
			List<int> listOfInts = new List<int>();
			while (num > 0) {
				int digit = num % 10;
				listOfInts.Add(digit);
				num = num / 10;
			}
			listOfInts.Reverse();
			return listOfInts;
		}
	}
}