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

namespace AoCCSharp
{
    internal class Day15 {

        internal void Solve() {
            string input = "0,8,15,2,12,1,4";
            int numberAt2020 = 30000000;

            var numbers = input.Split(",").Select(int.Parse).ToList();
            Dictionary<int, int> numberAge = numbers
                .Select((n, i) => (n, i))
                .ToDictionary(tuple => tuple.n, tuple => tuple.i);
            
            int currentNumber = 0; // initial numbers are unique
            int nextNumber;
            int count = numbers.Count;
            do {
                if (numberAge.ContainsKey(currentNumber)) {
                    nextNumber = count - numberAge[currentNumber];
                    numberAge[currentNumber] = count;
                } else {
                    nextNumber = 0;
                    numberAge[currentNumber] = count;
                }
                count += 1;
                if (count < numberAt2020) {
                    currentNumber = nextNumber;
                }
            } while (count < numberAt2020);

            Console.WriteLine(currentNumber);
        }
    }
}