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using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Advent_of_Code_2020 {
    internal class DayFifteen {
        internal static long Part1(string input) {
            string[] lines = input.Split(',');
            List<int> turns = new List<int>();
            foreach(string l in lines) {
                int v = int.Parse(l);
                turns.Add(v);
            }
            while(true) {
                int last = turns[turns.Count - 1]; // shove into list
                int next = GetDelay(turns, last); // get how long it has been
                turns.Add(next);
                if(turns.Count == 2020) {
                    return next;
                }
            }
        }

        private static int GetDelay(List<int> turns, int last) {
            // loop from end towards beginning (ignoring last index or we will always get 1)
            for(int i = 1; i < turns.Count; i++) {
                int pp = turns[turns.Count - 1 - i];
                if(pp == last) {
                    return i; // return how many values we had to look at
                }
            }
            return 0; // value is unique!
        }

        internal static long Part2(string input) {
            // we don't need the whole fekking list. We just need to know what turn
            // the value was last seen
            Dictionary<int, int> dict = new Dictionary<int, int>();
            string[] lines = input.Split(',');
            int turn = 0;
            foreach(string l in lines) {
                int v = int.Parse(l);
                dict.Add(v, turn);
                turn++;
            }
            // input values are guaranteed unique, so the most recent number is always
            // the first time it was said, so the first turn after input is always 0.
            int next = 0;
            int last = 0;
            while(true) {
                // read comments, ignore the goto statements
                goto label4;
            label3:
                // increment turn counter
                turn++;
                goto label5;
            label2:
                // compute the next number based on the last
                if(dict.ContainsKey(last)) {
                    next = turn - dict[last];
                }
                else {
                    next = 0;
                }
                goto label1;
            label4:
                // update variables
                last = next;
                goto label2;
            label1:
                // store the number
                if(dict.ContainsKey(last)) {
                    dict[last] = turn;
                }
                else {
                    dict.Add(last, turn);
                }
                goto label3;
            label5:
                // check for the end of the game
                if(turn == 30000000) { // 30 million
                    return last; // we need the "last" number spoken
                }
            }
        }
    }
}