Advent of Code 2018 Day 2

aoc2018_2.cs

	class Day2_2018 {
		const string INPUTFILE = @"2018\day2.txt";

		public void Part1() {
			var input = File.ReadAllLines(INPUTFILE);

			var twos = 0;
			var threes = 0;

			foreach (var line in input) {
				var letters = line.ToCharArray();
				var hasTwo = false;
				var hasThree = false;

				for (var charCode = 97; charCode <= 122; charCode++) {
					var charCount = letters.Count(letter => letter == charCode);

					if (charCount == 2 && !hasTwo) {
						twos++;
						hasTwo = true;
					}

					if (charCount == 3 && !hasThree) {
						threes++;
						hasThree = true;
					}
				}
			}

			Console.WriteLine($"{twos} x {threes} = {twos * threes}");
		}

		public void Part2() {
			var input = File.ReadAllLines(INPUTFILE);

			var sets = new List<char[]>();

			foreach (var line in input) {
				sets.Add(line.ToCharArray());
			}

			var winner = string.Empty;

			while (winner == string.Empty && sets.Any()) {
				var nextLine = sets.First();
				winner = RecursiveCheckSet(0, nextLine, sets);
				sets.Remove(nextLine);
			}

			Console.WriteLine(winner);
		}

		string RecursiveCheckSet(int index, char[] set, List<char[]> sets, int difference = -1) {
			// we have a winner!
			if (index == set.Count()) {
				return StringWithoutDroppedChar(set, difference);
			}

			var similarSets = sets.Where(otherSet => otherSet[index] == set[index]).ToList();

			// We found a different character
			if (similarSets.Count() == 1) {
				// This is the first different character, so give it another chance.
				if (difference < 0 && sets.Count > 1) {
					difference = index;
					similarSets = sets;
				}
				else {
					return string.Empty;
				}
			}

			index++;

			return RecursiveCheckSet(index, set, similarSets, difference);
		}

		string StringWithoutDroppedChar(char[] set, int dropped) {
			var returnValue = string.Empty;

			for (var i = 0; i < set.Count(); i++) {
				if (i != dropped) {
					returnValue += set[i];
				}
			}

			return returnValue;
		}
	}