adventofcode/2020/Day03.cs

using System;
using static System.Console;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Xunit;

namespace AdventOfCode {

  public class Day03 {

    static int CountTreeIntersections(string[] lines, int dx, int dy) {
      int count = 0, x = dx, y = dy;
      while (y < lines.Length) {
        string line = lines[y];
        char tile = line[x % line.Length];
        if (tile == '#') {
          count++;
        }
        x += dx;
        y += dy;
      }
      return count;
    }

    static int Part1() {
      string[] input = File.ReadAllLines(Util.RootDir + "day03.txt");
      return CountTreeIntersections(input, 3, 1);
    }

    static long Part2() {
      string[] input = File.ReadAllLines(Util.RootDir + "day03.txt");
      var deltas = new List<(int, int)> { (1, 1), (3, 1), (5, 1), (7, 1), (1, 2) };
      long result = 1;
      foreach ((int dx, int dy) in deltas) {
        result *= CountTreeIntersections(input, dx, dy);
      }
      return result;
    }

    [Fact]
    public static void Test() {
      string[] example =
@"..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#".Split('\n');

      Assert.Equal(7, CountTreeIntersections(example, 3, 1));
      Assert.Equal(218, Part1());
      Assert.Equal(3847183340, Part2());
    }
  }
}