adventofcode/2020/Day05.cs

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

namespace AdventOfCode {

  public class Day05 {

    static int FindRow(string boardingPass, int low, int high) {
      if (low == high) {
        return low;
      }
      int midpoint = (high + low) / 2;
      if (boardingPass[0] == 'F' || boardingPass[0] == 'L') {
        return FindRow(boardingPass.Substring(1), low, midpoint);
      } else {
        return FindRow(boardingPass.Substring(1), midpoint + 1, high);
      }
    }

    static int FindRow(string boardingPass) {
      return FindRow(boardingPass, 0, 127);
    }

    static int FindColumn(string boardingPass) {
      return FindRow(boardingPass.Substring(7), 0, 7);
    }

    static int FindSeatId(string boardingPass) {
      return FindRow(boardingPass) * 8 + FindColumn(boardingPass);
    }

    static int Part1() {
      string[] passes = File.ReadAllLines(Util.RootDir + "day05.txt");
      int maxId = 0;
      foreach (string boardingPass in passes) {
        int id = FindSeatId(boardingPass);
        if (id > maxId) {
          maxId = id;
        }
      }
      return maxId;
    }

    static int Part2() {
      string[] passes = File.ReadAllLines(Util.RootDir + "day05.txt");
      var seatIds = new HashSet<int>();
      foreach (string boardingPass in passes) {
        int id = FindSeatId(boardingPass);
        seatIds.Add(id);
      }
      for (int i = 1; i < 828; i++) {
        if (!seatIds.Contains(i) && seatIds.Contains(i - 1) && seatIds.Contains(i + 1)) {
          return i;
        }
      }
      return -1;
    }

    [Fact]
    public static void Test() {
      Assert.Equal(44, FindRow("FBFBBFFRLR"));
      Assert.Equal(5, FindColumn("FBFBBFFRLR"));
      Assert.Equal(357, FindSeatId("FBFBBFFRLR"));
      Assert.Equal(567, FindSeatId("BFFFBBFRRR"));
      Assert.Equal(119, FindSeatId("FFFBBBFRRR"));
      Assert.Equal(820, FindSeatId("BBFFBBFRLL"));

      Assert.Equal(828, Part1());
      WriteLine(Part2());
    }
  }
}