adventofcode/2020/Day08.cs

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

namespace AdventOfCode {

  public class Day08 {

    public class GameConsole {
      int accumulator = 0;
      int pointer = 0;

      // Returns a (bool, int) tuple, where the bool indicates whether the program terminated
      // normally (if false, it executed an infinite loop), and the int indicates the value of
      // the accumulator at the time of termination.
      public (bool, int) Execute(List<Instruction> code) {
        var instructionsExecuted = new HashSet<int>();
        while (pointer < code.Count()) {
          if (instructionsExecuted.Contains(pointer)) {
            return (false, accumulator);  // loop detected
          }
          instructionsExecuted.Add(pointer);
          Instruction instruction = code[pointer];
          switch (instruction.Op) {
            case "nop":
              pointer++;
              break;
            case "acc":
              pointer++;
              accumulator += instruction.Value;
              break;
            case "jmp":
              pointer += instruction.Value;
              break;
            default:
              throw new Exception("invalid op");
          }
        }
        return (true, accumulator);  // normal termination
      }
    }

    public record Instruction(string Op, int Value);

    public static int RepairBrokenInstruction(List<Instruction> code) {
      for (int i = 0; i < code.Count(); i++) {
        if (code[i].Op == "acc") {
          continue;  // this line can't be the broken one
        }
        List<Instruction> repaired = new List<Instruction>(code);
        if (repaired[i].Op == "nop") {
          repaired[i] = new Instruction("jmp", repaired[i].Value);
        } else {
          repaired[i] = new Instruction("nop", repaired[i].Value);
        }
        var (success, result) = new GameConsole().Execute(repaired);
        if (success) {
          return result;
        }
      }
      throw new Exception("didn't find a valid instruction to repair");
    }

    static List<Instruction> ParseInstructions(string[] code) {
      var result = new List<Instruction>();
      foreach (string line in code) {
        string[] tokens = line.Split(' ');
        result.Add(new Instruction(tokens[0], int.Parse(tokens[1])));
      }
      return result;
    }

    static int Part1() {
      string[] input = File.ReadAllLines(Util.RootDir + "day08.txt");
      List<Instruction> code = ParseInstructions(input);
      (bool finished, int result) = new GameConsole().Execute(code);
      return result;
    }

    static int Part2() {
      string[] input = File.ReadAllLines(Util.RootDir + "day08.txt");
      List<Instruction> code = ParseInstructions(input);
      return RepairBrokenInstruction(code);
    }

    [Fact]
    public static void Test() {
      string[] example =
@"nop +0
acc +1
jmp +4
acc +3
jmp -3
acc -99
acc +1
jmp -4
acc +6".Split('\n');
      List<Instruction> parsed = ParseInstructions(example);
      Assert.Equal(9, parsed.Count());
      Assert.Equal((false, 5), new GameConsole().Execute(parsed));
      Assert.Equal(1610, Part1());

      string[] example2 =
@"nop +0
acc +1
jmp +4
acc +3
jmp -3
acc -99
acc +1
nop -4
acc +6".Split('\n');
      List<Instruction> parsed2 = ParseInstructions(example2);
      Assert.Equal((true, 8), new GameConsole().Execute(parsed2));
      Assert.Equal(1703, Part2());
    }
  }
}