2020aoc/Day8.cs

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading;

namespace AoC {
    
    internal class Day8 {

        private enum Instruction {
            ACC, JMP, NOP
        }

        private record Command(Instruction ins, int value);

        private ImmutableDictionary<string, Instruction> InstructionFromString =
            new Dictionary<string, Instruction> {
                { "acc", Instruction.ACC },
                { "jmp", Instruction.JMP },
                { "nop", Instruction.NOP }
            }.ToImmutableDictionary();
        
        private Regex rx = new(@"^(?<ins>\w+) (?<value>(\+|\-)\d+)");
        
        private bool DoesItLoop(List<Command> commands, int index, ref int accumulator) {
            ISet<int> commandIndices = new HashSet<int>();
            while (index >= 0 && index < commands.Count && !commandIndices.Contains(index)) {
                commandIndices.Add(index);

                var command = commands[index];
                (accumulator, index) = command.ins switch {
                    Instruction.ACC => (accumulator + command.value, index + 1),
                    Instruction.JMP => (accumulator, index + command.value),
                    Instruction.NOP => (accumulator, index + 1),
                    _ => throw new ArgumentException("Du doofer Hund!")
                };
            }

            return commandIndices.Contains(index);
        }
        
        internal void Solve() {
            var readLines = File.ReadAllText("day8/input").Split("\n", StringSplitOptions.RemoveEmptyEntries);

            Command LineToCommandSelector(string line) {
                var groups = rx.Match(line).Groups;
                return new Command(InstructionFromString[groups["ins"].Value], int.Parse(groups["value"].Value));
            }

            var commands = readLines.Select(LineToCommandSelector).ToList();
            
            SortedSet<int> commandIndices = new SortedSet<int>();
            int index = 0;
            int accumulator = 0;
            while (!commandIndices.Contains(index)) {
                commandIndices.Add(index);

                var command = commands[index];
                (accumulator, index) = command.ins switch {
                    Instruction.ACC => (accumulator + command.value, index + 1),
                    Instruction.JMP => (accumulator, index + command.value),
                    Instruction.NOP => (accumulator, index + 1),
                    _ => throw new ArgumentException("Du doofer Hund!")
                };
            }
            
            Console.WriteLine(accumulator);
            
            foreach (var commandIndex in commandIndices.Reverse()) {
                index = 0;
                accumulator = 0;
                if (commands[commandIndex].ins == Instruction.ACC) {
                    continue;
                } 
                if (commands[commandIndex].ins == Instruction.JMP) {
                    commands[commandIndex] = new Command(Instruction.NOP, commands[commandIndex].value);
                    if (!DoesItLoop(commands, index, ref accumulator)) {
                        break;
                    }
                    commands[commandIndex] = new Command(Instruction.JMP, commands[commandIndex].value);
                } else {
                    commands[commandIndex] = new Command(Instruction.JMP, commands[commandIndex].value);
                    if (!DoesItLoop(commands, index, ref accumulator)) {
                        break;
                    }
                    commands[commandIndex] = new Command(Instruction.NOP, commands[commandIndex].value);
                }
            }
            
            Console.WriteLine(accumulator);
        }

        private record Info(int index, int accumulator);

        private static void ExecuteInstruction(List<Command> commands, ref int index, ref int accumulator) {
            var command = commands[index];
            (accumulator, index) = command.ins switch {
                Instruction.ACC => (accumulator + command.value, index + 1),
                Instruction.JMP => (accumulator, index + command.value),
                Instruction.NOP => (accumulator, index + 1),
                _ => throw new ArgumentException("Du doofer Hund!")
            };
        }

        private bool DoesItLoop(List<Command> commands, int index, ref int accumulator, ISet<int> visitedInstructions) {
            while (index >= 0 && index < commands.Count && !visitedInstructions.Contains(index)) {
                visitedInstructions.Add(index);
                ExecuteInstruction(commands, ref index, ref accumulator);
            }
            return visitedInstructions.Contains(index);
        }

        internal void SolveOn() {
            
            var readLines = File.ReadAllText("day8/input").Split("\n", StringSplitOptions.RemoveEmptyEntries);

            Command LineToCommandSelector(string line) {
                var groups = rx.Match(line).Groups;
                return new Command(InstructionFromString[groups["ins"].Value], int.Parse(groups["value"].Value));
            }

            var commands = readLines.Select(LineToCommandSelector).ToList();

            ISet<int> commandIndices = new HashSet<int>();
            Stack<Info> commandInfo = new Stack<Info>();
            int index = 0;
            int accumulator = 0;
            while (!commandIndices.Contains(index)) {
                commandIndices.Add(index);
                commandInfo.Push(new Info(index, accumulator));

                ExecuteInstruction(commands, ref index, ref accumulator);
            }
            
            Console.WriteLine(accumulator);

            Instruction switchInstruction(Instruction ins) => ins == Instruction.NOP ? Instruction.JMP : Instruction.NOP;
            
            int tempAccumulator = -1;
            while (commandInfo.Count > 0) {
                var current = commandInfo.Pop();
                if (commands[current.index].ins == Instruction.ACC) {
                    continue;
                }

                commands[current.index] = new Command(switchInstruction(commands[current.index].ins), commands[current.index].value);
                tempAccumulator = current.accumulator;
                commandIndices.Remove(current.index);
                if (!DoesItLoop(commands, current.index, ref tempAccumulator, commandIndices)) {
                    break;
                }
                commands[current.index] = new Command(switchInstruction(commands[current.index].ins), commands[current.index].value);
            }

            Console.WriteLine(tempAccumulator);
        }


    }
}