AdventOfCode/Days/Day17.cs

using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day17 : Day
{
    public override int Number => 17;
    public override string Name => "Chronospatial Computer";

    public override void RunPart1(bool display = true)
    {
        var (registerA, registerB, registerC, program) = ParseState();

        var toOutput = new List<int>();
        var instructionPointer = 0;

        while (instructionPointer < program.Length)
        {
            var opCode = program[instructionPointer];

            var isComboOperand = opCode switch
            {
                0 => true,
                1 => false,
                2 => true,
                3 => false,
                4 => false,
                5 => true,
                6 => true,
                7 => true,
                _ => throw new ArgumentOutOfRangeException(nameof(opCode))
            };

            var operand = (isComboOperand, program[instructionPointer + 1]) switch
            {
                (false, var literalValue) => literalValue,
                (true, 0) => 0,
                (true, 1) => 1,
                (true, 2) => 2,
                (true, 3) => 3,
                (true, 4) => registerA,
                (true, 5) => registerB,
                (true, 6) => registerC,
                _ => throw new ArgumentOutOfRangeException()
            };

            switch (opCode)
            {
                // adv
                case 0:
                    registerA = (int)(registerA / Math.Pow(2, operand));
                    break;

                // bxl
                case 1:
                    registerB = registerB ^ operand;
                    break;

                // bst
                case 2:
                    registerB = operand % 8;
                    break;

                // jnz
                case 3:
                    if (registerA is not 0)
                    {
                        instructionPointer = operand - 2;
                    }
                    break;

                // bxc
                case 4:
                    registerB = registerB ^ registerC;
                    break;

                // out
                case 5:
                    toOutput.Add(operand % 8);
                    break;

                // bdv
                case 6:
                    registerB = (int)(registerA / Math.Pow(2, operand));
                    break;

                // cdv
                case 7:
                    registerC = (int)(registerA / Math.Pow(2, operand));
                    break;
            }

            instructionPointer += 2;
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Output: [yellow]{string.Join(',', toOutput)}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var (_, originalRegisterB, originalRegisterC, program) = ParseState();
        var toOutput = new List<int>();
        var instructionPointer = 0;

        var finalRegisterA = 0;

        AnsiConsole.Status().Start("Computing (0)...", ctx =>
        {
            for (var initialRegisterA = 0; initialRegisterA < int.MaxValue; initialRegisterA++)
            {
                // Reset state
                var registerA = initialRegisterA;
                var registerB = originalRegisterB;
                var registerC = originalRegisterC;

                toOutput.Clear();
                instructionPointer = 0;

                while (instructionPointer < program.Length)
                {
                    var opCode = program[instructionPointer];

                    var isComboOperand = opCode switch
                    {
                        0 => true,
                        1 => false,
                        2 => true,
                        3 => false,
                        4 => false,
                        5 => true,
                        6 => true,
                        7 => true,
                        _ => throw new ArgumentOutOfRangeException(nameof(opCode))
                    };

                    var operand = (isComboOperand, program[instructionPointer + 1]) switch
                    {
                        (false, var literalValue) => literalValue,
                        (true, 0) => 0,
                        (true, 1) => 1,
                        (true, 2) => 2,
                        (true, 3) => 3,
                        (true, 4) => registerA,
                        (true, 5) => registerB,
                        (true, 6) => registerC,
                        _ => throw new ArgumentOutOfRangeException()
                    };

                    switch (opCode)
                    {
                        // adv
                        case 0:
                            registerA = (int)(registerA / Math.Pow(2, operand));
                            break;

                        // bxl
                        case 1:
                            registerB = registerB ^ operand;
                            break;

                        // bst
                        case 2:
                            registerB = operand % 8;
                            break;

                        // jnz
                        case 3:
                            if (registerA is not 0)
                            {
                                instructionPointer = operand - 2;
                            }
                            break;

                        // bxc
                        case 4:
                            registerB = registerB ^ registerC;
                            break;

                        // out
                        case 5:
                            toOutput.Add(operand % 8);
                            break;

                        // bdv
                        case 6:
                            registerB = (int)(registerA / Math.Pow(2, operand));
                            break;

                        // cdv
                        case 7:
                            registerC = (int)(registerA / Math.Pow(2, operand));
                            break;
                    }

                    instructionPointer += 2;
                }

                if (initialRegisterA % 1000 == 0)
                {
                    ctx.Status($"Computing ({initialRegisterA})...");
                }

                // Check if output is the program itself
                if (toOutput.SequenceEqual(program))
                {
                    finalRegisterA = initialRegisterA;

                    break;
                }
            }
        });

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Lowest possible value for register A to output itself: [yellow]{finalRegisterA}[/][/]");
        }
    }

    private (int RegisterA, int RegisterN, int RegisterC, int[] Program) ParseState()
    {
        var lineIterator = Input.AsSpan().EnumerateLines();

        lineIterator.MoveNext();
        var registerA = int.Parse(lineIterator.Current[(lineIterator.Current.IndexOf(':') + 2)..]);

        lineIterator.MoveNext();
        var registerB = int.Parse(lineIterator.Current[(lineIterator.Current.IndexOf(':') + 2)..]);

        lineIterator.MoveNext();
        var registerC = int.Parse(lineIterator.Current[(lineIterator.Current.IndexOf(':') + 2)..]);

        lineIterator.MoveNext();
        lineIterator.MoveNext();

        var program = lineIterator.Current[(lineIterator.Current.IndexOf(':') + 2)..]
            .ToString()
            .Split(',')
            .Select(int.Parse)
            .ToArray();

        return (registerA, registerB, registerC, program);
    }
}