AdventOfCode/Days/Day11.cs

using System.Collections.Immutable;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Monkey
{
    public int Number { get; }
    public long InspectionCount { get; private set; }

    private readonly Queue<long> _items;
    private readonly Func<long, long> _operation;
    private readonly long _testDivider;
    private readonly int _testFailMonkey;
    private readonly int _testSuccessMonkey;

    public Monkey(int number, IEnumerable<long> items, Func<long, long> operation, long testDivider, int testFailMonkey, int testSuccessMonkey)
    {
        Number = number;
        _operation = operation;
        _testDivider = testDivider;
        _testFailMonkey = testFailMonkey;
        _testSuccessMonkey = testSuccessMonkey;

        InspectionCount = 0;
        _items = new Queue<long>(items);
    }

    public void AddItem(long item)
    {
        _items.Enqueue(item);
    }

    public bool ThrowItem(IImmutableList<Monkey> monkeys, bool divideWorry = true)
    {
        // Return false if there's no item to throw
        if (_items.Count < 1)
        {
            return false;
        }

        // Increment inspection count
        InspectionCount++;

        // Take next item to throw
        var item = _items.Dequeue();

        // Apply the operation on worry level
        var newValue = _operation(item);

        // Divide worry level by 3 before test
        if (divideWorry)
        {
            newValue = newValue / 3;
        }

        // Even using this simplification, it's still needed to use longs because
        // they sometime overflow before being simplified in part 2
        newValue = newValue % (2*3*5*7*11*13*17*19);

        // Test is a success
        if (newValue % _testDivider == 0)
        {
            monkeys[_testSuccessMonkey].AddItem(newValue);
        }
        // Test is a fail
        else
        {
            monkeys[_testFailMonkey].AddItem(newValue);
        }

        return true;
    }
}

public class Day11 : Day
{
    public override int Number => 11;
    public override string Name => "Monkey in the Middle";
    public override void RunPart1(bool display = true)
    {
        const int roundsCount = 20;

        var monkeys = ParseMonkeys();

        // Do 20 rounds
        for (int round = 0; round < roundsCount; round++)
        {
            foreach (var monkey in monkeys)
            {
                // Throw items while there's some
                while (monkey.ThrowItem(monkeys)) { }
            }
        }

        var topMonkeys = monkeys.OrderByDescending(m => m.InspectionCount).Take(2).ToList();

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Monkey business: [yellow]{topMonkeys[0].InspectionCount * topMonkeys[1].InspectionCount}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        const int roundsCount = 10_000;

        var monkeys = ParseMonkeys();

        // Do 10 000 rounds
        for (int round = 0; round < roundsCount; round++)
        {
            foreach (var monkey in monkeys)
            {
                // Throw items while there's some
                while (monkey.ThrowItem(monkeys, false)) { }
            }
        }

        var topMonkeys = monkeys.OrderByDescending(m => m.InspectionCount).Take(2).ToList();

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Monkey business: [yellow]{topMonkeys[0].InspectionCount * topMonkeys[1].InspectionCount}[/][/]");
        }
    }

    private static IImmutableList<Monkey> ParseMonkeys()
    {
        var monkeys = new List<Monkey>();

        // Split by monkey description
        foreach (var monkey in Input.ReplaceLineEndings("\n").Split("\n\n"))
        {
            var split = monkey.Split('\n');
            var numberLine = split[0];
            var itemsLine = split[1];
            var operationLine = split[2];
            var testDividerLine = split[3];
            var testSuccessLine = split[4];
            var testFailLine = split[5];

            // Monkey attributes
            var number = int.Parse(numberLine[numberLine.LastIndexOf(' ')..^1]);
            var items = itemsLine[(itemsLine.IndexOf(':') + 2)..].Split(", ").Select(long.Parse);

            // Read operation
            operationLine = operationLine[(operationLine.IndexOf('=') + 2)..];
            long? rightOperand = operationLine.EndsWith("old") ? null : long.Parse(operationLine.Split('+', '*')[1].Trim());

            Func<long, long> operation = operationLine.Contains('+') switch
            {
                true => rightOperand is null
                    ? old => old + old
                    : old => old + (int)rightOperand,
                false => rightOperand is null
                    ? old => old * old
                    : old => old * (int)rightOperand
            };

            var testDivider = long.Parse(testDividerLine[testDividerLine.LastIndexOf(' ')..]);
            var testSuccessMonkey = int.Parse(testSuccessLine[testSuccessLine.LastIndexOf(' ')..]);
            var testFailMonkey = int.Parse(testFailLine[testFailLine.LastIndexOf(' ')..]);

            monkeys.Add(new Monkey(number, items, operation, testDivider, testFailMonkey, testSuccessMonkey));
        }

        return monkeys.ToImmutableList();
    }

    #region Input

    public const string Input =
    """
    Monkey 0:
      Starting items: 89, 95, 92, 64, 87, 68
      Operation: new = old * 11
      Test: divisible by 2
        If true: throw to monkey 7
        If false: throw to monkey 4

    Monkey 1:
      Starting items: 87, 67
      Operation: new = old + 1
      Test: divisible by 13
        If true: throw to monkey 3
        If false: throw to monkey 6

    Monkey 2:
      Starting items: 95, 79, 92, 82, 60
      Operation: new = old + 6
      Test: divisible by 3
        If true: throw to monkey 1
        If false: throw to monkey 6

    Monkey 3:
      Starting items: 67, 97, 56
      Operation: new = old * old
      Test: divisible by 17
        If true: throw to monkey 7
        If false: throw to monkey 0

    Monkey 4:
      Starting items: 80, 68, 87, 94, 61, 59, 50, 68
      Operation: new = old * 7
      Test: divisible by 19
        If true: throw to monkey 5
        If false: throw to monkey 2

    Monkey 5:
      Starting items: 73, 51, 76, 59
      Operation: new = old + 8
      Test: divisible by 7
        If true: throw to monkey 2
        If false: throw to monkey 1

    Monkey 6:
      Starting items: 92
      Operation: new = old + 5
      Test: divisible by 11
        If true: throw to monkey 3
        If false: throw to monkey 0

    Monkey 7:
      Starting items: 99, 76, 78, 76, 79, 90, 89
      Operation: new = old + 7
      Test: divisible by 5
        If true: throw to monkey 4
        If false: throw to monkey 5
    """;

    #endregion
}