AdventOfCode/Days/Day8.cs

using System.Buffers;
using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day8 : Day
{
    public override int Number => 8;
    public override string Name => "Playground";

    public override void RunPart1(bool display = true)
    {
        var boxPositions = ParseBoxPositions(Input);
        var boxPairDistances = FetchOrderedDistances(boxPositions);

        var circuits = new List<HashSet<Box>>();

        var connectionsCount = 0;

        while (connectionsCount < 1000)
        {
            // Fetch next minimal distance pair. This is always the last pair in our ordered list
            var (pairLeftBox, pairRightBox, _) = boxPairDistances[^1];
            boxPairDistances.RemoveAt(boxPairDistances.Count - 1);

            // Create a connection if those two boxes are not already connected
            if (!ConnectionExists(circuits, pairLeftBox, pairRightBox))
            {
                // Connect the boxes, eventually merging circuits
                ConnectBoxes(circuits, pairLeftBox, pairRightBox);
            }

            connectionsCount++;
        }

        var result = circuits
            .OrderByDescending(circuit => circuit.Count)
            .Take(3)
            .Aggregate(1, (product, circuit) => product * circuit.Count);

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Result is: [yellow]{result}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var boxPositions = ParseBoxPositions(Input);
        var boxPairDistances = FetchOrderedDistances(boxPositions);

        var circuits = new List<HashSet<Box>>();

        long result;
        while (true)
        {
            // Fetch next minimal distance pair. This is always the last pair in our ordered list
            var (pairLeftBox, pairRightBox, _) = boxPairDistances[^1];
            boxPairDistances.RemoveAt(boxPairDistances.Count - 1);

            // Create a connection if those two boxes are not already connected
            if (!ConnectionExists(circuits, pairLeftBox, pairRightBox))
            {
                // Connect the boxes, eventually merging circuits
                ConnectBoxes(circuits, pairLeftBox, pairRightBox);
            }

            // Check if there is a circuit with all boxes connected
            if (circuits[0].Count >= 1000)
            {
                result = pairLeftBox.X * pairRightBox.X;
                break;
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Result is: [yellow]{result}[/][/]");
        }
    }

    private static double BoxDistance(Box left, Box right)
    {
        return Math.Sqrt(
            (right.X - left.X) * (right.X - left.X) +
            (right.Y - left.Y) * (right.Y - left.Y) +
            (right.Z - left.Z) * (right.Z - left.Z)
        );
    }

    static bool ConnectionExists(List<HashSet<Box>> circuits, Box left, Box right)
    {
        foreach (var circuit in circuits)
        {
            if (circuit.Contains(left) && circuit.Contains(right))
            {
                return true;
            }
        }

        return false;
    }

    static void ConnectBoxes(List<HashSet<Box>> circuits, Box pairLeftBox, Box pairRightBox)
    {
        // Try to find circuit containing each box
        HashSet<Box>? leftCircuit = null;
        HashSet<Box>? rightCircuit = null;

        foreach (var circuit in circuits)
        {
            if (circuit.Contains(pairLeftBox))
            {
                leftCircuit = circuit;
            }

            if (circuit.Contains(pairRightBox))
            {
                rightCircuit = circuit;
            }
        }

        // None is in a circuit, create a new one
        if (leftCircuit is null && rightCircuit is null)
        {
            var circuit = new HashSet<Box>
            {
                pairLeftBox,
                pairRightBox
            };

            circuits.Add(circuit);
        }
        // One of the two is null
        else if (leftCircuit is not null && rightCircuit is null)
        {
            leftCircuit.Add(pairRightBox);
        }
        else if (leftCircuit is null && rightCircuit is not null)
        {
            rightCircuit.Add(pairLeftBox);
        }
        // Both are already in a circuit, merge them
        else
        {
            foreach (var box in rightCircuit!)
            {
                leftCircuit!.Add(box);
            }

            circuits.Remove(rightCircuit);
        }
    }

    private static List<Box> ParseBoxPositions(string input)
    {
        var positions = new List<Box>();

        foreach (var line in input.EnumerateLines())
        {
            var split = line.Split(',');

            split.MoveNext();
            var x = long.Parse(line[split.Current]);

            split.MoveNext();
            var y = long.Parse(line[split.Current]);

            split.MoveNext();
            var z = long.Parse(line[split.Current]);

            positions.Add(new Box(x, y, z));
        }

        return positions;
    }

    private static List<BoxPairDistance> FetchOrderedDistances(List<Box> boxes)
    {
        var boxPairDistances = new List<BoxPairDistance>();

        for (var i = 0; i < boxes.Count - 1; i++)
        {
            for (var j = i + 1; j < boxes.Count; j++)
            {
                var leftBox = boxes[i];
                var rightBox = boxes[j];

                var distance = BoxDistance(leftBox, rightBox);

                boxPairDistances.Add(new BoxPairDistance(leftBox, rightBox, distance));
            }
        }

        // Sort in reverse order
        boxPairDistances.Sort((left, right) => right.CompareTo(left));

        return boxPairDistances;
    }

    private record Box(long X, long Y, long Z);

    private record BoxPairDistance(Box Left, Box Right, double Distance) : IComparable<BoxPairDistance>
    {
        public int CompareTo(BoxPairDistance? other)
        {
            if (ReferenceEquals(this, other))
            {
                return 0;
            }

            if (other is null)
            {
                return 1;
            }

            return Distance.CompareTo(other.Distance);
        }
    }
}