AdventOfCode/Days/Day8.cs

using System.Collections.Frozen;
using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

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

    private const int GridSize = 50;

    public override void RunPart1(bool display = true)
    {
        HashSet<Point> antinodePositions = [];

        var frequencyAntennas = ParseAntennas();

        // Find antinodes for each frequency
        foreach (var (_, antennas) in frequencyAntennas)
        {
            // Take antennas by pair to find antinodes
            foreach (var firstAntenna in antennas)
            {
                foreach (var secondAntenna in antennas.Where(a => a != firstAntenna))
                {
                    var firstAntinode = firstAntenna + (firstAntenna - secondAntenna);
                    var secondAntinode = secondAntenna + (secondAntenna - firstAntenna);

                    if (firstAntinode is { X: >= 0 and < GridSize, Y: >= 0 and < GridSize })
                    {
                        antinodePositions.Add(firstAntinode);
                    }

                    if (secondAntinode is { X: >= 0 and < GridSize, Y: >= 0 and < GridSize })
                    {
                        antinodePositions.Add(secondAntinode);
                    }
                }
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Unique antinode positions count: [yellow]{antinodePositions.Count}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        HashSet<Point> antinodePositions = [];

        var frequencyAntennas = ParseAntennas();

        // Find antinodes for each frequency
        foreach (var (_, antennas) in frequencyAntennas)
        {
            // Take antennas by pair to find antinodes
            foreach (var firstAntenna in antennas)
            {
                foreach (var secondAntenna in antennas.Where(a => a != firstAntenna))
                {
                    // Always add antinodes on antennas if a pair is formed
                    antinodePositions.Add(firstAntenna);
                    antinodePositions.Add(secondAntenna);

                    // Compute potential antinode positions
                    var firstAntinodeOffset = firstAntenna - secondAntenna;
                    var secondAntinodeOffset = secondAntenna - firstAntenna;

                    for (var i = 1; ; i++)
                    {
                        var firstAntinode = firstAntenna + (i * firstAntinodeOffset);

                        if (firstAntinode is { X: >= 0 and < GridSize, Y: >= 0 and < GridSize })
                        {
                            antinodePositions.Add(firstAntinode);
                        }
                        else
                        {
                            break;
                        }
                    }

                    for (var i = 1; ; i++)
                    {
                        var secondAntinode = secondAntenna + (i * secondAntinodeOffset);

                        if (secondAntinode is { X: >= 0 and < GridSize, Y: >= 0 and < GridSize })
                        {
                            antinodePositions.Add(secondAntinode);
                        }
                        else
                        {
                            break;
                        }
                    }
                }
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Unique antinode positions count: [yellow]{antinodePositions.Count}[/][/]");
        }
    }

    private FrozenDictionary<char, List<Point>> ParseAntennas()
    {
        var antennas = new Dictionary<char, List<Point>>();

        var y = 0;
        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            var x = 0;

            foreach (var frequency in line)
            {
                if (frequency is '.')
                {
                    x++;

                    continue;
                }

                if (!antennas.TryGetValue(frequency, out var list))
                {
                    list = [];
                    antennas[frequency] = list;
                }

                list.Add(new Point(x, y));

                x++;
            }

            y++;
        }

        return antennas.ToFrozenDictionary();
    }

    private readonly record struct Point(int X, int Y)
        : IAdditionOperators<Point, Point, Point>,
            ISubtractionOperators<Point, Point, Point>,
            IMultiplyOperators<Point, int, Point>
    {
        public static Point operator +(Point left, Point right)
        {
            return new Point(left.X + right.X, left.Y + right.Y);
        }

        public static Point operator -(Point left, Point right)
        {
            return new Point(left.X - right.X, left.Y - right.Y);
        }

        public static Point operator *(Point left, int right)
        {
            return new Point(left.X * right, left.Y * right);
        }

        public static Point operator *(int left, Point right)
        {
            return new Point(right.X * left, right.Y * left);
        }
    }
}