Advent of Code - 2024 - Day 8

var input = await File.ReadAllLinesAsync("input.txt");

var gridSize = input.Length;
var antennas = new Dictionary<char, List<Position>>();

for (var row = 0; row < gridSize; row++)
{
    for (var col = 0; col < gridSize; col++)
    {
        var cell = input[row][col];
        if (cell != '.')
        {
            if (!antennas.TryGetValue(cell, out _))
            {
                antennas[cell] = [];
            }
            antennas[cell].Add(new Position(row, col));
        }
    }
}

var antinodes1 = new HashSet<Position>();
var antinodes2 = new HashSet<Position>();

foreach (var frequency in antennas.Keys)
{
    var locations = antennas[frequency];

    foreach (var a in locations)
    {
        foreach (var b in locations)
        {
            if (a == b)
            {
                continue;
            }

            var rowDiff = a.Row - b.Row;
            var colDiff = a.Col - b.Col;

            var antinodeA = a.Move(rowDiff, colDiff);

            if (!antinodeA.OutOfBounds(gridSize))
            {
                antinodes1.Add(antinodeA);
            }

            while (!antinodeA.OutOfBounds(gridSize))
            {
                antinodes2.Add(antinodeA);
                antinodeA = antinodeA.Move(rowDiff, colDiff);
            }

            var antinodeB = b.Move(-rowDiff, -colDiff);

            if (!antinodeB.OutOfBounds(gridSize))
            {
                antinodes1.Add(antinodeB);
            }

            while (!antinodeB.OutOfBounds(gridSize))
            {
                antinodes2.Add(antinodeB);
                antinodeB = antinodeB.Move(-rowDiff, -colDiff);
            }

            antinodes2.Add(a);
            antinodes2.Add(b);
        }
    }
}

Console.WriteLine($"Part 1: {antinodes1.Count}");
Console.WriteLine($"Part 2: {antinodes2.Count}");


internal record Position(int Row, int Col)
{
    public Position Move(int row, int col)
    {
        return new Position(Row + row, Col + col);
    }

    public bool OutOfBounds(int size)
    {
        return OutOfBounds(0, size);
    }

    public bool OutOfBounds(int start, int end)
    {
        return Row < start || Row >= end || Col < start || Col >= end;
    }
}