AdventOfCode23/Day21/Map.cs

namespace Day21;

public class Map
{
    private readonly int _width;
    private readonly int _height;
    private readonly Tile[,] _map;
    private readonly Tile? _start;

    public Map(int width, int height, IEnumerable<Tile> tiles)
    {
        _width = width;
        _height = height;
        _map = new Tile[width, height];
        var i = 0;
        foreach (var t in tiles)
        {
            if (t.IsStart)
            {
                _start = t;
            }
            t.Position = new Vec(i % width, i / width);
            _map[t.Position.X, t.Position.Y] = t;
            i++;
        }

        if (_start == null)
        {
            throw new Exception("No start tile found");
        }
    }

    public void Print(ISet<Vec>? mark = null)
    {
        for (int y = 0; y < _height; y++)
        {
            for (int x = 0; x < _width; x++)
            {
                if (mark != null && mark.Contains(new Vec(x, y)))
                {
                    Console.Write('O');
                }
                else
                {
                    Console.Write(_map[x, y]);
                }
            }

            Console.WriteLine();
        }

        Console.WriteLine();
    }

    public long ReachableTiles(int steps)
    {
        var reachable = new HashSet<Vec>();
        reachable.Add(_start!.Position);
        
        for (int i = 0; i < steps; i++)
        {
            var next = new HashSet<Vec>();
            foreach (var p in reachable)
            {
                foreach (var neighbor in Surrounding(p).Where(n => At(n)?.IsWalkable ?? false))
                {
                    next.Add(neighbor);
                }
            }

            reachable = next;
        }
        
        Print(reachable);

        return reachable.Count;
    }

    private IEnumerable<Vec> Surrounding(Vec middle)
    {
        yield return middle.Add(ScreenSpace.Up);
        yield return middle.Add(ScreenSpace.Right);
        yield return middle.Add(ScreenSpace.Down);
        yield return middle.Add(ScreenSpace.Left);
    }

    private Tile? At(Vec p)
    {
        if (p.X < 0 || p.X >= _width || p.Y < 0 || p.Y >= _height)
        {
            return null;
        }

        return _map[p.X, p.Y];
    }
}