public struct Point : IEquatable<Point>
{
  public int X { get; set; }
  public int Y { get; set; }

  public Point(int x = 0, int y = 0) {
    X = x;
    Y = y;
  }

  public static Point operator+(Point l, Point r) {
    return new Point(l.X + r.X, l.Y + r.Y);
  }
  public bool Equals(Point other) {
    return X == other.X && Y == other.Y;
  }
}

Point size = new Point(0, 0);
HashSet<Point> obstructions = new HashSet<Point>();
Point guard = new Point(0, 0);

enum Direction
{
  Up = 0,
  Right,
  Down,
  Left
}

public void Input(IEnumerable<string> lines)
{
  size = new Point(lines.First().Length, lines.Count());
  char[] map = string.Join("", lines).ToCharArray();

  for (int y = 0; y < size.Y; ++y)
    for (int x = 0; x < size.X; ++x)
    {
      int pos = y * size.X + x;
      char at = map[pos];
      if (at == '#')
        obstructions.Add(new Point(x, y));
      else if (at == '^')
        guard = new Point(x, y);
    }
}

List<Point> path = new List<Point>();
public void PreCalc()
{
  path = WalkArea().path.Distinct().ToList();
}

public void Part1()
{
  Console.WriteLine($"Visited {path.Count} points");
}
public void Part2()
{
  int loopPoints = path.AsParallel().Where(p => !p.Equals(guard) && WalkArea(p).loop).Count();
  Console.WriteLine($"Valid loop points: {loopPoints}");
}

(IEnumerable<Point> path, bool loop) WalkArea(Point? obstruction = null)
{
  HashSet<(Point, Direction)> loopDetect = new HashSet<(Point, Direction)>();

  Point at = guard;
  Direction dir = Direction.Up;
  while (true)
  {
    if (!loopDetect.Add((at, dir)))
      return (loopDetect.Select(p => p.Item1), true);

    Point next = at;
    switch(dir)
    {
    case Direction.Up: next += new Point(0, -1); break;
    case Direction.Right: next += new Point(1, 0); break;
    case Direction.Down: next += new Point(0, 1); break;
    case Direction.Left: next += new Point(-1, 0); break;
    }

    if (next.X < 0 || next.Y < 0 || next.X >= size.X || next.Y >= size.Y)
      break;
    else if (obstructions.Contains(next) || (obstruction?.Equals(next) ?? false))
      dir = (Direction)(((int)dir + 1) % 4);
    else
      at = next;
  }

  return (loopDetect.Select(p => p.Item1), false);
}