2017-12-07_10-43-13.cs

int Part1() => GetSpiralCells()
	.Skip(PuzzleInput - 1)
	.Select(t => Math.Abs(t.x) + Math.Abs(t.y))
	.First();

int Part2()
{
	var values = new Dictionary<(int x, int y), int>();
	values[(0, 0)] = 1;

	foreach (var t in GetSpiralCells().Skip(1))
	{
		var v = t.GetNeighbors().Sum(n => values.GetOrDefault(n));
		
		if (v > PuzzleInput) 
			return v;
		
		values[t] = v;
	}
	
	throw new Exception();
}

IEnumerable<(int x, int y)> GetSpiralCells()
{
	var pos = (0,0);
	
	foreach(var run in GetSpiralSegments())
	{
		for(var i = 0; i < run.length; i++)
		{
			yield return pos;
			pos = pos.Apply(run.dir);
		}
	}
}

IEnumerable<(Direction dir, int length)> GetSpiralSegments()
{
	var d = Direction.East;
	var i = 2;
	while(true)
	{
		yield return (d, i / 2);
		i += 1;
		d = d.GetCounterclockwise();
	}
}

enum Direction {North, East, South, West}

static class ext
{	
	public static Direction GetCounterclockwise(this Direction d) => d == Direction.North ? Direction.West : ((Direction)d - 1);
		
	public static (int x, int y) Apply(this (int x, int y) t, Direction dir, int d = 1)
	{
		switch(dir)
		{
			case Direction.North: return (t.x, t.y - d);
			case Direction.East: return (t.x + d, t.y);
			case Direction.South: return (t.x, t.y+d);
			case Direction.West: return (t.x-d, t.y);
		}
		throw new Exception();
	}

	public static IEnumerable<(int x, int y)> GetNeighbors(this (int x, int y) origin) =>
		new[] {-1, 0, 1}
			.CartesianProduct()
			.Where(t => t.a != 0 || t.b != 0)
			.Select(t => (origin.x + t.a, origin.y + t.b));
	

	public static IEnumerable<(T a, T b)> CartesianProduct<T>(this IEnumerable<T> ts)
	{
		var tsa = ts.ToArray();
		foreach (var a in tsa)
			foreach (var b in tsa)
				yield return (a, b);
	}
	
	public static V GetOrDefault<K,V>(this IDictionary<K,V> d, K k, V def = default(V)) => d.TryGetValue(k, out V v) ? v : def;
}