aoc3.exs

defmodule AoC3 do
  def coord_stream do
    directions = [{1,0}, {0,1}, {-1,0}, {0, -1}]
    Stream.unfold({{0,0}, {0,0,0,0}, directions},
                  fn {{x,y}, {minx, miny, maxx, maxy}, dirs = [{dx,dy} | rest]} ->
                    {x_,y_} = {x+dx, y+dy}
                    {minx, miny, maxx, maxy, dirs} = 
                      if (x_ > maxx) || (x_< minx) || (y_ > maxy) || (y_ < miny) do
                        {min(minx, x_), min(miny, y_), max(maxx, x_), max(maxy, y_), rest ++ [{dx,dy}]}
                      else
                        {minx, miny, maxx, maxy, dirs}
                      end
                    {{x_, y_}, {{x_, y_}, {minx, miny, maxx, maxy}, dirs}}
                  end)
  end

  def sums do
    board = %{{0,0} => 1}
    Stream.transform(coord_stream(), board, fn {x,y}, board ->
      neighbors = for dx <- -1..1, dy <- -1..1, do: {x+dx, y+dy}
      sum = neighbors |> Enum.map(fn n -> Map.get(board, n, 0) end) |> Enum.sum
      {[sum], Map.put(board, {x,y}, sum)}
    end)
  end

  def sums_until(n) do
    sums()
    |> Stream.transform(false, fn(x, hit) ->
      if hit do
        {:halt, x}
      else
        {[x], x > n}
      end
    end)
  end
end

input = 277678
IO.inspect(AoC3.coord_stream |> Enum.at(input - 2) |> fn {x,y} -> abs(x) + abs(y) end.())
IO.inspect(AoC3.sums_until(input) |> Enum.at(-1))