Day 18

Part 1

# {input, size, to_take} = {File.stream!("18/example.txt"), 7, 12}
{input, size, to_take} = {File.stream!("18/input.txt"), 71, 1024}

defmodule U do
  def parse(input) do
    Enum.map(input, fn l ->
      [_, x, y] = Regex.run(~r"(\d+),(\d+)", l)
      {x |> String.to_integer(), y |> String.to_integer()}
    end)
  end
end

defmodule P1 do
  def find(map, size) do
    find(%{{0,0} => 0}, MapSet.new(), map, size)
  end

  def find(front, _, _, _) when front == %{}, do: nil

  def find(front, expanded, map, size) do
    {node, cost} = cheapest(front)
    front = Map.delete(front, node)
    if node == {size - 1, size - 1} do
      cost
    else
      expanded = MapSet.put(expanded, node)
      front = Enum.reduce(neighbours(node, map, size), front, fn n, front ->
        if MapSet.member?(expanded, n) do
          front
        else
          cost = cost + 1
          case front[n] do
            nil -> Map.put(front, n, cost)
            {in_front_cost, _} when cost < in_front_cost -> Map.put(front, n, cost)
            _ -> front
          end
        end
      end)
      find(front, expanded, map, size)
    end
    
  end

  def neighbours({x, y}, map, size) do
    for {x, y} = pos <- [{x + 1, y}, {x, y + 1}, {x - 1, y}, {x, y - 1}],
        x >= 0 &amp;&amp; x < size &amp;&amp; y >= 0 &amp;&amp; y < size &amp;&amp; !MapSet.member?(map, pos) do
      pos
    end
  end

  def cheapest(front) do
    [x | _ ] = Enum.sort(front, fn {_, cost1}, {_, cost2} -> cost1 <= cost2 end)
    x
  end
end

P1.find(input |> Stream.take(to_take) |> U.parse() |> MapSet.new(), size)

Enum.reduce_while(input |> U.parse(), MapSet.new(), fn pos, acc ->
  acc = MapSet.put(acc, pos)
  if P1.find(acc, size) do
    {:cont, acc}
  else
    {:halt, pos}
  end
end)