aoc2017_13_part2_optimized.ex

defmodule Day13 do
  def part2(), do:
    scanners() |> count(-1, 0) |> IO.inspect

  def scanners(), do:
    "input.txt"
    |> File.stream!()
    |> Stream.map(&String.trim/1)
    |> Stream.map(fn row -> row |> String.split(": ") |> Enum.map(&String.to_integer/1) end)
    |> Enum.map(fn [pos, depth] -> {{-pos, 2 * (depth - 1)}, nil} end)
    |> Enum.sort()
    |> :gb_trees.from_orddict

  defp count(scanners, previous, count) do
    {{pos, interval}, nil, scanners} = :gb_trees.take_smallest(scanners)
    new_pos = pos + interval
    scanners = :gb_trees.enter({new_pos, interval}, nil, scanners)

    cond do
      pos < 0 -> count(scanners, previous, count)
      pos == previous -> count(scanners, previous, count)
      pos == previous + 1 -> count(scanners, pos, count + 1)
      true -> count
    end
  end
end

Day13.part2()