defmodule Day6 do
  def part1 do
    File.read!("lib/day6/input.txt")
    |> parse_orbits()
    |> total_orbits()
  end

  def part2 do
    # orbits = parse_orbits("COM)B\nB)C\nC)D\nD)E\nE)F\nB)G\nG)H\nD)I\nE)J\nJ)K\nK)L\nK)YOU\nI)SAN")
    orbits =
      File.read!("lib/day6/input.txt")
      |> parse_orbits()

    you_path = path_to_root(orbits, "YOU") |> Enum.reverse()
    san_path = path_to_root(orbits, "SAN") |> Enum.reverse()

    {rem1, rem2} = remaining_after_lowest_common_element(you_path, san_path)
    length(rem1) - 1 + (length(rem2) - 1)
  end

  def parse_orbits(str) do
    str
    |> String.split("\n")
    |> Enum.map(fn line -> String.split(line, ")") end)
    |> Enum.reduce(%{}, fn [orbitted, object], orbits ->
      Map.put(orbits, object, orbitted)
    end)
  end

  def total_orbits(orbits) do
    orbits
    |> Map.keys()
    |> Enum.reduce(0, fn object, acc ->
      acc + orbit_count(orbits, object)
    end)
  end

  def orbit_count(orbits, object) do
    case Map.get(orbits, object) do
      nil ->
        0

      orbitted ->
        1 + orbit_count(orbits, orbitted)
    end
  end

  def path_to_root(graph, start) do
    case Map.get(graph, start) do
      nil ->
        []

      parent ->
        [parent | path_to_root(graph, parent)]
    end
  end

  def remaining_after_lowest_common_element([_, one | _] = list1, [_, two | _] = list2)
      when one != two do
    {list1, list2}
  end

  def remaining_after_lowest_common_element([_ | rest1], [_ | rest2]) do
    remaining_after_lowest_common_element(rest1, rest2)
  end
end