defmodule AOC.Day13 do
  def parse_input do
    [depart, buses] = AOC.Util.input_lines(13, 1)
    depart = String.to_integer(depart)

    buses =
      buses
      |> String.split(",")
      |> Enum.map(fn
        bus when bus == "x" -> :x
        bus -> String.to_integer(bus)
      end)

    {depart, buses}
  end

  def find_earliest_bus(depart, buses), do: find_earliest_bus(depart, buses, {nil, :infinity})
  def find_earliest_bus(_, [], earliest), do: earliest

  def find_earliest_bus(depart, [bus | tl], {_, earliest} = acc) do
    rounds = div(depart, bus)
    rounds = if rounds * bus < depart, do: rounds + 1, else: rounds
    wait = rounds * bus - depart

    acc = if wait < earliest, do: {bus, wait}, else: acc

    find_earliest_bus(depart, tl, acc)
  end

  def find_offsets(buses) do
    buses
    |> Enum.with_index()
    |> Enum.reject(&(elem(&1, 0) |> is_atom()))
  end

  def parts do
    {depart, buses} = parse_input()
    {bus, earliest} = find_earliest_bus(depart, Enum.reject(buses, &is_atom/1))
    part1 = bus * earliest

    offsets = find_offsets(buses)

    {part1, offsets}
  end
end