defmodule Day15 do
  def part1(example \\ false) do
    find_number_spoken_at(example, 2020)
  end

  def part2(example \\ false) do
    find_number_spoken_at(example, 30_000_000)
  end

  def find_number_spoken_at(example, final_turn) do
    inputs = parse_input(example)

    # inputs
    # |> Enum.reverse()
    # |> step_times(final_turn - length(inputs))
    # |> List.first()

    # numbers = parse_input(example)

    inputs
    |> Enum.with_index()
    |> Enum.map(fn {e, i} -> {e, i + 1} end)
    |> Map.new()
    |> do_van_eck(List.last(inputs), length(inputs) + 1, final_turn)

    # |> step_until_turn(final_turn, length(numbers) + 1)
    # |> what_was_spoken_on_turn(final_turn)
  end

  def parse_input(example) do
    case example do
      1 -> "0,3,6"
      2 -> "1,3,2"
      3 -> "2,1,3"
      4 -> "1,2,3"
      5 -> "2,3,1"
      6 -> "3,2,1"
      7 -> "3,1,2"
      _ -> "9,12,1,4,17,0,18"
    end
    |> String.split(",")
    |> Enum.map(&String.to_integer/1)
  end

  def do_van_eck(_, prev, turn, count) when turn == count + 1 do
    prev
  end

  def do_van_eck(map, prev, turn, count) do
    # IO.inspect(map)

    new_val =
      if Map.has_key?(map, prev) do
        turn - map[prev]
      else
        0
      end

    # IO.inspect(map)
    IO.puts("turn: #{turn} \tprev: #{prev} \tnew: #{new_val}")

    do_van_eck(Map.put(map, prev, turn - 1), new_val, turn + 1, count)
  end

  def step_van_eck([prev | rest] = sequence) do
    next =
      case Enum.find_index(rest, &(&1 == prev)) do
        nil -> 0
        index_in_rest -> index_in_rest + 1
      end

    [next | sequence]
  end

  def step_times(sequence, 0) do
    sequence
  end

  def step_times(sequence, count) do
    step_times(step_van_eck(sequence), count - 1)
  end

  # def step(map, turn) do
  #   # IO.puts("current turn: #{turn}")

  #   {prev_spoken, prev_spoken_on_turns} = what_was_spoken_on_turn(map, turn - 1)

  #   # IO.puts("prev spoken: #{prev_spoken}")
  #   # IO.puts("prev spoken on turns: #{inspect(prev_spoken_on_turns)}")

  #   number_to_speak =
  #     case prev_spoken_on_turns do
  #       [_] ->
  #         0

  #       [most_recent, second_most_recent | _] ->
  #         most_recent - second_most_recent
  #         # List.last(prev_spoken_on_turns) - Enum.at(prev_spoken_on_turns, -2)
  #     end

  #   # IO.puts("will speak #{number_to_speak}")

  #   Map.update(map, number_to_speak, [turn], fn turns -> [turn | turns] end)
  # end

  # def step_until_turn(map, final_turn, turn) when final_turn + 1 == turn do
  #   map
  # end

  # def step_until_turn(map, final_turn, turn) do
  #   # IO.inspect(map)
  #   step_until_turn(step(map, turn), final_turn, turn + 1)
  # end

  # # def step(numbers, turn) do
  # #   # prev = Map.get(map, cur - 1)
  # #   {prev, prev_turn} = Enum.find(numbers, &(elem(&1, 1) == turn - 1))

  # #   IO.puts("turn #{turn}")
  # #   IO.puts("prev #{prev}")

  # #   new =
  # #     case Enum.find(numbers, &(elem(&1, 0) == prev)) |> IO.inspect() do
  # #       nil -> 0
  # #       {_, ^prev_turn} -> 0
  # #       {_, last_spoken_turn} -> turn - last_spoken_turn
  # #     end

  # #   # IO.inspect({prev, new})
  # #   IO.puts("speak #{new}")

  # #   [{new, turn} | Enum.reject(numbers, fn {n, _} -> n == new end)]
  # #   |> IO.inspect()
  # # end
end