defmodule Day1 do
  @example_1 """
  1721
  979
  366
  299
  675
  1456
  """

  def part1(example \\ false) do
    if(example, do: @example_1, else: File.read!("lib/day1/input.txt"))
    |> String.trim()
    |> String.split("\n")
    |> Enum.map(&String.to_integer/1)
    |> all_pairs()
    |> find_sum_to_2020()
  end

  def part2(example \\ false) do
    if(example, do: @example_1, else: File.read!("lib/day1/input.txt"))
    |> String.trim()
    |> String.split("\n")
    |> Enum.map(&String.to_integer/1)
    |> all_triples()
    |> find_sum_to_2020()
  end

  def all_pairs(items) do
    items
    |> Enum.drop(-1)
    |> Enum.with_index()
    |> Enum.flat_map(fn {a, a_index} ->
      {_, rest} = Enum.split(items, a_index + 1)

      Enum.map(rest, fn b ->
        {a, b}
      end)
    end)
  end

  def all_triples(items) do
    items
    |> Enum.drop(-1)
    |> Enum.with_index()
    |> Enum.flat_map(fn {a, a_index} ->
      {_, rest} = Enum.split(items, a_index + 1)

      rest
      |> Enum.drop(-1)
      |> Enum.with_index()
      |> Enum.flat_map(fn {b, b_index} ->
        {_, rest_rest} = Enum.split(rest, b_index + 1)

        Enum.map(rest_rest, fn c ->
          {a, b, c}
        end)
      end)
    end)
  end

  def find_sum_to_2020(groups) do
    Enum.find_value(groups, fn items ->
      list = Tuple.to_list(items)

      if Enum.reduce(list, 0, &(&1 + &2)) == 2020 do
        Enum.reduce(list, 1, &(&1 * &2))
      else
        nil
      end
    end)
  end
end