defmodule AOC.Day9 do
  use AOC.Day, day: 9

  def parse_input(lines) do
    Enum.map(lines, fn line ->
      line
      |> String.split(" ")
      |> Enum.map(&String.to_integer/1)
    end)
  end

  def part1(histories) do
    histories
    |> Enum.map(&predict_next_value/1)
    |> Enum.sum()
  end

  def part2(histories) do
    Enum.map(histories, &predict_previous_value/1)
    |> Enum.sum()
  end

  def predict_next_value(history) do
    history
    |> generate_differences()
    |> Enum.map(&List.last/1)
    |> Enum.sum()
  end

  def predict_previous_value(history) do
    history
    |> generate_differences()
    |> Enum.map(&List.first/1)
    |> Enum.reduce(&Kernel.-/2)
  end

  def generate_differences(history), do: generate_differences(history, [history])

  def generate_differences(numbers, acc) do
    if Enum.all?(numbers, &(&1 == 0)) do
      acc
    else
      differences =
        numbers
        |> Enum.chunk_every(2, 1, :discard)
        |> Enum.map(fn [val1, val2] -> val2 - val1 end)

      generate_differences(differences, [differences | acc])
    end
  end
end