defmodule Aoc2023.Day9 do
  use Aoc2023

  defp differences(lst) do
    Enum.zip_with(lst, Enum.drop(lst, 1), &(&2 - &1))
  end

  defp prediction(lst) do
    diffs = differences(lst)

    if Enum.all?(diffs, &(&1 == 0)) do
      List.first(lst)
    else
      List.last(lst) + prediction(diffs)
    end
  end

  def parse(input) do
    input
    |> lines
    |> Enum.map(pipe(String.split() |> Enum.map(&String.to_integer/1)))
  end

  def part1(input) do
    input
    |> Enum.map(&prediction/1)
    |> Enum.sum
  end

  def part2(input) do
    input
    |> Enum.map(pipe(Enum.reverse |> prediction))
    |> Enum.sum
  end
end