Day 9

Mix.install([:kino])

Get input

input =
  Kino.Input.textarea("Paste input here",
    default: """
    0 3 6 9 12 15
    1 3 6 10 15 21
    10 13 16 21 30 45
    """
  )

parsed_input =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)
  |> Stream.map(&String.split(&1, " ", trim: true))
  |> Enum.map(fn ns -> Enum.map(ns, &String.to_integer/1) end)

defmodule MirageMaintenance do
  def get_next(sequence) do
    [head | tail] = sequence

    if Enum.all?(tail, &(&1 === head)) do
      # all elements are the same, so we know the next value
      head
    else
      differences = get_differences(sequence)
      List.last(tail) + get_next(differences)
    end
  end

  def get_differences(sequence) do
    Stream.chunk_every(sequence, 2, 1, :discard)
    |> Enum.map(fn [a, b] -> b - a end)
  end
end

Part 1

parsed_input
|> Stream.map(&MirageMaintenance.get_next/1)
|> Enum.sum()
|> dbg()

Part 2

parsed_input
|> Stream.map(&Enum.reverse/1)
|> Stream.map(&MirageMaintenance.get_next/1)
|> Enum.sum()