AOC 2023 - Day 9

Mix.install([
  {:kino_aoc, "~> 0.1"}
])

Input

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2023", "9", System.fetch_env!("LB_AOC_SESSION"))

test_input = """
0 3 6 9 12 15
1 3 6 10 15 21
10 13 16 21 30 45
"""

Code

sequences =
  puzzle_input
  |> String.split("\n")
  |> Enum.filter(&(&1 != ""))
  |> Enum.map(fn line ->
    line
    |> String.split()
    |> Enum.map(&String.to_integer(&1))
  end)

defmodule Day9 do
  def get_step_differences(numbers, current_diffs) do
    if Enum.all?(numbers, &(&1 == 0)) do
      current_diffs ++ [numbers]
    else
      last_idx = length(numbers) - 2

      differences =
        0..last_idx
        |> Enum.map(fn idx ->
          current = Enum.at(numbers, idx)
          next = Enum.at(numbers, idx + 1)
          next - current
        end)

      get_step_differences(differences, current_diffs ++ [numbers])
    end
  end

  def extrapolate_next_step([], processed), do: processed

  def extrapolate_next_step(sequences, []) do
    {rest, [last]} = Enum.split(sequences, -1)
    extrapolate_next_step(rest, [[0 | last]])
  end

  def extrapolate_next_step(sequences, processed) do
    {rest, [last]} = Enum.split(sequences, -1)
    last_number = List.last(last)

    number_to_add = processed |> List.first() |> List.last()

    extrapolate_next_step(rest, [last ++ [last_number + number_to_add] | processed])
  end
end

Part 1 - Solution

sequences_with_differences =
  sequences
  |> Enum.map(fn seq ->
    Day9.get_step_differences(seq, [])
  end)

Enum.map(sequences_with_differences, fn seq ->
  Day9.extrapolate_next_step(seq, [])
  |> List.first()
  |> List.last()
end)
|> Enum.sum()

Part 2 - Solution

sequences_with_differences =
  sequences
  |> Enum.map(fn seq ->
    Day9.get_step_differences(Enum.reverse(seq), [])
  end)

Enum.map(sequences_with_differences, fn seq ->
  Day9.extrapolate_next_step(seq, [])
  |> List.first()
  |> List.last()
end)
|> Enum.sum()