🎄 Year 2023 🔔 Day 08

Setup

filename = "#{__DIR__}/../../../inputs/2023/day08.txt"

[instructions, maps] = File.read!(filename) |> String.split("\n\n")
instructions = String.split(instructions, "", trim: true)

maps =
  maps
  |> String.split("\n", trim: true)
  |> Enum.map(fn
    <<location::bitstring-size(24)>> <>
        " = (" <>
        <<left::bitstring-size(24)>> <>
        ", " <> <<right::bitstring-size(24)>> <> ")" ->
      {location, {left, right}}
  end)
  |> Map.new()

Part 1

Stream.cycle(instructions)
|> Stream.scan("AAA", fn
  "L", acc ->
    Map.fetch!(maps, acc) |> elem(0)

  "R", acc ->
    Map.fetch!(maps, acc) |> elem(1)
end)
|> Enum.find_index(&(&1 == "ZZZ"))
|> then(&(&1 + 1))

Part 2

starters = maps |> Enum.map(&elem(&1, 0)) |> Enum.filter(&String.ends_with?(&1, "A"))

intervals =
  Enum.map(starters, fn starter ->
    Stream.cycle(instructions)
    |> Stream.scan(starter, fn
      "L", acc ->
        Map.fetch!(maps, acc) |> elem(0)

      "R", acc ->
        Map.fetch!(maps, acc) |> elem(1)
    end)
    |> Enum.find_index(&String.ends_with?(&1, "Z"))
    |> then(&(&1 + 1))
  end)

Enum.reduce(intervals, fn a, b ->
  div(a * b, Integer.gcd(a, b))
end)