Advent of code day 08

Mix.install([
  {:kino, "~> 0.5.0"}
])

Setup input

example = Kino.Input.textarea("Please paste your input example:")

input = Kino.Input.textarea("Please paste your real input:")

[dirs | nodes] =
  example
  |> Kino.Input.read()
  |> String.split(["\n"], trim: true)

original_dirs = String.split(dirs, "", trim: true)

nodes =
  Enum.reduce(nodes, Map.new(), fn node, acc ->
    [key, _, children | _] = String.split(node, [" = ", "(", ")"])
    [left, right | _] = String.split(children, ", ", trim: true)

    acc
    |> Map.put(key, [left, right])
  end)

Part 01

start = "AAA"

Stream.iterate(0, &(&1 + 1))
|> Enum.reduce_while({original_dirs, start, 0}, fn _move, {dirs, node, moves} ->
  cond do
    node == "ZZZ" -> {:halt, moves}
    dirs == [] -> {:cont, {original_dirs, node, moves}}
    hd(dirs) == "L" -> {:cont, {tl(dirs), Map.get(nodes, node) |> Enum.at(0), moves + 1}}
    hd(dirs) == "R" -> {:cont, {tl(dirs), Map.get(nodes, node) |> Enum.at(1), moves + 1}}
  end
end)

Part 02

starts = Map.keys(nodes) |> Enum.filter(fn node -> String.ends_with?(node, "A") end)

# tried simulation the same way as part 01. It was too slow.
defmodule Solver do
  def lcm(a, b), do: div(a * b, Integer.gcd(a,b))

  def find_cycle_length(start, dirs, nodes) do
    Stream.cycle(dirs)
    |> Enum.reduce_while({start, 0}, fn dir, {node, steps} ->
      cond do
        String.ends_with?(node, "Z") -> {:halt, steps}
        dir == "L" -> {:cont, { Map.get(nodes, node) |> Enum.at(0), steps + 1}}
        dir == "R" -> {:cont, { Map.get(nodes, node) |> Enum.at(1), steps + 1}}
      end
    end)
  end
end

starts
|> Enum.map(&Solver.find_cycle_length(&1, original_dirs, nodes))
|> Enum.reduce(&Solver.lcm/2)