{replacements, medicine} =
  File.stream!("inputs/day19.txt")
  |> Stream.map(&String.trim/1)
  |> Stream.filter(&(&1 != ""))
  |> Stream.map(fn line ->
    case Regex.run(~r/(\w+) => (\w+)/, line) do
      [_, from, to] -> {:replacement, from, to}
      _ -> {:start, line}
    end
  end)
  |> Enum.reduce({[], nil}, fn
    {:replacement, from, to}, {replacements, start} -> {[{from, to} | replacements], start}
    {:start, start}, {replacements, _start} -> {replacements, start}
  end)

nextGen = fn start ->
  Enum.flat_map(0..(String.length(start) - 1), fn offset ->
    {beforeReplace, replaceStart} = String.split_at(start, offset)

    Enum.filter(replacements, fn {from, _} -> String.starts_with?(replaceStart, from) end)
    |> Enum.map(fn {from, to} ->
      {^from, afterReplace} = String.split_at(replaceStart, String.length(from))
      <<beforeReplace <> to <> afterReplace>>
    end)
  end)
  |> Enum.uniq()
end

calibration = nextGen.(medicine)

IO.puts("Calibration size: #{length(calibration)}")

medicineGeneration =
  Stream.iterate(["e"], fn gen -> Enum.flat_map(gen, &nextGen.(&1)) |> Enum.uniq() end)
  |> Enum.find_index(fn gen -> Enum.any?(gen, &(&1 == medicine)) end)

IO.puts("The Medicine generation: #{medicineGeneration}")