5.exs

defmodule Polymer do
  def scan(p, status \\ :ok)

  def scan(p, _) when is_bitstring(p) do
    p = "%#{p}"

    p
    |> String.to_charlist()
    |> scan(:ok)
    |> elem(0)
    |> to_string
    |> String.slice(1..-1)
  end

  def scan([], s), do: {[], s}

  def scan([f | [f | t]], s) do
    {nl, ns} = scan([f | t], s)

    case ns do
      :backtrace -> scan([f | nl], :ok)
      _ -> {[f | nl], :ok}
    end
  end

  def scan([f | [n | t]], _) do
    fs = [f] |> to_string |> String.upcase()
    ns = [n] |> to_string |> String.upcase()

    cond do
      ns == fs ->
        {t, :backtrace}

      true ->
        {next_list, next_status} = scan([n | t], :ok)

        case next_status do
          :backtrace ->
            scan([f | next_list], :ok)

          _ ->
            {[f | next_list], :ok}
        end
    end
  end

  def scan([f | t], s) do
    {[f | t], s}
  end
end

polymer = File.read("5.txt") |> elem(1)

# Part 1
polymer |> Polymer.scan() |> String.length() |> IO.inspect()

# Part 2
shortest =
  Enum.reduce(?a..?z, nil, fn letter, len ->
    next =
      Regex.compile([letter] |> to_string, "i")
      |> elem(1)
      |> Regex.replace(polymer, "")
      |> Polymer.scan()
      |> String.length()

    cond do
      len == nil -> next
      len > next -> next
      true -> len
    end
  end)

IO.inspect(shortest)