day5_part1.ex

defmodule Day5 do
  @doc """
    Removes adjacents when they have opposite polarity (determined by case Aa), it recurses over result until there are no further adyacent oposites
      iex> Day5.slice_polarity_matches("dabAcCaCBAcCcaDA", "")
      "dabCBAcaDA"
  """
  def slice_polarity_matches(string, prev_string) when is_binary(prev_string) do
    cond do
      String.length(string) == String.length(prev_string) ->
        string
      true ->
        process_string(string)
        |> slice_polarity_matches(:second_pass)
        |> slice_polarity_matches(string)
    end
  end

  def slice_polarity_matches(string, :second_pass) do
    {first_char, rest } = String.split_at(string, 1)
    first_char <> process_string(rest)
  end

  defp process_string(string) do
    String.to_charlist(string)
    |> Enum.chunk_every(2)
    |> Enum.filter(&are_compatible/1)
    |> Enum.flat_map(&(&1))
    |> List.to_string
  end

  defp are_compatible([left, right]) do
    abs(left - right) != 32
  end

  defp are_compatible(_) do
    true
  end
end