defmodule Runlength do
  def encode(<< first :: binary-size(1) , rest :: binary>>) do
    encode({first, 1}, rest, "")
  end

  def encode({letter, count}, << letter :: binary-size(1) , rest :: binary>>, result) do
    encode({letter, count + 1}, rest, result)
  end
  def encode({letter, count}, << diff :: binary-size(1) , rest :: binary>>, result) do
    result = result <> to_string(count) <> letter
    encode({diff, 1}, rest, result)
  end

  def encode({letter, count}, <<>>, result) do
    result <> to_string(count) <> letter
  end
end