Flatten a nested map

flatten.exs

data = %{"key1" => %{sub_key: :a}, "key2" => %{"sub_key1" => %{"sub_key" => "a"}, "sub_key2" => :a}}

defmodule Flatten do
  def flat(data, acc) when is_bitstring(data) or is_atom(data), do: {acc, data}
  def flat({key, value}, acc) do
    flat(value,
      case acc do
        "" -> key
        _ -> acc <> "." <> "#{key}"
      end
    )
  end
  def flat(data, acc) do
    case Enum.map(data, fn x ->
      flat(x, acc)
    end) do
      [result|[]] -> result
      [result] -> result
      result -> result
    end
  end
end

data
|> Enum.map(fn entry ->
  Flatten.flat(entry, "")
end)
|> IO.inspect

# [{"key1.sub_key", :a}, [{"key2.sub_key1.sub_key", "a"}, {"key2.sub_key2", :a}]]