defmodule ETL do
  @doc """
  Transform an index into an inverted index.

  ## Examples

  iex> ETL.transform(%{"a" => ["ABILITY", "AARDVARK"], "b" => ["BALLAST", "BEAUTY"]})
  %{"ability" => "a", "aardvark" => "a", "ballast" => "b", "beauty" =>"b"}
  """
  @spec transform(map) :: map
  def transform(input) do
    Enum.reduce(input, %{}, fn {count, words}, acc ->
      Enum.reduce(words, acc, &Map.put(&2, String.downcase(&1), count))
    end)
  end
end