defmodule Config.Helper do
  @doc """
  Get a map of environment variables.

  `env_list` contains a list of environment variables to load, either as a single
  binary (e.g. "PORT") or a tuple of a binary and an informative message to
  display if the variable is missing (e.g. {"PORT", "It should be around 4000"}).

  The map key is the environment variable converted to lower-case
  """
  def load_all(env_list) when is_list(env_list) do
    env_list
    |> Map.new(fn
      {env, msg} -> {env |> to_env_dict_key, load_from_env(env, msg)}
      env when is_binary(env) -> {env |> to_env_dict_key, load_from_env(env)}
    end)
  end

  def load_from_env(env, msg \\ "") when is_binary(env) and is_binary(msg),
    do: System.get_env(env) || raise("environment variable #{env} is missing. \n#{msg}")

  defp to_env_dict_key(env) when is_binary(env), do: env |> String.downcase() |> String.to_atom()
end