defmodule ConfigExt do
  @moduledoc """
  A helper module, which contains common functions used around loading config at runtime.
  """

  @doc """
  Looks for dynamic patterns in input, when found - evals them - otherwise passes input forward. Supported input formats are:

      {:system, KEY}
      {:system, KEY, default}

  Returns a tuple with two elements:

  - `{:ok, value}` for matched pattern if the environment variable is present, or default is present, or if the pattern wasn't found.
  - `{:error, message}` if given environment variable was empty and there was no default value.

  ## Examples

  Given `CONFIG_EXT_TEST=foo` is set in environment.

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST"})
      {:ok, "foo"}

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST", "bar"})
      {:ok, "foo"}

  When `CONFIG_EXT_TEST` is not set.

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST"})
      {:error, ""}

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST", "bar"})
      {:ok, "bar"}

  For input without pattern.

      iex> ConfigExt.load(:error)
      {:ok, :error} # for example logger level

      iex> ConfigExt.load("baz")
      {:ok, "baz"}
  """
  def load({:system, key}) when is_bitstring(key) do
    case System.get_env(key) do
      nil -> error(key)
      ""  -> error(key)
      val -> {:ok, val}
    end
  end

  def load({:system, key}), do: {:error, "ENV Key: #{inspect(key)}, is not a string"}
  def load({:system, key, default}), do: load({:system, key}, default)
  def load(value), do: {:ok, value}

  defp error(key), do: {:error, "ENV Key: #{key} is missing"}

  @doc """
  An extension to `ConfigExt.load/1` function, which accepts default value as a second argument.

  Returns:

  - `{:ok, value}` as in `ConfigExt.load/1`
  - `{:ok, default}` in case of:
    - pattern failure
    - `nil` input value

  ## Examples

  Given `CONFIG_EXT_TEST=foo` is set in environment.

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST"}, "baz")
      {:ok, "foo"}

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST", "bar"}, "baz")
      {:ok, "foo"}

  When `CONFIG_EXT_TEST` is not set, default value is used.

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST"}, "baz")
      {:ok, "baz"}

  When pattern comes with default, it takes precedence before the given one.

      iex> ConfigExt.load({:system, "CONFIG_EXT_TEST", "bar"}, "baz")
      {:ok, "bar"}

  For input without pattern.

      iex> ConfigExt.load(:error, "baz")
      {:ok, :error} # as :error is a valid input

      iex> ConfigExt.load("foo", "bar")
      {:ok, "foo"}
  """
  def load({:system, key}, default) do
    case load({:system, key}) do
      {:error, _} -> {:ok, default}
      {:ok, val}  -> {:ok, val}
    end
  end

  def load({:system, key, user_default}, _default) do
    load({:system, key}, user_default)
  end

  def load(nil, default), do: {:ok, default}
  def load(value, _default), do: {:ok, value}
end