defmodule Envy do
  @moduledoc """
  Provides explicit and auto loading of env files.

  ## Example

  The following will set the `FOO` environment variable with the value of `bar`.

  ```
  foo=bar
  ```

  You can define comments with `#` but can't use `#` in values without wrapping
  the value in double quotes.

  ```
  foo="#bar" # Comment
  ```
  """

  @doc """
  Loads the `.env` and the `Mix.env` specific env file.

  eg: If `Mix.env` is `test` then envy will attempt to load `.env.test`.
  """
  def auto_load do
    Application.ensure_started(:mix)
    current_env = Mix.env |> to_string |> String.downcase

    [".env", ".env.#{current_env}"] |> load
  end

  @doc """
  Loads a list of env files.
  """
  def load(env_files) do
    for path <- env_files do
      if File.exists?(path) do
        File.read!(path) |> parse
      end
    end
  end

  @doc """
  Parses env formatted file.
  """
  def parse(content) do
    content |> get_pairs |> load_env
  end

  defp get_pairs(content) do
    content
    |> String.split("\n")
    |> Enum.reject(&blank_entry?/1)
    |> Enum.map(&parse_line/1)
  end

  defp parse_line(line) do
    [key, value] = line
      |> String.strip
      |> String.split("=")

    [key, parse_value(value)]
  end

  defp parse_value(value) do
    if String.starts_with?(value, "\"") do
      unquote_string(value)
    else
      value |> String.split("#", parts: 2) |> List.first
    end
  end

  defp unquote_string(value) do
    value
    |> String.split(~r{(?<!\\)"}, parts: 3)
    |> Enum.drop(1)
    |> List.first
    |> String.replace(~r{\\"}, ~S("))
  end

  defp load_env(pairs) when is_list(pairs) do
    Enum.each(pairs, fn([key, value]) ->
      System.put_env(String.upcase(key), value)
    end)
  end

  defp blank_entry?(string) do
    string == ""
  end
end