defmodule WechatPay do
  @moduledoc """
  WechatPay provide toolkit for Wechat Payment Platform.

  ### Setup

  You need to define you own pay module, then `use` WechatPay, with an `:otp_app`
  option.

  ```elixir
  defmodule MyApp.Pay do
    use WechatPay, otp_app: :my_app
  end
  ```

  Then you can config your app with:

  ```elixir
  config :my_app, MyApp.Pay,
    env: :production,
    appid: "wx8888888888888888",
    mch_id: "1900000109",
    apikey: "192006250b4c09247ec02edce69f6a2d",
    ssl_cacert: File.read!("fixture/certs/rootca.pem"),
    ssl_cert: File.read!("fixture/certs/apiclient_cert.pem"),
    ssl_key: File.read!("fixture/certs/apiclient_key.pem")
  ```

  > NOTE: If your are using the `:sandbox` environment,
  > You need to use `mix wechat_pay.get_sandbox_signkey` to
  > fetch the Sandbox API Key.

  ### Payment methods

  When `use` WechatPay in `MyApp.Pay` module, it will generate following
  payment method modules for you:

  - `MyApp.Pay.App` - Implements the `WechatPay.PaymentMethod.App` behaviour
  - `MyApp.Pay.JSAPI` - Implements the `WechatPay.PaymentMethod.JSAPI` behaviour
  - `MyApp.Pay.Native` - Implements the `WechatPay.PaymentMethod.Native` behaviour

  ### Plug

  We will also generate some [Plugs](https://github.com/elixir-plug/plug) to
  simplify the process of handling notification from Wechat's Payment Gateway:

  - `MyApp.Pay.Plug.Payment` - Implements the `WechatPay.Plug.Payment` behaviour

  #### Phoenix Example

  In `lib/my_app_web/router.ex`:

  ```elixir
  post "/pay/cb/payment", MyApp.Pay.Plug.Payment, [handler: MyApp.PaymentHandler]
  ```
  """

  @typedoc """
  The Configuration

  - `env` - `:sandbox` or `:production`
  - `appid` - APP ID
  - `mch_id` - Merchant ID
  - `apikey` - API key
  - `ssl_cacert` - CA Root certificate in PEM
  - `ssl_cert` - Certificate in PEM
  - `ssl_key` - Private key in PEM
  """
  @type config :: [
    env: :sandbox | :production,
    appid: String.t,
    mch_id: String.t,
    apikey: String.t,
    ssl_cacert: String.t,
    ssl_cert: String.t,
    ssl_key: String.t
  ]

  defmacro __using__(opts) do
    quote bind_quoted: [opts: opts] do
      otp_app = Keyword.fetch!(opts, :otp_app)

      @doc false
      @spec get_config :: WechatPay.config
      def get_config do
        __MODULE__
        |> WechatPay.build_config(unquote(otp_app))
      end

      # define module `MyModule.App`
      __MODULE__
      |> Module.concat(:App)
      |> Module.create(
        quote do
          use WechatPay.PaymentMethod.App, mod: unquote(__MODULE__)
        end,
        Macro.Env.location(__ENV__)
      )

      # define module `MyModule.JSAPI`
      __MODULE__
      |> Module.concat(:JSAPI)
      |> Module.create(
        quote do
          use WechatPay.PaymentMethod.JSAPI, mod: unquote(__MODULE__)
        end,
        Macro.Env.location(__ENV__)
      )

      # define module `MyModule.Native`
      __MODULE__
      |> Module.concat(:Native)
      |> Module.create(
        quote do
          use WechatPay.PaymentMethod.Native, mod: unquote(__MODULE__)
        end,
        Macro.Env.location(__ENV__)
      )

      # define module `MyModule.Plug.Payment`
      if Code.ensure_loaded?(Plug) do
        [__MODULE__, :Plug, :Payment]
        |> Module.concat()
        |> Module.create(
          quote do
            use WechatPay.Plug.Payment, mod: unquote(__MODULE__)
          end,
          Macro.Env.location(__ENV__)
        )
      end
    end
  end

  @doc false
  def build_config(module, otp_app) do
    otp_app
    |> Application.fetch_env!(module)
  end
end