defmodule Canary.Plugs do
  require Logger
  import Canada.Can, only: [can?: 3]
  import Ecto.Query
  import Keyword, only: [has_key?: 2]

  @moduledoc """
  Plug functions for loading and authorizing resources for the current request.

  Plugs all store data in conn.assigns (in Phoenix applications, keys in conn.assigns can be accessed with `@key_name` in both views and controllers)

  In order to use the plug functions, you must `use Canary`.

  You must also specify the Ecto repo to use in your configuration:
  ```
  config :canary, repo: Project.Repo
  ```
  If you wish, you may also specify the key where Canary will look for the current user record to authorize against:
  ```
  config :canary, current_user: :some_current_user
  ```
  """

  @doc """
  Load the given resource.

  Load the resource with id given by `conn.params["id"]` (or `conn.params[opts[:id_name]]` if `opts[:id_name]` is specified)
  and ecto model given by `opts[:model]` into `conn.assigns.resource_name`.

  `resource_name` is either inferred from the model name or specified in the plug declaration with the `:as` key.
  To infer the `resource_name`, the most specific(right most) name in the model's
  module name will be used, converted to underscore case.

  For example, `load_resource model: Some.Project.BlogPost` will load the resource into
  `conn.assigns.blog_post`

  If the resource cannot be fetched, `conn.assigns.resource_name` is set
  to nil.

  If the action is `:index`, all records from the specified model will be loaded.

  Currently, `:new` and `:create` actions are ignored, and `conn.assigns.resource_name`
  will be set to nil for these actions.

  Required opts:

  * `:model` - Specifies the module name of the model to load resources from

  Optional opts:

  * `:as` - Specifies the `resource_name` to use
  * `:only` - Specifies which actions to authorize
  * `:except` - Specifies which actions for which to skip authorization
  * `:preload` - Specifies association(s) to preload
  * `:id_name` - Specifies the name of the id in `conn.params`, defaults to "id"

  Examples:
  ```
  plug :load_resource, model: Post

  plug :load_resource, model: User, preload: :posts, as: :the_user

  plug :load_resource, model: User, only: [:index, :show], preload: :posts, as: :person

  plug :load_resource, model: User, except: [:destroy]
  ```
  """
  def load_resource(conn, opts) do
    conn
    |> action_valid?(opts)
    |> case do
      true  -> _load_resource(conn, opts)
      false -> conn
    end
  end

  defp _load_resource(conn, opts) do
    loaded_resource = case get_action(conn) do
      :index  ->
        fetch_all(conn, opts)
      :new    ->
        nil
      :create ->
        nil
      _       ->
        fetch_resource(conn, opts)
    end

    %{conn | assigns: Map.put(conn.assigns, resource_name(conn, opts), loaded_resource)}
  end

  @doc """
  Authorize the current user for the given resource.

  In order to use this function,

    1) `conn.assigns[Application.get_env(:canary, :current_user, :current_user)]` must be an ecto
    struct representing the current user

    2) `conn.private` must be a map (this should not be a problem unless you explicitly modified it)

  If authorization succeeds, sets `conn.assigns.authorized` to true.

  If authorization fails, sets `conn.assigns.authorized` to false.

  For the `:index`, `:new`, and `:create` actions, the resource in the `Canada.Can` implementation
  should be the module name of the model rather than a struct.

  For example:

    use
    ```
    def can?(%User{}, :index, Post), do: true
    ```
    instead of
    ```
    def can?(%User{}, :index, %Post{}), do: true
    ```

  Required opts:

  * `:model` - Specifies the module name of the model to authorize access to

  Optional opts:

  * `:only` - Specifies which actions to authorize
  * `:except` - Specifies which actions for which to skip authorization
  * `:preload` - Specifies association(s) to preload
  * `:id_name` - Specifies the name of the id in `conn.params`, defaults to "id"

  Examples:
  ```
  plug :authorize_resource, model: Post

  plug :authorize_resource, model: User, preload: :posts

  plug :authorize_resource, model: User, only: [:index, :show], preload: :posts
  ```
  """
  def authorize_resource(conn, opts) do
    conn
    |> action_valid?(opts)
    |> case do
      true  -> _authorize_resource(conn, opts)
      false -> conn
    end
  end

  defp _authorize_resource(conn, opts) do
    current_user_name = opts[:current_user] || Application.get_env(:canary, :current_user, :current_user)
    current_user = Dict.fetch! conn.assigns, current_user_name
    action = get_action(conn)

    resource = cond do
      action in [:index, :new, :create] ->
        opts[:model]
      true      ->
        fetch_resource(conn, opts)
    end

    case current_user |> can? action, resource do
      true  ->
        %{conn | assigns: Map.put(conn.assigns, :authorized, true)}
      false ->
        %{conn | assigns: Map.put(conn.assigns, :authorized, false)}
    end
  end

  @doc """
  Authorize the given resource and then load it if
  authorization succeeds.

  If the resource cannot be loaded or authorization
  fails, conn.assigns.resource_name is set to nil.

  The result of the authorization (true/false) is
  assigned to conn.assigns.authorized.

  Also, see the documentation for load_resource/2 and
  authorize_resource/2.

  Required opts:

  * `:model` - Specifies the module name of the model to load resources from

  Optional opts:

  * `:as` - Specifies the `resource_name` to use
  * `:only` - Specifies which actions to authorize
  * `:except` - Specifies which actions for which to skip authorization
  * `:preload` - Specifies association(s) to preload
  * `:id_name` - Specifies the name of the id in `conn.params`, defaults to "id"

  Examples:
  ```
  plug :load_and_authorize_resource, model: Post

  plug :load_and_authorize_resource, model: User, preload: :posts, as: :the_user

  plug :load_and_authorize_resource, model: User, only: [:index, :show], preload: :posts, as: :person

  plug :load_and_authorize_resource, model: User, except: [:destroy]
  ```
  """
  def load_and_authorize_resource(conn, opts) do
    conn
    |> action_valid?(opts)
    |> case do
      true  -> _load_and_authorize_resource(conn, opts)
      false -> conn
    end
  end

  defp _load_and_authorize_resource(conn, opts) do
    conn
    |> load_resource(opts)
    |> authorize_resource(opts)
    |> purge_resource_if_unauthorized(opts)
  end

  defp purge_resource_if_unauthorized(conn = %{assigns: %{authorized: true}}, _opts), do: conn
  defp purge_resource_if_unauthorized(conn = %{assigns: %{authorized: false}}, opts) do
    %{conn | assigns: Map.put(conn.assigns, resource_name(conn, opts), nil)}
  end

  defp fetch_resource(conn, opts) do
    repo = Application.get_env(:canary, :repo)

    id = get_resource_id(conn, opts)

    conn.assigns
    |> Map.fetch(resource_name(conn, opts))
    |> case do
      :error ->
        repo.get(opts[:model], id)
        |> preload_if_needed(repo, opts)
      {:ok, nil} ->
        repo.get(opts[:model], id)
        |> preload_if_needed(repo, opts)
      {:ok, resource} -> # if there is already a resource loaded onto the conn
        case (resource.__struct__ == opts[:model]) do
          true  ->
            resource
          false ->
            repo.get(opts[:model], id)
            |> preload_if_needed(repo, opts)
        end
    end
  end

  defp get_resource_id(conn, opts) do
    case opts[:id_name] do
      nil ->
        conn.params["id"]
      id_name ->
        conn.params[id_name]
    end
  end

  defp fetch_all(conn, opts) do
    repo = Application.get_env(:canary, :repo)

    conn
    |> Map.fetch(resource_name(conn, opts))
    |> case do
      :error ->
        from(m in opts[:model]) |> select([m], m) |> repo.all |> preload_if_needed(repo, opts)
      {:ok, resource} ->
        case (resource.__struct__ == opts[:model]) do
          true  ->
            resource
          false ->
            from(m in opts[:model]) |> select([m], m) |> repo.all |> preload_if_needed(repo, opts)
        end
    end
  end

  defp get_action(conn) do
    conn.assigns
    |> Map.fetch(:canary_action)
    |> case do
      {:ok, action} -> action
      _             ->
        conn.assigns
        |> Map.fetch(:action)
        |> case do
          {:ok, action} ->
            Logger.warn("WARNING: The conn.assigns.action key is deprecated in favour of conn.assigns.canary_action")
            action
          _             -> conn.private.phoenix_action
        end
    end
  end

  defp action_exempt?(conn, opts) do
    action = get_action(conn)

    (is_list(opts[:except]) && action in opts[:except])
    |> case do
      true  -> true
      false -> action == opts[:except]
    end
  end

  defp action_included?(conn, opts) do
    action = get_action(conn)

    (is_list(opts[:only]) && action in opts[:only])
    |> case do
      true  -> true
      false -> action == opts[:only]
    end
  end

  defp action_valid?(conn, opts) do
    cond do
      has_key?(opts, :except) && has_key?(opts, :only) ->
        false
      has_key?(opts, :except) ->
        !action_exempt?(conn, opts)
      has_key?(opts, :only) ->
        action_included?(conn, opts)
      true ->
        true
    end
  end

  defp resource_name(conn, opts) do
    case opts[:as] do
      nil ->
        opts[:model]
        |> Module.split
        |> List.last
        |> Mix.Utils.underscore
        |> pluralize_if_needed(conn)
        |> String.to_atom
      as -> as
    end
  end

  defp pluralize_if_needed(name, conn) do
    case get_action(conn) in [:index] do
      true -> name <> "s"
      _    -> name
    end
  end

  defp preload_if_needed(nil, _repo, _opts), do: nil

  defp preload_if_needed(records, repo, opts) do
    case opts[:preload] do
      nil ->
        records
      models ->
        repo.preload(records, models)
    end
  end
end