defmodule Kiq do
  @moduledoc """
  Kiq is a robust and extensible job processing queue that aims for
  compatibility with Sidekiq Enterprise.

  Job queuing, processing and reporting are all built on GenStage. That means
  maximum parallelism with the safety of backpressure as jobs are processed.

  ## Usage

  Kiq isn't an application that must be started. Similarly to Ecto, you define
  one or more Kiq modules within your application. This allows multiple
  supervision trees with entirely different configurations.

  Define a Kiq module for your application:

      defmodule MyApp.Kiq do
        use Kiq, queues: [default: 25, events: 50]

        @impl Kiq
        def init(_reason, opts) do
          for_env = Application.get_env(:my_app, :kiq)

          opts =
            opts
            |> Keyword.merge(for_env)
            |> Keyword.put(:client_opts, [redis_url: System.get_env("REDIS_URL")])

          {:ok, opts}
        end
      end

  Include the module in your application's supervision tree:


      defmodule MyApp.Application do
        @moduledoc false

        use Application

        alias MyApp.{Endpoint, Kiq, Repo}

        def start(_type, _args) do
          children = [
            {Repo, []},
            {Endpoint, []},
            {Kiq, []}
          ]

          Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
        end
      end

  TODO: Configuration
  TODO: Workers
  TODO: Testing
  TODO: Error Handling
  """

  alias Kiq.{Client, Job, Timestamp}

  @type job_args :: map() | Keyword.t() | Job.t()
  @type job_opts :: [in: pos_integer(), at: DateTime.t()]

  @doc """
  Starts the client and possibly the supervision tree, returning `{:ok, pid}` when startup is
  successful.

  Returns `{:error, {:already_started, pid}}` if the tree is already started or `{:error, term}`
  in case anything else goes wrong.

  ## Options

  Any options passed to `start_link` will be merged with those provided in the `use` block.
  """
  @callback start_link(opts :: Keyword.t()) :: Supervisor.on_start()

  @doc """
  A callback executed when the supervision tree is started and possibly when configuration is
  read.

  The first argument is the context of the callback being invoked. In most circumstances this
  will be `:supervisor`. The second argument is a keyword list of the combined options passed
  to `use/1` and `start_link/1`.

  Application configuration is _not_ passed into the `init/2` callback. To use application
  config the callback must be overridden and merged manually.
  """
  @callback init(reason :: :supervisor, opts :: Keyword.t()) :: {:ok, Keyword.t()} | :ignore

  @doc """
  Clear all enqueued, scheduled and backup jobs.

  All known queues are cleared, even if they aren't listed in the current configuration.
  """
  @callback clear_all() :: :ok

  @doc """
  Enqueue a job to be processed asynchronously.

  Jobs can be enqueued from `Job` structs, maps or keyword lists.

  ## Options

  * `in` - The amount of time in seconds to wait before processing the job. This must be a
    positive integer.
  * `at` - A specific `DateTime` in the future when the job should be processed.

  ## Examples

      # Enqueue a job to be processed immediately
      MyJob.new([1, 2]) |> MyKiq.enqueue()

      # Enqueue a job in one minute
      MyJob.new([1, 2]) |> MyKiq.enqueue(in: 60)

      # Enqueue a job some time in the future
      MyJob.new([1, 2]) |> MyKiq.enqueue(at: ~D[2020-09-20 12:00:00])

      # Enqueue a job from scratch, without using a worker module
      MyKiq.enqueue(class: "ExternalWorker", args: [1])
  """
  @callback enqueue(job_args(), job_opts()) :: {:ok, Job.t()} | {:error, Exception.t()}

  @doc false
  defmacro __using__(opts) do
    quote do
      @behaviour Kiq

      @client_name Module.concat(__MODULE__, "Client")
      @reporter_name Module.concat(__MODULE__, "Reporter")
      @supervisor_name Module.concat(__MODULE__, "Supervisor")

      @opts unquote(opts)
            |> Keyword.put(:main, __MODULE__)
            |> Keyword.put(:name, @supervisor_name)
            |> Keyword.put(:client_name, @client_name)
            |> Keyword.put(:reporter_name, @reporter_name)

      @doc false
      def child_spec(opts) do
        %{id: __MODULE__, start: {__MODULE__, :start_link, [opts]}, type: :supervisor}
      end

      @impl Kiq
      def start_link(opts \\ []) do
        @opts
        |> Keyword.merge(opts)
        |> Kiq.Supervisor.start_link()
      end

      @impl Kiq
      def init(reason, opts) when is_atom(reason) and is_list(opts) do
        client_opts = [redis_url: System.get_env("REDIS_URL")]

        {:ok, Keyword.put(opts, :client_opts, client_opts)}
      end

      @impl Kiq
      def clear_all do
        Kiq.Client.clear_all(@client_name)
      end

      @impl Kiq
      def enqueue(job_args, job_opts \\ []) when is_map(job_args) or is_list(job_args) do
        Kiq.enqueue(@client_name, job_args, job_opts)
      end

      defoverridable child_spec: 1, enqueue: 2, init: 2, start_link: 1
    end
  end

  @doc false
  def enqueue(client, job_args, job_opts) do
    job =
      job_args
      |> to_job()
      |> with_opts(job_opts)

    Client.enqueue(client, job)
  end

  defp to_job(%Job{} = job), do: job
  defp to_job(args), do: Job.new(args)

  defp with_opts(job, []), do: job
  defp with_opts(job, at: timestamp), do: %Job{job | at: timestamp}
  defp with_opts(job, in: seconds), do: %Job{job | at: Timestamp.unix_in(seconds)}
end