defmodule Crawly.Spider do
  @moduledoc """
  A behavior module for implementing a Crawly Spider

  A Spider is a module which is responsible for defining:
  1. `init/0` function, which must return a keyword list with start_urls/start_requests list
  2. `init/1` same as init, but also takes a list of options sent from Engine
  3. `base_url/0` function responsible for filtering out requests not related to
      a given website
  4. `override_settings/0` function that is called each time a setting is referenced internally. Allows overriding of Crawly configuration at the spider-level.
  5. `parse_item/1` function which is responsible for parsing the downloaded
     request and converting it into items which can be stored and new requests
     which can be scheduled
  6. `custom_settings/0` an optional callback which can be used in order to
      provide custom spider specific settings. Should define a list with custom
      settings and their values. These values will take precedence over the
      global settings defined in the config.

  @callback init() :: [start_urls: list(), start_requests: list()]
  @callback init(options: keyword()) :: [
              start_urls: list(),
              start_requests: list()

  @callback base_url() :: binary()

  @callback parse_item(response :: HTTPoison.Response.t()) ::

  @callback override_settings() :: Crawly.Settings.t()
  defmacro __using__(_opts) do
    quote do
      require Logger
      @behaviour Crawly.Spider

      def override_settings(), do: []

      # This line is needed to keep the backward compatibility, so all spiders
      # with init/0 will still work normally.
      def init(_options), do: init()

      def init() do
        Logger.error("Using default spider init, without start urls")
        %{start_urls: []}

      def parse_item(_response), do: %{}

      defoverridable override_settings: 0, init: 1, init: 0, parse_item: 1