defmodule FeedParser do
  @moduledoc """
  FeedParser is a for parsing feeds of various formats into a unified representation.

  The currently supported formats are:
  - RSS 2.0
  - Atom
  - JSON Feed
  - RSS-in-JSON
  """

  @default_parsers [
    FeedParser.Parser.RSS2,
    FeedParser.Parser.Atom,
    FeedParser.Parser.JSONFeed,
    FeedParser.Parser.RSSInJSON
  ]

  @doc """
  Parses a feed from the given data string and MIME type.

  An extended set of parsers may also be provided, otherwise the default set (supporting RSS 2.0, Atom, JSON Feed, and RSS-in-JSON) will be used. Parsers are modules that implement the `FeedParser.Parser` behaviour.
  """
  @spec parse(data :: String.t(), content_type :: String.t(), parsers :: [module()]) ::
          {:ok, feed :: FeedParser.Feed.t()} | {:error, reason :: term()}
  def parse(data, content_type, parsers \\ @default_parsers)

  def parse(nil, _, _) do
    {:error, :no_data}
  end

  def parse(data, content_type, parsers) when is_binary(data) do
    parsers
    |> Enum.reduce_while(false, fn parser, acc ->
      case parser.accepts(data, content_type) do
        {true, result} ->
          {:halt, {parser, result}}

        _ ->
          {:cont, acc}
      end
    end)
    |> case do
      {parser, result} ->
        parser.parse_feed(result)

      false ->
        {:error, "no parser matched the given content type ('#{content_type}') and data"}
    end
  end
end