defmodule FeedParser.Parser.Atom do
  @moduledoc """
  A `FeedParser.Parser` that handles [Atom feeds](https://validator.w3.org/feed/docs/atom.html).
  """

  alias FeedParser.XML
  require XML

  @behaviour FeedParser.Parser

  @impl FeedParser.Parser
  def accepts(data, content_type) do
    case content_type do
      "application/atom+xml" ->
        true

      _ when content_type in ["text/xml", "application/xml"] ->
        doc = XML.parse(data)

        if XML.xmlElement(doc, :name) == :feed do
          {true, doc}
        else
          false
        end

      _ ->
        false
    end
  end

  @impl FeedParser.Parser
  def parse_feed(feed) do
    title = text('/feed/title/text()', feed)
    link = attr('/feed/link/@href', feed)
    icon = text('/feed/icon/text()', feed)

    items =
      :xmerl_xpath.string('/feed/entry', feed)
      |> Enum.map(fn entry ->
        id = text('/entry/id/text()', entry)
        title = text('/entry/title/text()', entry)
        link = attr('/entry/link/@href', entry)

        updated =
          text('/entry/updated/text()', entry)
          |> Timex.parse("{ISO:Extended}")
          |> case do
            {:ok, date} -> date
            _ -> nil
          end

        content = text('/entry/content/text()', entry) || text('/entry/summary/text()', entry)

        %FeedParser.Item{
          guid: id,
          title: title,
          url: link,
          content: content,
          date: updated
        }
      end)

    {:ok,
     %FeedParser.Feed{
       site_url: link,
       title: title,
       image_url: icon,
       items: items
     }}
  end

  defp text(xpath, element) do
    case :xmerl_xpath.string(xpath, element) do
      [el] ->
        XML.xmlText(el, :value) |> List.to_string() |> String.trim()

      _ ->
        nil
    end
  end

  defp attr(xpath, element) do
    case :xmerl_xpath.string(xpath, element) do
      [attr] ->
        XML.xmlAttribute(attr, :value) |> List.to_string() |> String.trim()

      _ ->
        nil
    end
  end
end