defmodule Frenzy.OPML.Importer do
  import Record

  @typedoc """
  The data created from an OPML import.
  A list of groups. Each group has a name (String) or :default (if the OPML specified no group) and a list of feed URLs.
  """
  @type import_data() :: %{optional(String.t() | :default) => [String.t()]}

  defrecord :xmlElement, extract(:xmlElement, from_lib: "xmerl/include/xmerl.hrl")
  defrecord :xmlAttribute, extract(:xmlAttribute, from_lib: "xmerl/include/xmerl.hrl")

  @spec parse_opml(String.t()) :: import_data()
  def parse_opml(text) do
    {doc, _} =
      text
      |> String.trim()
      |> :binary.bin_to_list()
      |> :xmerl_scan.string()

    outline_elements = :xmerl_xpath.string('/opml/body/outline', doc)

    outline_elements
    |> Enum.flat_map(&get_feeds/1)
    |> Enum.reduce(%{}, fn {group, feed_url}, acc ->
      Map.update(acc, group, [feed_url], fn feeds -> [feed_url | feeds] end)
    end)
  end

  defp get_feeds(outline_el) do
    attributes = xmlElement(outline_el, :attributes)

    # if the <outline> contains a xmlUrl attribute, it is a top level feed (uses the :default group)
    # otherwise, it is a group of feeds

    if Enum.any?(attributes, fn attr -> xmlAttribute(attr, :name) == :xmlUrl end) do
      [{:default, get_feed_from_outline(outline_el)}]
    else
      get_feeds_from_group(outline_el)
    end
  end

  defp get_feed_from_outline(feed_el) do
    [attr] = :xmerl_xpath.string('//@xmlUrl', feed_el)

    xmlAttribute(attr, :value)
    |> List.to_string()
  end

  defp get_feeds_from_group(group_el) do
    [title_attr] = :xmerl_xpath.string('/outline/@title', group_el)

    group_title =
      xmlAttribute(title_attr, :value)
      |> List.to_string()

    :xmerl_xpath.string('/outline/outline/@xmlUrl', group_el)
    |> Enum.map(fn attr ->
      feed_url =
        xmlAttribute(attr, :value)
        |> List.to_string()

      {group_title, feed_url}
    end)
  end
end