defmodule Gpex do
  defstruct ~w(tracks)a

  alias Gpex.Track

  def parse(text) when is_binary(text) do
    {:ok, {"gpx", attrs, children}} = Saxy.SimpleForm.parse_string(text)
    new(attrs, children)
  end

  defp new(_attrs, children) when is_list(children) do
    tracks =
      children
      |> Enum.map(fn
        {"trk", attrs, children} ->
          Track.new(attrs, children)
        _ ->
          nil
      end)
      |> Enum.filter(& &1)

    %__MODULE__{tracks: tracks}
  end

  defimpl String.Chars do
    def to_string(gpx) do
      """
      <?xml version="1.0" encoding="UTF-8"?>
      <gpx
        xmlns="http://www.topografix.com/GPX/1/1"
        xmlns:topografix="http://www.topografix.com/GPX/Private/TopoGrafix/0/1"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        version="1.1"
        creator="OpenTracks"
        xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd http://www.topografix.com/GPX/Private/TopoGrafix/0/1 http://www.topografix.com/GPX/Private/TopoGrafix/0/1/topografix.xsd"
      >
        #{ gpx.tracks |> Enum.map(&Kernel.to_string/1) |> Enum.join("") }
      </gpx>
      """
    end
  end
end