defmodule Meta.OpenNebula.Schema do
  @moduledoc """
  Helpers used to process OpenNebula's XSD schema.

                 ,.
            /,,;';;.  ,;;;..  ,,;.    '
          .',''   `::;:' ``;;;;'  `..'
          `      ,,/'     ,,//

  Here be dragons! We import the records from Erlang header files (.hrl)
  generated by erlsom as macros in the Records module. The only way I found to
  programatically call a macro (apply/3 only works with functions) is by
  playing around with the AST to template said call...

  I'm not fond of this as it makes things more complex than I would like them
  to be, but likely the easiest way to user erlsom's data binder from elixir
  (and it's not *that* bad). The ideal would be to generate elixir structs from
  source XSD files, but it would take much more work.

  Heavily inspired from Gary Poster's:
  http://codesinger.blogspot.com/2015/12/elixir-erlang-records-and-erlsom-xml.html

  I stole the dragon art from ASCII.co.uk.
  """

  require Logger
  alias Meta.OpenNebula.Schema
  alias Meta.OpenNebula.Schema.Records

  @opennebula_release "5.12"

  ##
  # Data-binding: access and extract XSD models.

  defp get_raw(type, object) do
    priv_dir = :code.priv_dir(:meta) |> to_string
    {data_dir, extension} = case type do
      :xsd -> {"xsd-src", ".xsd"}
      :hrl -> {"xsd-records", ".hrl"}
    end

    raw_path = [priv_dir, "opennebula", @opennebula_release, data_dir, object <> extension]
    raw_path |> Path.join()
  end

  def get_xsd_for(object), do: get_raw(:xsd, object)
  def get_hrl_for(object), do: get_raw(:hrl, object)

  def generate_model_for(object) do
    {:ok, model} = object
                   |> get_xsd_for()
                   |> :erlsom.compile_xsd_file()

    model
  end


  ##
  # Data-binding: transform erlsom records to Elixir maps.

  def scan(raw, object_type) do
    {:ok, data, _rest} = :erlsom.scan(raw, Records.get_model_for(object_type))
    data
  end

  defp transform_value({k, l}) when is_list(l) do
    {k, Enum.map(l, fn v -> map_record(v) end)}
  end
  defp transform_value({k, v}), do: {k, map_record(v)}

  # Here be dragons! See moduledoc.
  def map_record(data) when is_tuple(data) do
    schema = elem(data, 0)
    if schema in Keyword.keys(Schema.Records.__info__(:macros)) do
      call = quote do
        require Meta.OpenNebula.Schema.Records
        Meta.OpenNebula.Schema.Records."XSD"(:data)
      end
      replace = fn
        :XSD, {model, data} -> {model, {model, data}}
        :data, {model, data} -> {data, {model, data}}
        node, stubs -> {node, stubs}
      end

      {{inserted, _raw}, []} = call
      |> Macro.prewalk({schema, Macro.escape(data)}, replace)
      |> Code.eval_quoted()

      Enum.into(inserted, Map.new, &transform_value/1)
    else
      Logger.debug("Ignoring unknown OpenNebula Model #{schema}.")
      data
    end
  end
  def map_record(data = [first | _rest]) when is_integer(first), do:
    List.to_string(data)
  def map_record(:undefined), do: nil
  def map_record(data), do: data
end