##
# DiffReader reads OSM diffs and applies them to the database.
#
# Uses the streaming LibXML "Reader" interface to cut down on memory
# usage, so hopefully we can process fairly large diffs.
class DiffReader
  include ConsistencyValidations

  # maps each element type to the model class which handles it
  MODELS = { 
    "node"     => Node, 
    "way"      => Way, 
    "relation" => Relation
  }

  ##
  # Construct a diff reader by giving it a bunch of XML +data+ to parse
  # in OsmChange format. All diffs must be limited to a single changeset
  # given in +changeset+.
  def initialize(data, changeset)
    @reader = XML::Reader.new data
    @changeset = changeset
  end

  ##
  # An element-block mapping for using the LibXML reader interface. 
  #
  # Since a lot of LibXML reader usage is boilerplate iteration through
  # elements, it would be better to DRY and do this in a block. This
  # could also help with error handling...?
  def with_element
    # skip the first element, which is our opening element of the block
    @reader.read
    # loop over all elements. 
    # NOTE: XML::Reader#read returns 0 for EOF and -1 for error.
    while @reader.read == 1
      break if @reader.node_type == 15 # end element
      next unless @reader.node_type == 1 # element
      yield @reader.name
    end
  end

  ##
  # An element-block mapping for using the LibXML reader interface. 
  #
  # Since a lot of LibXML reader usage is boilerplate iteration through
  # elements, it would be better to DRY and do this in a block. This
  # could also help with error handling...?
  def with_model
    with_element do |model_name|
      model = MODELS[model_name]
      raise "Unexpected element type #{model_name}, " +
        "expected node, way, relation." if model.nil?
      yield model, @reader.expand
      @reader.next
    end
  end

  ##
  # Checks a few invariants. Others are checked in the model methods
  # such as save_ and delete_with_history.
  def check(model, xml, new)
    raise OSM::APIBadXMLError.new(model, xml) if new.nil?
    unless new.changeset_id == @changeset.id 
      raise OSM::APIChangesetMismatchError.new(new.changeset_id, @changeset.id)
    end
  end

  ##
  # Consume the XML diff and try to commit it to the database. This code
  # is *not* transactional, so code which calls it should ensure that the
  # appropriate transaction block is in place.
  #
  # On a failure to meet preconditions (e.g: optimistic locking fails) 
  # an exception subclassing OSM::APIError will be thrown.
  def commit

    node_ids, way_ids, rel_ids = {}, {}, {}
    ids = { :node => node_ids, :way => way_ids, :relation => rel_ids}

    result = OSM::API.new.get_xml_doc

    # loop at the top level, within the <osmChange> element (although we
    # don't actually check this...)
    with_element do |action_name|
      if action_name == 'create'
        # create a new element. this code is agnostic of the element type
        # because all the elements support the methods that we're using.
        with_model do |model, xml|
          new = model.from_xml_node(xml, true)
          check(model, xml, new)

          # when this element is saved it will get a new ID, so we save it
          # to produce the mapping which is sent to other elements.
          placeholder_id = xml['id'].to_i
          raise OSM::APIBadXMLError.new(model, xml) if placeholder_id.nil?

          # some elements may have placeholders for other elements in the
          # diff, so we must fix these before saving the element.
          new.fix_placeholders!(ids)

          # create element given user
          new.create_with_history(@changeset.user)
          
          # save placeholder => allocated ID map
          ids[model.to_s.downcase.to_sym][placeholder_id] = new.id

          # add the result to the document we're building for return.
          xml_result = XML::Node.new model.to_s.downcase
          xml_result["old_id"] = placeholder_id.to_s
          xml_result["new_id"] = new.id.to_s
          xml_result["new_version"] = new.version.to_s
          result.root << xml_result
        end
        
      elsif action_name == 'modify'
        # modify an existing element. again, this code doesn't directly deal
        # with types, but uses duck typing to handle them transparently.
        with_model do |model, xml|
          # get the new element from the XML payload
          new = model.from_xml_node(xml, false)
          check(model, xml, new)

          # and the old one from the database
          old = model.find(new.id)

          new.fix_placeholders!(ids)
          old.update_from(new, @changeset.user)

          xml_result = XML::Node.new model.to_s.downcase
          xml_result["old_id"] = old.id.to_s
          xml_result["new_id"] = new.id.to_s
          xml_result["new_version"] = new.version.to_s
          result.root << xml_result
        end

      elsif action_name == 'delete'
        # delete action. this takes a payload in API 0.6, so we need to do
        # most of the same checks that are done for the modify.
        with_model do |model, xml|
          new = model.from_xml_node(xml, false)
          check(model, xml, new)

          old = model.find(new.id)

          # can a delete have placeholders under any circumstances?
          # if a way is modified, then deleted is that a valid diff?
          new.fix_placeholders!(ids)
          old.delete_with_history!(new, @changeset.user)

          xml_result = XML::Node.new model.to_s.downcase
          xml_result["old_id"] = old.id.to_s
          result.root << xml_result
        end

      else
        # no other actions to choose from, so it must be the users fault!
        raise OSM::APIChangesetActionInvalid.new(action_name)
      end
    end

    # return the XML document to be rendered back to the client
    return result
  end

end