module ActiveFedora::SemanticNode

def self.included(klass)
  klass.extend(ClassMethods)
end

def add_relationship(relationship)
  # Only accept ActiveFedora::Relationships as input arguments
  assert_kind_of 'relationship',  relationship, ActiveFedora::Relationship
  register_triple(relationship.subject, relationship.predicate, relationship.object)
end

def assert_kind_of(n, o,t)
  raise "Assertion failure: #{n}: #{o} is not of type #{t}" unless o.kind_of?(t)
end

def outbound_relationships()
  if !internal_uri.nil? && !relationships[internal_uri].nil?
    return relationships[:self].merge(relationships[internal_uri]) 
  else
    return relationships[:self]
  end
end

def register_predicate(subject, predicate)
  register_subject(subject)
  if !relationships[subject].has_key?(predicate) 
    relationships[subject][predicate] = []
  end
end

def register_subject(subject)
  if !relationships.has_key?(subject) 
      relationships[subject] = {} 
  end
end

def register_triple(subject, predicate, object)
  register_subject(subject)
  register_predicate(subject, predicate)
  relationships[subject][predicate] << object
end

def relationships
  @relationships ||= relationships_from_class
end

def relationships_from_class
  rels = {}
  self.class.relationships.each_pair do |subj, pred|
    rels[subj] = {}
    pred.each_key do |pred_key|
      rels[subj][pred_key] = []
    end
  end
  #puts "rels from class: #{rels.inspect}"
  return rels
end

def to_rels_ext(pid, relationships=self.relationships)
  starter_xml = <<-EOL
  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
    <rdf:Description rdf:about="info:fedora/#{pid}">
    </rdf:Description>
  </rdf:RDF>
  EOL
  xml = REXML::Document.new(starter_xml)

  # Iterate through the hash of predicates, adding an element to the RELS-EXT for each "object" in the predicate's corresponding array.
  # puts ""
  # puts "Iterating through a(n) #{self.class}"
  # puts "=> whose relationships are #{self.relationships.inspect}"
  # puts "=> and whose outbound relationships are #{self.outbound_relationships.inspect}"
  self.outbound_relationships.each do |predicate, targets_array|
    targets_array.each do |target|
      # puts ". #{predicate} #{target}"
      xml.root.elements["rdf:Description"].add_element(self.class.predicate_lookup(predicate), {"xmlns" => "info:fedora/fedora-system:def/relations-external#", "rdf:resource"=>target})
    end
  end
  xml.to_s
end