Experimental get_inverse_cardinality proposal. See #1904.

src/ifcopenshell-python/ifcopenshell/file.py

@@ -391,6 +391,16 @@ class file(object):
             return inverses
         return set(inverses)
 
+    def get_inverse_cardinality(self, inst):
+        """Returns the number of entities that reference this entity
+
+        :param inst: The entity instance to get inverse relationships
+        :type inst: ifcopenshell.entity_instance.entity_instance
+        :returns: 0 if no references, 1 if one reference, or 2 if more than one reference
+        :rtype: int
+        """
+        return self.wrapped_data.get_inverse_cardinality(inst.wrapped_data)
+
     def remove(self, inst):
         """Deletes an IFC object in the file.
 

src/ifcparse/IfcFile.h

@@ -249,6 +249,7 @@ public:
 	aggregate_of_instance::ptr traverse_breadth_first(IfcUtil::IfcBaseClass* instance, int max_level=-1);
 
 	aggregate_of_instance::ptr getInverse(int instance_id, const IfcParse::declaration* type, int attribute_index);
+	int getInverseCardinality(int instance_id);
 
 	template <class T>
 	typename T::list::ptr getInverse(int instance_id, int attribute_index) {

src/ifcparse/IfcParse.cpp

@@ -2387,6 +2387,25 @@ aggregate_of_instance::ptr IfcFile::getInverse(int instance_id, const IfcParse::
 	return return_value;
 }
 
+
+int IfcFile::getInverseCardinality(int instance_id) {
+	auto lower = byref_excl.lower_bound({ instance_id,-1,-1 });
+	auto upper = byref_excl.upper_bound({ instance_id, std::numeric_limits<int>::max(), std::numeric_limits<int>::max() });
+
+	int total_references = 0;
+	for (auto it = lower; it != upper; it++) {
+		for (auto& i : it->second) {
+			total_references++;
+			if (total_references > 1) {
+				return total_references;
+			}
+		}
+	}
+
+	return total_references;
+}
+
+
 void IfcFile::setDefaultHeaderValues() {
 	const std::string empty_string = "";
 	std::vector<std::string> file_description, schema_identifiers, empty_vector;

src/ifcwrap/IfcParseWrapper.i

@@ -88,6 +88,9 @@ static IfcUtil::ArgumentType helper_fn_attribute_type(const IfcUtil::IfcBaseClas
 	aggregate_of_instance::ptr get_inverse(IfcUtil::IfcBaseClass* e) {
 		return $self->getInverse(e->data().id(), 0, -1);
 	}
+	int get_inverse_cardinality(IfcUtil::IfcBaseClass* e) {
+		return $self->getInverseCardinality(e->data().id());
+	}
 
 	void write(const std::string& fn) {
 		std::ofstream f(IfcUtil::path::from_utf8(fn).c_str());