Short-circuit when combining a registry with itself.

This is useful when combining some base registry with further
resources -- e.g. in jsonschema we do JSONSCHEMA_SPECS.combine(...)
and if no additional resources are provided that's meant to be a
no-op.

Doing this seems to be a huge speed-up, taking the test run from ~25s
to 9s (though even that is very slow compared to the non-compliant
implementation, so some performance optimization will come regardless).

But as a micro-optimization, this seems definitely useful.

Author: Julian

referencing/_core.py

@@ -339,6 +339,8 @@ def combine(self, *registries: Registry[D]) -> Registry[D]:
         """
         Combine together one or more other registries, producing a unified one.
         """
+        if registries == (self,):
+            return self
         anchors = self._anchors
         resources = self._resources
         uncrawled = self._uncrawled

referencing/tests/test_core.py

@@ -218,6 +218,18 @@ def test_combine(self):
             ),
         ).with_resource("http://example.com/foo/bar", two)
 
+    def test_combine_self(self):
+        """
+        Combining a registry with itself short-circuits.
+
+        This is a performance optimization -- otherwise we do lots more work
+        (in jsonschema this seems to correspond to making the test suite take
+         *3x* longer).
+        """
+
+        registry = Registry({"urn:foo": "bar"})
+        assert registry.combine(registry) is registry
+
     def test_combine_with_uncrawled_resources(self):
         one = Resource.opaque(contents={})
         two = ID_AND_CHILDREN.create_resource({"foo": "bar"})