bpo-27575: port set intersection logic into dictview intersection (GH-7696)

Objects/dictobject.c

@@ -4169,24 +4169,97 @@ dictviews_sub(PyObject* self, PyObject *other)
     return result;
 }
 
-PyObject*
+static int
+dictitems_contains(_PyDictViewObject *dv, PyObject *obj);
+
+PyObject *
 _PyDictView_Intersect(PyObject* self, PyObject *other)
 {
-    PyObject *result = PySet_New(self);
+    PyObject *result;
+    PyObject *it;
+    PyObject *key;
+    Py_ssize_t len_self;
+    int rv;
+    int (*dict_contains)(_PyDictViewObject *, PyObject *);
     PyObject *tmp;
-    _Py_IDENTIFIER(intersection_update);
 
+    /* Python interpreter swaps parameters when dict view
+       is on right side of & */
+    if (!PyDictViewSet_Check(self)) {
+        PyObject *tmp = other;
+        other = self;
+        self = tmp;
+    }
+
+    len_self = dictview_len((_PyDictViewObject *)self);
+
+    /* if other is a set and self is smaller than other,
+       reuse set intersection logic */
+    if (Py_TYPE(other) == &PySet_Type && len_self <= PyObject_Size(other)) {
+        _Py_IDENTIFIER(intersection);
+        return _PyObject_CallMethodIdObjArgs(other, &PyId_intersection, self, NULL);
+    }
+
+    /* if other is another dict view, and it is bigger than self,
+       swap them */
+    if (PyDictViewSet_Check(other)) {
+        Py_ssize_t len_other = dictview_len((_PyDictViewObject *)other);
+        if (len_other > len_self) {
+            PyObject *tmp = other;
+            other = self;
+            self = tmp;
+        }
+    }
+
+    /* at this point, two things should be true
+       1. self is a dictview
+       2. if other is a dictview then it is smaller than self */
+    result = PySet_New(NULL);
     if (result == NULL)
         return NULL;
 
+    it = PyObject_GetIter(other);
+
+    _Py_IDENTIFIER(intersection_update);
     tmp = _PyObject_CallMethodIdOneArg(result, &PyId_intersection_update, other);
     if (tmp == NULL) {
         Py_DECREF(result);
         return NULL;
     }
-
     Py_DECREF(tmp);
+
+    if (PyDictKeys_Check(self)) {
+        dict_contains = dictkeys_contains;
+    }
+    /* else PyDictItems_Check(self) */
+    else {
+        dict_contains = dictitems_contains;
+    }
+
+    while ((key = PyIter_Next(it)) != NULL) {
+        rv = dict_contains((_PyDictViewObject *)self, key);
+        if (rv < 0) {
+            goto error;
+        }
+        if (rv) {
+            if (PySet_Add(result, key)) {
+                goto error;
+            }
+        }
+        Py_DECREF(key);
+    }
+    Py_DECREF(it);
+    if (PyErr_Occurred()) {
+        Py_DECREF(result);
+        return NULL;
+    }
     return result;
+
+error:
+    Py_DECREF(it);
+    Py_DECREF(result);
+    Py_DECREF(key);
+    return NULL;
 }
 
 static PyObject*