Issue #8685: Speed up set difference `a - b` when source set `a` is
much larger than operand `b`. Patch by Andrew Bennetts.
diff --git a/Objects/setobject.c b/Objects/setobject.c
index dfafefd..478b5cb 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1525,6 +1525,20 @@
"Remove all elements of another set from this set.");
static PyObject *
+set_copy_and_difference(PySetObject *so, PyObject *other)
+{
+ PyObject *result;
+
+ result = set_copy(so);
+ if (result == NULL)
+ return NULL;
+ if (set_difference_update_internal((PySetObject *) result, other) != -1)
+ return result;
+ Py_DECREF(result);
+ return NULL;
+}
+
+static PyObject *
set_difference(PySetObject *so, PyObject *other)
{
PyObject *result;
@@ -1532,13 +1546,13 @@
Py_ssize_t pos = 0;
if (!PyAnySet_Check(other) && !PyDict_CheckExact(other)) {
- result = set_copy(so);
- if (result == NULL)
- return NULL;
- if (set_difference_update_internal((PySetObject *)result, other) != -1)
- return result;
- Py_DECREF(result);
- return NULL;
+ return set_copy_and_difference(so, other);
+ }
+
+ /* If len(so) much more than len(other), it's more efficient to simply copy
+ * so and then iterate other looking for common elements. */
+ if ((PySet_GET_SIZE(so) >> 2) > PyObject_Size(other)) {
+ return set_copy_and_difference(so, other);
}
result = make_new_set_basetype(Py_TYPE(so), NULL);
@@ -1560,6 +1574,7 @@
return result;
}
+ /* Iterate over so, checking for common elements in other. */
while (set_next(so, &pos, &entry)) {
int rv = set_contains_entry((PySetObject *)other, entry);
if (rv == -1) {