bpo-26163: Frozenset hash improvement (#5194)

diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-01-14-20-32-47.bpo-26163.xv9Iuv.rst b/Misc/NEWS.d/next/Core and Builtins/2018-01-14-20-32-47.bpo-26163.xv9Iuv.rst
new file mode 100644 (file)
index 0000000..163b9e0
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-01-14-20-32-47.bpo-26163.xv9Iuv.rst
@@ -0,0 +1,2 @@
+Improved frozenset() hash to create more distinct hash values when faced
+with datasets containing many similar values.

diff --git a/Objects/setobject.c b/Objects/setobject.c
index c920fb9853406b2bdaff01e87e9d2fc450bb9801..4bc1020d56f77a741a19cd9191994890161ac6b0 100644 (file)
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -795,6 +795,7 @@ frozenset_hash(PyObject *self)
     hash ^= ((Py_uhash_t)PySet_GET_SIZE(self) + 1) * 1927868237UL;
 
     /* Disperse patterns arising in nested frozensets */
+    hash ^= (hash >> 11) ^ (~hash >> 25);
     hash = hash * 69069U + 907133923UL;
 
     /* -1 is reserved as an error code */