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-rw-r--r--Lib/test/test_random.py8
-rw-r--r--Misc/NEWS3
-rw-r--r--Modules/_randommodule.c19
3 files changed, 28 insertions, 2 deletions
diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index df82990..caa4f28 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -57,6 +57,14 @@ class TestBasicOps(unittest.TestCase):
self.assertRaises(TypeError, self.gen.jumpahead) # needs an arg
self.assertRaises(TypeError, self.gen.jumpahead, 2, 3) # too many
+ def test_jumpahead_produces_valid_state(self):
+ # From http://bugs.python.org/issue14591.
+ self.gen.seed(199210368)
+ self.gen.jumpahead(13550674232554645900)
+ for i in range(500):
+ val = self.gen.random()
+ self.assertLess(val, 1.0)
+
def test_sample(self):
# For the entire allowable range of 0 <= k <= N, validate that
# the sample is of the correct length and contains only unique items
diff --git a/Misc/NEWS b/Misc/NEWS
index 9839903..94da277 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -75,6 +75,9 @@ Core and Builtins
Library
-------
+- Issue #14591: Fix bug in Random.jumpahead that could produce an invalid
+ Mersenne Twister state on 64-bit machines.
+
- Issue #5346: Preserve permissions of mbox, MMDF and Babyl mailbox
files on flush().
diff --git a/Modules/_randommodule.c b/Modules/_randommodule.c
index 08c2277..8bb9e37 100644
--- a/Modules/_randommodule.c
+++ b/Modules/_randommodule.c
@@ -400,7 +400,7 @@ random_jumpahead(RandomObject *self, PyObject *n)
long i, j;
PyObject *iobj;
PyObject *remobj;
- unsigned long *mt, tmp;
+ unsigned long *mt, tmp, nonzero;
if (!PyInt_Check(n) && !PyLong_Check(n)) {
PyErr_Format(PyExc_TypeError, "jumpahead requires an "
@@ -427,8 +427,23 @@ random_jumpahead(RandomObject *self, PyObject *n)
mt[j] = tmp;
}
- for (i = 0; i < N; i++)
+ nonzero = 0;
+ for (i = 1; i < N; i++) {
mt[i] += i+1;
+ mt[i] &= 0xffffffffUL; /* for WORDSIZE > 32 machines */
+ nonzero |= mt[i];
+ }
+
+ /* Ensure the state is nonzero: in the unlikely event that mt[1] through
+ mt[N-1] are all zero, set the MSB of mt[0] (see issue #14591). In the
+ normal case, we fall back to the pre-issue 14591 behaviour for mt[0]. */
+ if (nonzero) {
+ mt[0] += 1;
+ mt[0] &= 0xffffffffUL; /* for WORDSIZE > 32 machines */
+ }
+ else {
+ mt[0] = 0x80000000UL;
+ }
self->index = N;
Py_INCREF(Py_None);