Closes #14591: Random.jumpahead could produce an invalid MT state on 64-bit machines.
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 @@
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 @@
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);