Issue 9816: Random.jumpahead(n) didn't work well for small values of n.

1 files changed, 13 insertions, 0 deletions

diff --git a/Lib/random.py b/Lib/random.py
index f919718a6d..01e1420df6 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -46,6 +46,7 @@ from math import log as _log, exp as _exp, pi as _pi, e as _e, ceil as _ceil
 from math import sqrt as _sqrt, acos as _acos, cos as _cos, sin as _sin
 from os import urandom as _urandom
 from binascii import hexlify as _hexlify
+import hashlib as _hashlib
 
 __all__ = ["Random","seed","random","uniform","randint","choice","sample",
            "randrange","shuffle","normalvariate","lognormvariate",
@@ -141,6 +142,18 @@ class Random(_random.Random):
                              "Random.setstate() of version %s" %
                              (version, self.VERSION))
 
+    def jumpahead(self, n):
+        """Change the internal state to one that is likely far away
+        from the current state.  This method will not be in Py3.x,
+        so it is better to simply reseed.
+        """
+        # The super.jumpahead() method uses shuffling to change state,
+        # so it needs a large and "interesting" n to work with.  Here,
+        # we use hashing to create a large n for the shuffle.
+        s = repr(n) + repr(self.getstate())
+        n = int(_hashlib.new('sha512', s).hexdigest(), 16)
+        super(Random, self).jumpahead(n)
+
 ## ---- Methods below this point do not need to be overridden when
 ## ---- subclassing for the purpose of using a different core generator.