From 231ee4dc9da8d85aceb9ef18aa7197dd0023534e Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <python@rcn.com>
Date: Sat, 2 Feb 2013 11:24:43 -0800
Subject: Issue 16398:  Add assertions to show why memcmp is safe.

---
 Modules/_collectionsmodule.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index e0c6f0c..17b1ad6 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -425,8 +425,9 @@ _deque_rotate(dequeobject *deque, Py_ssize_t n)
         else if (n < -halflen)
             n += len;
     }
-
     assert(deque->len > 1);
+    assert((n < len / 2) || (n > len / -2));
+
     deque->state++;
     for (i=0 ; i<n ; ) {
         if (deque->leftindex == 0) {
@@ -446,6 +447,8 @@ _deque_rotate(dequeobject *deque, Py_ssize_t n)
         if (m > deque->leftindex)
             m = deque->leftindex;
         assert (m > 0);
+        assert (deque->leftblock != deque->rightblock ||
+                deque->leftindex < deque->rightindex - m + 1);
         memcpy(&deque->leftblock->data[deque->leftindex - m],
                &deque->rightblock->data[deque->rightindex - m + 1],
                m * sizeof(PyObject *));
-- 
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