add some randomness to lrn interval

anki/sched.py

@@ -294,7 +294,11 @@ limit %d""" % self.reportLimit, lim=self.dayCutoff)
                 card.grade += 1
             else:
                 card.grade = 0
-            card.due = time.time() + self._delayForGrade(conf, card.grade)
+            delay = self._delayForGrade(conf, card.grade)
+            if card.due < time.time():
+                # not collapsed; add some randomness
+                delay *= random.uniform(1, 1.25)
+            card.due = time.time() + delay
             heappush(self.lrnQueue, (card.due, card.id))
         self._logLrn(card, ease, conf, leaving, type)
 

tests/test_sched.py

@@ -65,7 +65,8 @@ def test_learn():
     # fail it
     d.sched.answerCard(c, 1)
     # it should by due in 30 seconds
-    assert round(c.due - time.time()) == 30
+    t = round(c.due - time.time())
+    assert t >= 30 and t <= 40
     # and have 1 cycle, but still a zero grade
     assert c.grade == 0
     assert c.cycles == 1