Minor cleanup.

bigjoyints/lil_divmod.py

@@ -18,17 +18,31 @@ def lil_divmod(A, B):
 
 def find_greatest(low, high, f):
     '''
-    Return the highest number n: low <= n <= high
+    Return the highest number n: low <= n <= high for which:
-    for which f(n) and not f(n+1) is True.
+
-    The greatest n which makes f(n) True.
+        for all i in low..n f(i) is True
+        -and-
+        for all i in (n+1)..high f(i) is False.
+
+    If the function f::int->bool doesn't behave like above
+    then I don't know what this function will do.
+
+    There must be some number in the range or this function will
+    fail with an assertion error.  (If you turn off assertions
+    it will succeed and return low in such a case.)
     '''
     assert low <= high
     if f(high):
         return high
     pivot = (low + high) >> 1
+    # If there isn't a pivot between low and high that means there's only
+    # two numbers it could be: low or high, and we already know it isn't
+    # high from the test above so it must be low.
+    if low == pivot:
+        assert f(low) and not f(low + 1)
+        return low
+    assert low < pivot < high
     return (
-        low
-        if low == pivot else
         find_greatest(pivot, high - 1, f)
         if f(pivot) else
         find_greatest(low, pivot - 1, f)