diff --git a/src/tools/int_seq.py b/src/tools/int_seq.py
index 8db57da..cd2f52b 100644
--- a/src/tools/int_seq.py
+++ b/src/tools/int_seq.py
@@ -1,12 +1,20 @@
-import math
-
-
-def factorial(n: int) -> int:
+def factorial(n: int, start: int = 1) -> int:
     """
     n! = 1 * 2 * 3 * 4 * ... * n
     1, 1, 2, 6, 24, 120, 720, ...
+
+    If you're looking for efficiency with start == 1, just use math.factorial(n)
+    which takes just 25% of the compute time on average, but this is the fastest
+    pure python implementation I could come up with and it allows for partial
+    multiplications, like 5 * 6 * 7 * 8 * .... * 17
     """
-    return math.factorial(n)
+    if start == n:
+        return n
+    if n - start == 1:
+        return n * start
+
+    middle = start + (n - start) // 2
+    return factorial(middle, start) * factorial(n, middle + 1)
 
 
 def fibonacci(n: int) -> int:
diff --git a/src/tools/stopwatch.py b/src/tools/stopwatch.py
index c3d517c..833d1f6 100644
--- a/src/tools/stopwatch.py
+++ b/src/tools/stopwatch.py
@@ -20,7 +20,9 @@ class StopWatch:
         self.stopped = perf_counter_ns()
         self.total_elapsed += self.elapsed()
 
-    reset = start
+    def reset(self):
+        self.total_elapsed = 0
+        self.start()
 
     def elapsed(self) -> int:
         if self.stopped is None: