avoid while loop[ in BusinessDay.apply

pandas/tseries/offsets.py

@@ -532,28 +532,31 @@ def get_str(td):
     def apply(self, other):
         if isinstance(other, datetime):
             n = self.n
+            wday = other.weekday()
 
-            if n == 0 and other.weekday() > 4:
-                n = 1
-
-            result = other
-
-            # avoid slowness below
-            if abs(n) > 5:
-                k = n // 5
-                result = result + timedelta(7 * k)
-                if n < 0 and result.weekday() > 4:
-                    n += 1
-                n -= 5 * k
-                if n == 0 and result.weekday() > 4:
-                    n -= 1
+            # avoid slowness below by operating on weeks first
+            weeks = n // 5
+            if n <= 0 and wday > 4:
+                # roll forward
+                n += 1
 
-            while n != 0:
-                k = n // abs(n)
-                result = result + timedelta(k)
-                if result.weekday() < 5:
-                    n -= k
+            n -= 5 * weeks
+
+            # n is always >= 0 at this point
+            if n == 0 and wday > 4:
+                # roll back
+                days = 4 - wday
+            elif wday > 4:
+                # roll forward
+                days = (7 - wday) + (n - 1)
+            elif wday + n <= 4:
+                # shift by n days without leaving the current week
+                days = n
+            else:
+                # shift by n days plus 2 to get past the weekend
+                days = n + 2
 
+            result = other + timedelta(days=7 * weeks + days)
             if self.offset:
                 result = result + self.offset
             return result