PERF: categorical rank GH#15498

no need to rename categories where they are already ordered

Author: jeet63

pandas/core/algorithms.py

@@ -974,6 +974,9 @@ def _get_data_algo(values, func_map):
 
     f = None
 
+    if is_categorical_dtype(values):
+        values = values._values_for_rank()
+
     if is_float_dtype(values):
         f = func_map['float64']
         values = _ensure_float64(values)
@@ -990,10 +993,6 @@ def _get_data_algo(values, func_map):
         f = func_map['uint64']
         values = _ensure_uint64(values)
 
-    elif is_categorical_dtype(values):
-        f = func_map['float64']
-        values = values._values_for_rank()
-
     else:
         values = _ensure_object(values)
 

pandas/core/categorical.py

@@ -1420,8 +1420,8 @@ def _values_for_rank(self):
         if self.ordered:
             values = self.codes
             mask = values == -1
-            values = values.astype('float64')
             if mask.any():
+                values = values.astype('float64')
                 values[mask] = np.nan
         elif self.categories.is_numeric():
             values = np.array(self)