Speed up decode_cf_datetime

cchwala · cchwala · commit d7d7c01f3e2f · 2017-05-18T22:41:57.000+02:00
Instead of casting the input numeric dates to float, they are
casted to nanoseconds as integer which makes `pd.to_timedelta()`
work much faster (x100 speedup on my machine)
diff --git a/xarray/conventions.py b/xarray/conventions.py
@@ -126,11 +126,14 @@ def decode_cf_datetime(num_dates, units, calendar=None):
     operations, which makes it much faster than netCDF4.num2date. In such a
     case, the returned array will be of type np.datetime64.
 
+    Note that time unit in `units` must not be smaller than microseconds and
+    not larger than days.
+
     See also
     --------
     netCDF4.num2date
     """
-    num_dates = np.asarray(num_dates, dtype=float)
+    num_dates = np.asarray(num_dates)
     flat_num_dates = num_dates.ravel()
     if calendar is None:
         calendar = 'standard'
@@ -155,10 +158,24 @@ def decode_cf_datetime(num_dates, units, calendar=None):
         pd.to_timedelta(flat_num_dates.min(), delta) + ref_date
         pd.to_timedelta(flat_num_dates.max(), delta) + ref_date
 
-        dates = (pd.to_timedelta(flat_num_dates, delta) + ref_date).values
+        # Cast input dates to integers of nanoseconds because `pd.to_datetime`
+        # works much faster when dealing with integers
+        ns_per_time_delta = {'us': 1e3,
+                             'ms': 1e6,
+                             's': 1e9,
+                             'm': 1e9 * 60,
+                             'h': 1e9 * 60 * 60,
+                             'D': 1e9 * 60 * 60 * 24}
+        flat_num_dates_ns_int = (flat_num_dates *
+                                 ns_per_time_delta[delta]).astype(np.int64)
+
+        dates = (pd.to_timedelta(flat_num_dates_ns_int, 'ns') +
+                 ref_date).values
 
     except (OutOfBoundsDatetime, OverflowError):
-        dates = _decode_datetime_with_netcdf4(flat_num_dates, units, calendar)
+        dates = _decode_datetime_with_netcdf4(flat_num_dates.astype(np.float),
+                                              units,
+                                              calendar)
 
     return dates.reshape(num_dates.shape)
 
