Adding in more accurate fillvals in xarray_to_cdf (#255)

* Adding in more accurate fillvals

* Fixing bug with previous changes

cdflib/xarray/xarray_to_cdf.py

@@ -52,6 +52,26 @@
     "CDF_UCHAR": 52,
 }
 
+DATATYPE_FILLVALS = {
+    1: np.int8(-128),
+    2: np.int16(-32768),
+    4: np.int32(-2147483648),
+    8: np.int64(-9223372036854775808),
+    11: np.uint8(255),
+    12: np.uint16(65535),
+    14: np.uint32(4294967295),
+    21: np.float32(-1e30),
+    22: np.float64(-1e30),
+    31: np.float64(-1e30),
+    32: np.complex128(complex(-1e30, -1e30)),
+    33: np.datetime64(-9223372036854775808, "ns"),
+    41: np.int8(-128),
+    44: np.float32(-1e30),
+    45: np.float64(-1e30),
+    51: np.str_(" "),
+    52: np.str_(" "),
+}
+
 
 class ISTPError(Exception):
     """
@@ -171,18 +191,12 @@ def _dtype_to_cdf_type(var: xr.Dataset, terminate_on_warning: bool = False) -> T
     return STRINGS_TO_DATATYPES[cdf_data_type], element_size
 
 
-def _dtype_to_fillval(dtype: np.dtype, terminate_on_warning: bool = False) -> Union[float, int, str, None]:
-    if dtype == np.int8 or dtype == np.int16 or dtype == np.int32 or dtype == np.int64:
-        return -9223372036854775808  # Default FILLVAL of 'CDF_INT8'
-    elif dtype == np.float64 or dtype == np.float32 or dtype == np.float16:
-        return -1e30  # Default FILLVAL of 'CDF_DOUBLE'
-    elif dtype == np.uint8 or dtype == np.uint16 or dtype == np.uint32 or dtype == np.uint64:
-        return 4294967294  # Default FILLVAL of 'CDF_UNIT4'
-    elif dtype.type == np.str_:
-        return " "  # Default FILLVAL of 'CDF_CHAR'
+def _dtype_to_fillval(var: xr.Dataset, terminate_on_warning: bool = False) -> Union[np.number, np.str_, np.datetime64, object]:
+    datatype, _ = _dtype_to_cdf_type(var, terminate_on_warning=terminate_on_warning)
+    if datatype in DATATYPE_FILLVALS:
+        return DATATYPE_FILLVALS[datatype]
     else:
-        _warn_or_except(f"Data type of {dtype} not supported", terminate_on_warning)
-        return None
+        return np.str_(" ")
 
 
 def _verify_depend_dimensions(
@@ -616,7 +630,7 @@ def _variable_attribute_checker(dataset: xr.Dataset, epoch_list: List[str], term
             if "FILLVAL" not in d[var].attrs:
                 if var_type.lower() == "data":
                     _warn_or_except(f"ISTP Compliance Warning: FILLVAL required for variable {var}", terminate_on_warning)
-                    fillval = _dtype_to_fillval(d[var].dtype)
+                    fillval = _dtype_to_fillval(d[var])
                     d[var].attrs["FILLVAL"] = fillval
                     _warn_or_except(
                         f"ISTP Compliance Action: Automatically set FILLVAL to {fillval} for variable {var}", terminate_on_warning
@@ -625,7 +639,7 @@ def _variable_attribute_checker(dataset: xr.Dataset, epoch_list: List[str], term
                     if len(dataset[var].dims) > 0:
                         if d[var].dims[0] in epoch_list:
                             _warn_or_except(f"ISTP Compliance Warning: FILLVAL required for variable {var}", terminate_on_warning)
-                            fillval = _dtype_to_fillval(d[var].dtype)
+                            fillval = _dtype_to_fillval(d[var])
                             d[var].attrs["FILLVAL"] = fillval
                             _warn_or_except(
                                 f"ISTP Compliance Action: Automatically set FILLVAL to {fillval} for variable {var}",