Thresholded statistics (closes #68)

climix/etc/metadata.yml

@@ -37,6 +37,23 @@ index_functions:
       reducer:
         kind: reducer
 
+  thresholded_statistics:
+    description: |
+      Calculates a simple statistic of the data for which some condition is met.
+      First, the threshold is transformed to the same standard_name and units as
+      the input data.
+      Then the thresholding is performed as condition(data, threshold), ie
+      if condition is <, data < threshold.
+      Finally, the statistic is calculated for those data values that fulfill
+      the condition.
+    parameters:
+      threshold:
+        kind: quantity
+      condition:
+        kind: operator
+      reducer:
+        kind: reducer
+
 indices:
   cdd:
     reference: ETCCDI

climix/index_functions.py

@@ -59,3 +59,20 @@ class SpellLength:
             else:
                 res = self.reducer(candidates)
         return float(res)
+
+
+class ThresholdedStatistics:
+    def __init__(self, threshold, condition, reducer):
+        self.threshold = threshold
+        self.condition = condition
+        self.reducer = reducer
+
+    def prepare(self, input_cube):
+        change_units(self.threshold, input_cube.units, input_cube.standard_name)
+        self.standard_name = input_cube.standard_name
+        self.units = input_cube.units
+
+    def __call__(self, raw_data):
+        comb = self.condition(raw_data, self.threshold.data)
+        res = self.reducer(raw_data[comb])
+        return float(res)

setup.py

@@ -51,6 +51,7 @@ setuptools.setup(
         'climix.index_functions': [
             'count_occurrences=climix.index_functions:CountOccurences',
             'spell_length=climix.index_functions:SpellLength',
+            'thresholded_statistics=climix.index_functions:ThresholdedStatistics',
         ],
     },
     project_urls={