Index function percentile (closes #80)

climix/etc/metadata.yml

@@ -123,6 +123,13 @@ index_functions:
       reducer:
         kind: reducer
 
+  percentile:
+    description: |
+      Calculates percentiles, default method is "linear" option of numpy and dask.
+    parameters:
+      percentiles:
+        kind: quantity
+
   temperature_sum:
     description: |
       Calculates the temperature sum above/below a threshold. First, the threshold

climix/index_functions/__init__.py

@@ -8,6 +8,7 @@ from .index_functions import (  # noqa: F401
     ExtremeTemperatureRange,
     FirstOccurrence,
     LastOccurrence,
+    Percentile,
     SpellLength,
     Statistics,
     ThresholdedStatistics,

climix/index_functions/index_functions.py

@@ -286,6 +286,39 @@ class ThresholdedStatistics(ThresholdMixin, ReducerMixin, IndexFunction):
         return res.astype('float32')
 
 
+class Percentile(IndexFunction):
+    def __init__(self, percentiles, interpolation='linear'):
+        super().__init__(units=Unit('days'))
+        points = percentiles.points
+        assert np.all(points > 0)
+        assert np.all(points < 100)
+        self.percentiles = percentiles
+        self.interpolation = interpolation
+        self.units = '%'
+
+    def prepare(self, input_cubes):
+        super().prepare(input_cubes)
+        ref_cube = next(iter(input_cubes.values()))
+        self.standard_name = ref_cube.standard_name
+        self.units = ref_cube.units
+
+    def call_func(self, data, axis, **kwargs):
+        axis = normalize_axis(axis, data.ndim)
+        res = np.percentile(data, q=self.percentiles, axis=axis,
+                            interpolation=self.interpolation)
+        return res.astype('float32')
+
+    def lazy_func(self, data, axis, **kwargs):
+        axis = normalize_axis(axis, data.ndim)
+
+        def percentile(arr):
+            return np.percentile(arr,
+                                 q=self.percentiles.points,
+                                 interpolation=self.interpolation)
+        res = da.apply_along_axis(percentile, axis=axis, arr=data).squeeze()
+        return res.astype('float32')
+
+
 class TemperatureSum(ThresholdMixin, IndexFunction):
     def __init__(self, threshold, condition):
         super().__init__(threshold, condition, units=Unit('days'))

setup.py

@@ -58,6 +58,7 @@ setuptools.setup(
             'last_occurrence=climix.index_functions:LastOccurrence',
             'spell_length=climix.index_functions:SpellLength',
             'statistics=climix.index_functions:Statistics',
+            'percentile=climix.index_functions:Percentile',
             'thresholded_statistics=climix.index_functions:ThresholdedStatistics',
             'temperature_sum=climix.index_functions:TemperatureSum',
             'diurnal_temperature_range=climix.index_functions:DiurnalTemperatureRange',