Add count_level_crossings index function (closes #151)

climix/etc/metadata.yml

@@ -6,6 +6,19 @@ drop:
   - history
 
 index_functions:
+  count_level_crossings:
+    description: |
+      This index function takes two inputs, low_data and high_data, together
+      with one parameter, the threshold.
+      It calculates the number of times low_data is below threshold while
+      high_data is above threshold.
+      First, the threshold is transformed to the same standard_name and units as
+      the input data, then the thresholding is performed, and finally, the number
+      of occurrences is counted.
+    parameters:
+      threshold:
+        kind: quantity
+
   count_occurrences:
     description: |
       Calculates the number of times some condition is met.

climix/index_functions/__init__.py

 # -*- coding: utf-8 -*-
 
 from .index_functions import (  # noqa: F401
+    CountLevelCrossings,
     CountOccurrences,
     FirstOccurrence,
     LastOccurrence,

climix/index_functions/index_functions.py

@@ -9,6 +9,34 @@ import numpy as np
 from .support import (normalize_axis,
                       IndexFunction,
                       ThresholdMixin, ReducerMixin)
+from ..util import change_units
+
+
+class CountLevelCrossings(IndexFunction):
+    def __init__(self, threshold):
+        super().__init__(units=Unit('days'))
+        self.threshold = threshold
+        self.extra_coords.append(threshold.copy())
+
+    def prepare(self, input_cubes):
+        props = {(cube.dtype, cube.units, cube.standard_name)
+                 for cube in input_cubes}
+        assert len(props) == 1
+        dtype, units, standard_name = props.pop()
+        threshold = self.threshold
+        threshold.points = threshold.points.astype(dtype)
+        if threshold.has_bounds():
+            threshold.bounds = threshold.bounds.astype(dtype)
+        change_units(threshold, units, standard_name)
+        super().prepare(input_cubes)
+
+    def call_func(self, data, axis, **kwargs):
+        cond = da.logical_and(data['low_data'] < self.threshold.points,
+                              self.threshold.points < data['high_data'])
+        res = np.count_nonzero(cond, axis=axis)
+        return res.astype('float32')
+
+    lazy_func = call_func
 
 
 class CountOccurrences(ThresholdMixin, IndexFunction):

setup.py

@@ -51,6 +51,7 @@ setuptools.setup(
             'climix-editor=climix.editor:main [editor]',
         ],
         'climix.index_functions': [
+            'count_level_crossings=climix.index_functions:CountLevelCrossings',
             'count_occurrences=climix.index_functions:CountOccurrences',
             'first_occurrence=climix.index_functions:FirstOccurrence',
             'last_occurrence=climix.index_functions:LastOccurrence',