fix: guard against ZeroDivisionError in Comparitor._calc_stats

changelog.d/compare-annotations-empty.rst

@@ -0,0 +1,2 @@
+**Allow empty annotations in compare_annotations**
+  ``compare_annotations`` (``Comparitor.compare``) no longer fails when the reference or test annotations are empty, for example when a detector finds no beats. The sensitivity or positive predictivity that cannot be computed is NaN.

src/wfdb/processing/evaluate.py

@@ -99,7 +99,7 @@ def _calc_stats(self):
         ]
         # Test annotation indices that were unmatched to a reference annotation
         self.unmatched_test_inds = np.setdiff1d(
-            np.array(range(self.n_test)),
+            np.arange(self.n_test),
             self.matched_test_inds,
             assume_unique=True,
         )
@@ -118,8 +118,13 @@ def _calc_stats(self):
         self.fn = self.n_ref - self.tp
         # No tn attribute
 
-        self.sensitivity = float(self.tp) / float(self.tp + self.fn)
-        self.positive_predictivity = float(self.tp) / self.n_test
+        denom_sens = self.tp + self.fn
+        self.sensitivity = (
+            float(self.tp) / denom_sens if denom_sens > 0 else float("nan")
+        )
+        self.positive_predictivity = (
+            float(self.tp) / self.n_test if self.n_test > 0 else float("nan")
+        )
 
     def compare(self):
         """Main comparison function.

tests/test_processing.py

@@ -1,3 +1,4 @@
+import math
 from unittest import mock
 
 import numpy as np
@@ -334,6 +335,28 @@ def test_random_annotations(self):
             assert np.all(np.abs(ref[matches != -1] - test[matched]) < window_width)
             assert 0 <= comparitor.fp <= len(test)
 
+    def test_compare_annotations_empty_ref(self):
+        """When reference annotations are empty, sensitivity should be NaN."""
+        comparitor = processing.compare_annotations(
+            np.array([]), np.array([10, 20, 30]), 5
+        )
+        assert math.isnan(comparitor.sensitivity)
+        assert comparitor.positive_predictivity == 0.0
+
+    def test_compare_annotations_empty_test(self):
+        """When test annotations are empty, PPV should be NaN."""
+        comparitor = processing.compare_annotations(
+            np.array([10, 20, 30]), np.array([]), 5
+        )
+        assert comparitor.sensitivity == 0.0
+        assert math.isnan(comparitor.positive_predictivity)
+
+    def test_compare_annotations_both_empty(self):
+        """When both annotation arrays are empty, both metrics should be NaN."""
+        comparitor = processing.compare_annotations(np.array([]), np.array([]), 5)
+        assert math.isnan(comparitor.sensitivity)
+        assert math.isnan(comparitor.positive_predictivity)
+
 
 class TestQrs:
     """Testing QRS detectors"""