RDPS precip dif can't be negative (#4001)

api/app/tests/weather_models/test_precip_rdps_model.py

@@ -32,6 +32,28 @@ def test_difference_identity():
     assert np.allclose(res, np.zeros(precip_raster.shape))
 
 
+def test_negative_precip_diff_raises_value_error():
+    """
+    Verify ValueError raised if raster subtraction contains a negative value.
+    """
+    later_precip = TemporalPrecip(datetime.fromisoformat("2024-06-10T18:42:49"), np.zeros(1))
+    earlier_precip = TemporalPrecip(datetime.fromisoformat("2024-06-09T18:42:49"), np.ones(1))
+    with pytest.raises(ValueError):
+        compute_precip_difference(later_precip, earlier_precip)
+
+
+def test_trivial_negative_precip_diff_returns_zero():
+    """
+    Verify that a negative precip dif between -0.01 and 0 returns 0.
+    """
+    early_array = np.empty(1)
+    early_array[0] = 0.005
+    later_precip = TemporalPrecip(datetime.fromisoformat("2024-06-10T18:42:49"), np.zeros(1))
+    earlier_precip = TemporalPrecip(datetime.fromisoformat("2024-06-09T18:42:49"), early_array)
+    result = compute_precip_difference(later_precip, earlier_precip)
+    assert result[0] == 0
+
+
 @pytest.mark.parametrize(
     "later_datetime,earlier_datetime",
     [

api/app/weather_models/precip_rdps_model.py

@@ -141,7 +141,18 @@ def compute_precip_difference(later_precip: TemporalPrecip, earlier_precip: Temp
 
 
 def _diff(value_a: float, value_b: float):
-    return value_a - value_b
+    """
+    Subtract value_a from value_b.
+    :param value_a: The first value
+    :param value_b: The second value
+    :raises ValueError: If difference is less than -0.01 (ie. negative precip not allowed)
+    :return: Return value_a minus value_b if the value is >= 0. If the difference is slightly negative (-0.01 <= value < 0), due to floating
+    point math for example, return 0. If the value is truly negative, raise an error.
+    """
+    result = value_a - value_b
+    if result < -0.01:
+        raise ValueError("Precip difference cannot be negative")
+    return result if result > 0 else 0
 
 
 vectorized_diff = vectorize(_diff)