Fix dest→source multithread chunk sizing and add single-vs-multithread parity coverage

src/interpcore/kernels.py

@@ -151,8 +151,9 @@ def interpolate_block(
         # Initialize output for destination points
         interpolated = np.zeros([neighbours_coords.shape[0], config.num_components])
     else:
-        # Initialize output for destination points (same size as chunked_coords)
-        interpolated = np.zeros([chunked_coords.shape[0], config.num_components])
+        # Initialize output only for the current destination chunk
+        block_size = max(chunk_idx.stop - chunk_idx.start, 0)
+        interpolated = np.zeros([block_size, config.num_components])
 
     unmapped = np.zeros([1, config.num_components])
 

tests/test_kernels.py

@@ -14,6 +14,7 @@
     INTERPOLATED_LOAD_TYPE,
     INTERPOLATION_KERNEL,
 )
+from interpcore.dest_tree import DestinationTree
 from interpcore.errors import InterpolationError
 
 
@@ -294,3 +295,95 @@ def test_interpolate_block_coincident_node(self):
         assert interpolated[2, 0] == 0.0
         # Nothing should be unmapped
         assert np.all(unmapped == 0)
+
+    def test_destination_to_source_multithread_handles_empty_blocks(self, monkeypatch):
+        """Dest-to-source multithread should handle blocks larger than points without shape errors"""
+        dest_coords = np.array([[0.0, 0.0, 0.0], [10.0, 0.0, 0.0]])
+        src_coords = np.array([[0.0, 0.0, 0.0], [10.0, 0.0, 0.0]])
+        dest_ids = np.array([1, 2])
+        src_values = np.array([[100.0], [200.0]])
+
+        config = InterpolationConfig(
+            kernel=INTERPOLATION_KERNEL.CLOSEST,  # Dest-to-source mode
+            max_distance=1.0,
+            coincidence_tolerance=1e-9,
+            method=QUERY_TYPE.K,
+            param=1,
+            multithread=True,
+            interpolated_load=INTERPOLATED_LOAD_TYPE.HEAT_FLUX,
+        )
+
+        # Force more worker blocks than destination points (creates empty trailing blocks)
+        monkeypatch.setattr("interpcore.dest_tree.os.cpu_count", lambda: 4)
+
+        tree = DestinationTree(
+            dest_coordinates=dest_coords,
+            src_coordinates=src_coords,
+            dest_ids=dest_ids,
+            config=config,
+        )
+        interpolated, unmapped = tree.interpolate(src_values)
+
+        np.testing.assert_array_equal(interpolated, src_values)
+        np.testing.assert_array_equal(unmapped, np.zeros((1, 1)))
+
+    def test_destination_to_source_multithread_matches_single_core(self, monkeypatch):
+        """Dest-to-source interpolation should match between multithread and single-core runs"""
+        dest_coords = np.array(
+            [
+                [0.0, 0.0, 0.0],
+                [2.0, 0.0, 0.0],
+                [5.0, 0.0, 0.0],
+                [9.0, 0.0, 0.0],
+            ]
+        )
+        src_coords = np.array(
+            [
+                [0.1, 0.0, 0.0],
+                [1.9, 0.0, 0.0],
+                [5.2, 0.0, 0.0],
+                [8.8, 0.0, 0.0],
+            ]
+        )
+        dest_ids = np.array([101, 102, 103, 104])
+        src_values = np.array([[10.0], [20.0], [30.0], [40.0]])
+
+        single_core_config = InterpolationConfig(
+            kernel=INTERPOLATION_KERNEL.CLOSEST,
+            max_distance=1.0,
+            coincidence_tolerance=1e-9,
+            method=QUERY_TYPE.K,
+            param=1,
+            multithread=False,
+            interpolated_load=INTERPOLATED_LOAD_TYPE.HEAT_FLUX,
+        )
+        multithread_config = InterpolationConfig(
+            kernel=INTERPOLATION_KERNEL.CLOSEST,
+            max_distance=1.0,
+            coincidence_tolerance=1e-9,
+            method=QUERY_TYPE.K,
+            param=1,
+            multithread=True,
+            interpolated_load=INTERPOLATED_LOAD_TYPE.HEAT_FLUX,
+        )
+
+        monkeypatch.setattr("interpcore.dest_tree.os.cpu_count", lambda: 8)
+
+        single_core_tree = DestinationTree(
+            dest_coordinates=dest_coords,
+            src_coordinates=src_coords,
+            dest_ids=dest_ids,
+            config=single_core_config,
+        )
+        multithread_tree = DestinationTree(
+            dest_coordinates=dest_coords,
+            src_coordinates=src_coords,
+            dest_ids=dest_ids,
+            config=multithread_config,
+        )
+
+        interpolated_single, unmapped_single = single_core_tree.interpolate(src_values)
+        interpolated_multi, unmapped_multi = multithread_tree.interpolate(src_values)
+
+        np.testing.assert_array_equal(interpolated_multi, interpolated_single)
+        np.testing.assert_array_equal(unmapped_multi, unmapped_single)