Issue #236 fix indexing in mode method regridding

docs/changelog.rst

@@ -6,6 +6,18 @@ All notable changes to this project will be documented in this file.
 The format is based on `Keep a Changelog`_, and this project adheres to
 `Semantic Versioning`_.
 
+[Unreleased]
+------------
+
+Fixed
+~~~~~
+
+- Fixed indexing bug in the ``"mode"`` method in
+  :class:`xugrid.CentroidLocatorRegridder`, :class:`xugrid.OverlapRegridder`,
+  :class:`xugrid.RelativeOverlapRegridder`, which gave the method the tendency
+  to repeat the first value in the source grid across the target grid.
+
+
 [0.9.0] 2024-02-15
 ------------------
 

tests/test_regrid/test_reduce.py

@@ -55,6 +55,21 @@ def test_mode(args):
     # The weights shouldn't be mutated!
     assert np.allclose(weights, 0.5)
 
+    values = np.array([-1, 1, 1, 3, 4])
+    indices = np.array([1, 2, 3])
+    weights = np.array([1.0, 1.0, 1.0])
+    args = (values, indices, weights)
+    actual = reduce.mode(*args)
+    assert np.allclose(actual, 1.0)
+
+    values = np.array([99, 1, 2, 3, 4, 5, 6, 7, 8])
+    indices = np.array([4, 5, 6])
+    weights = np.array([0.5, 0.5, 0.5])
+    args = (values, indices, weights)
+    actual = reduce.mode(*args)
+    assert np.allclose(actual, 4)
+    assert np.allclose(weights, 0.5)
+
 
 def test_median(args):
     actual = reduce.median(*args)

tests/test_regrid/test_regridder.py

@@ -103,6 +103,16 @@ def test_overlap_regridder_structured(
     assert broadcasted.dims == ("layer", "y", "x")
     assert (broadcasted.isel(layer=0) == expected_results_overlap).any()
 
+    # Test if "mode" method doesn't repeat first values again
+    # https://github.com/Deltares/xugrid/issues/236
+    grid_data_adapted = grid_data_a.copy()
+    grid_data_adapted[0, 0] = 99
+    regridder = OverlapRegridder(
+        source=grid_data_adapted, target=grid_data_a, method="mode"
+    )
+    result = regridder.regrid(grid_data_adapted)
+    assert not np.all(result == 99.0)
+
 
 def test_overlap_regridder(disk, quads_1):
     square = quads_1

xugrid/regrid/reduce.py

@@ -122,8 +122,7 @@ def mode(values, indices, weights):
         return np.nan
     else:  # Find value with highest frequency
         w_max = 0
-        for i in range(accum.size):
-            w_accum = accum[i]
+        for w_accum, i in zip(accum, indices):
             if w_accum > w_max:
                 w_max = w_accum
                 v = values[i]