feat: implement DifferenceFilter to compute the difference between two variables

src/anemoi/transform/filters/difference.py

@@ -0,0 +1,74 @@
+# (C) Copyright 2024 Anemoi contributors.
+#
+# This software is licensed under the terms of the Apache Licence Version 2.0
+# which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# In applying this licence, ECMWF does not waive the privileges and immunities
+# granted to it by virtue of its status as an intergovernmental organisation
+# nor does it submit to any jurisdiction.
+
+
+from typing import Literal
+
+import earthkit.data as ekd
+
+from anemoi.transform.filters import filter_registry
+from anemoi.transform.filters.matching import MatchingFieldsFilter
+from anemoi.transform.filters.matching import matching
+
+
+@filter_registry.register("difference")
+class DifferenceFilter(MatchingFieldsFilter):
+    """A filter to compute a simple difference between two variables.
+
+    This filter converts two fields X and Y to a field representing the difference between the two (X - Y).
+
+    """
+
+    @matching(select="param", forward=("x", "y"), backward=("diff",))
+    def __init__(
+        self,
+        *,
+        x: str,
+        y: str,
+        diff: str,
+        return_inputs: Literal["all", "none"] | list[str] = "all",
+    ) -> None:
+        """Initialize the Diff filter.
+
+        Parameters
+        ----------
+        x : str
+            Name of the first variable.
+        y : str
+            Name of the second variable.
+        diff : str
+            Name of the difference variable.
+        """
+
+        self.x = x
+        self.y = y
+        self.diff = diff
+        self.return_inputs = return_inputs
+
+    def forward_transform(self, x: ekd.Field, y: ekd.Field) -> ekd.Field:
+        """Compute the difference between two fields (X - Y).
+
+        Parameters
+        ----------
+        x : ekd.Field
+            The first variable.
+        y : ekd.Field
+            The second variable.
+
+        Returns
+        -------
+        ekd.Field
+            The difference field.
+        """
+
+        diff = x.to_numpy() - y.to_numpy()
+        yield self.new_field_from_numpy(diff, template=x, param=self.diff)
+
+    def backward(self, diff: ekd.FieldList) -> ekd.FieldList:
+        raise NotImplementedError("Difference filter is not reversible")