implement the IMPORT-REF mechanism

If I understood the spec correctly, this simply extends the pool of
objects that are locally referenceable via ODXLINK (i.e., without
having to specify the DOCREF attribute), and can thus be seen as a
kind of "poor man's inheritance" mechanism.

Signed-off-by: Andreas Lauser <andreas.lauser@mbition.io>
Signed-off-by: Gerrit Ecke <gerrit.ecke@mbition.io>

odxtools/diaglayer.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: MIT
 import re
 import warnings
+from copy import copy
 from dataclasses import dataclass
 from functools import cached_property
 from itertools import chain
@@ -76,6 +77,46 @@ def _build_odxlinks(self) -> Dict[OdxLinkId, Any]:
     def _resolve_odxlinks(self, odxlinks: OdxLinkDatabase) -> None:
         """Recursively resolve all references."""
 
+        # deal with the import references: these basically extend the
+        # pool of objects that are referenceable without having to
+        # explicitly specify the DOCREF attribute in the
+        # reference. This mechanism can thus be seen as a kind of
+        # "poor man's inheritance".
+        if self.import_refs:
+            imported_links: Dict[OdxLinkId, Any] = {}
+            for import_ref in self.import_refs:
+                imported_dl = odxlinks.resolve(import_ref, DiagLayer)
+
+                odxassert(
+                    imported_dl.variant_type == DiagLayerType.ECU_SHARED_DATA,
+                    f"Tried to import references from diagnostic layer "
+                    f"'{imported_dl.short_name}' of type {imported_dl.variant_type.value}. "
+                    f"Only ECU-SHARED-DATA layers may be referenced using the "
+                    f"IMPORT-REF mechanism")
+
+                # TODO: ensure that the imported diagnostic layer has
+                # not been referenced in any PARENT-REF of the current
+                # layer or any of its parents.
+
+                # TODO: detect and complain about cyclic IMPORT-REFs
+
+                # TODO (?): detect conflicts with locally-defined
+                # objects
+
+                imported_dl_links = imported_dl._build_odxlinks()
+                for link_id, obj in imported_dl_links.items():
+                    # the imported objects shall behave as if they
+                    # where defined by the importing layer. IOW, they
+                    # must be visible in the same document fragments.
+                    link_id = OdxLinkId(link_id.local_id, self.odx_id.doc_fragments)
+                    imported_links[link_id] = obj
+
+            odxlinks = copy(odxlinks)
+            odxlinks.update(imported_links)
+
+            self.diag_layer_raw._resolve_odxlinks(odxlinks)
+            return
+
         self.diag_layer_raw._resolve_odxlinks(odxlinks)
 
     def _finalize_init(self, odxlinks: OdxLinkDatabase) -> None: