Add several helper methods returning SWHIDs

This adds several helper methods returning SWHIDs to model objects,
namely:

- DirectoryEntry.swhid()
- SnapshotBranch.swhid()
- Release.target_swhid() and Release.parent_swhids()

swh/model/model.py

@@ -910,6 +910,15 @@ class SnapshotBranch(BaseModel):
     def from_dict(cls, d):
         return cls(target=d["target"], target_type=TargetType(d["target_type"]))
 
+    def swhid(self) -> Optional[CoreSWHID]:
+        """Returns a SWHID for the current branch or None if the branch has no
+        target or is an alias."""
+        if self.target is None or self.target_type == TargetType.ALIAS:
+            return None
+        return CoreSWHID(
+            object_id=self.target, object_type=SwhidObjectType[self.target_type.name]
+        )
+
 
 @attr.s(frozen=True, slots=True, field_transformer=optimize_all_validators)
 class Snapshot(HashableObject, BaseModel):
@@ -1005,6 +1014,14 @@ class Release(HashableObjectWithManifest, BaseModel):
         """Returns a SWHID representing this object."""
         return CoreSWHID(object_type=SwhidObjectType.RELEASE, object_id=self.id)
 
+    def target_swhid(self) -> Optional[CoreSWHID]:
+        """Returns the SWHID for the target of this release or None if unset."""
+        if self.target is None:
+            return None
+        return CoreSWHID(
+            object_id=self.target, object_type=SwhidObjectType[self.target_type.name]
+        )
+
     def anonymize(self) -> "Release":
         """Returns an anonymized version of the Release object.
 
@@ -1133,6 +1150,19 @@ class Revision(HashableObjectWithManifest, BaseModel):
         """Returns a SWHID representing this object."""
         return CoreSWHID(object_type=SwhidObjectType.REVISION, object_id=self.id)
 
+    def directory_swhid(self) -> CoreSWHID:
+        """Returns the SWHID for the directory referenced by the revision."""
+        return CoreSWHID(
+            object_type=SwhidObjectType.DIRECTORY, object_id=self.directory
+        )
+
+    def parent_swhids(self) -> List[CoreSWHID]:
+        """Returns a list of SWHID for the parent revisions."""
+        return [
+            CoreSWHID(object_type=SwhidObjectType.REVISION, object_id=parent)
+            for parent in self.parents
+        ]
+
     def anonymize(self) -> "Revision":
         """Returns an anonymized version of the Revision object.
 
@@ -1159,6 +1189,12 @@ class DirectoryEntry(BaseModel):
     perms = attr.ib(type=int, validator=generic_type_validator, converter=int, repr=oct)
     """Usually one of the values of `swh.model.from_disk.DentryPerms`."""
 
+    DIR_ENTRY_TYPE_TO_SWHID_OBJECT_TYPE = {
+        "file": SwhidObjectType.CONTENT,
+        "dir": SwhidObjectType.DIRECTORY,
+        "rev": SwhidObjectType.REVISION,
+    }
+
     @name.validator
     def check_name(self, attribute, value):
         if value.__class__ is not bytes:
@@ -1166,6 +1202,13 @@ class DirectoryEntry(BaseModel):
         if b"/" in value:
             raise ValueError(f"{value!r} is not a valid directory entry name.")
 
+    def swhid(self) -> CoreSWHID:
+        """Returns a SWHID for this directory entry"""
+        return CoreSWHID(
+            object_type=DirectoryEntry.DIR_ENTRY_TYPE_TO_SWHID_OBJECT_TYPE[self.type],
+            object_id=self.target,
+        )
+
 
 @attr.s(frozen=True, slots=True, field_transformer=optimize_all_validators)
 class Directory(HashableObjectWithManifest, BaseModel):

swh/model/tests/test_model.py

@@ -1092,6 +1092,42 @@ def test_directory_from_possibly_duplicated_entries__preserve_manifest():
     assert dir_.raw_manifest == b"blah"
 
 
+@pytest.fixture
+def directory_with_every_possible_type():
+    return Directory.from_dict(
+        {
+            "entries": [
+                {
+                    "type": "file",
+                    "perms": 33188,
+                    "name": b"README",
+                    "target": hash_to_bytes("37ec8ea2110c0b7a32fbb0e872f6e7debbf95e21"),
+                },
+                {
+                    "type": "dir",
+                    "perms": 16384,
+                    "name": b"src",
+                    "target": hash_to_bytes("61e6e867f5d7ba3b40540869bc050b0c4fed9e95"),
+                },
+                {
+                    "type": "rev",
+                    "perms": 57344,
+                    "name": b"submodule",
+                    "target": hash_to_bytes("3d531e169db92a16a9a8974f0ae6edf52e52659e"),
+                },
+            ],
+        }
+    )
+
+
+def test_directory_entry_swhids(directory_with_every_possible_type):
+    assert [entry.swhid() for entry in directory_with_every_possible_type.entries] == [
+        CoreSWHID.from_string("swh:1:cnt:37ec8ea2110c0b7a32fbb0e872f6e7debbf95e21"),
+        CoreSWHID.from_string("swh:1:dir:61e6e867f5d7ba3b40540869bc050b0c4fed9e95"),
+        CoreSWHID.from_string("swh:1:rev:3d531e169db92a16a9a8974f0ae6edf52e52659e"),
+    ]
+
+
 # Release
 
 
@@ -1129,6 +1165,13 @@ def test_release_raw_manifest(release):
     release2.check()
 
 
+def test_release_target_swhid():
+    release = Release.from_dict(release_example)
+    assert release.target_swhid() == CoreSWHID.from_string(
+        "swh:1:rev:741b2252a5e14d6c60a913c77a6099abe73a854a"
+    )
+
+
 # Revision
 
 
@@ -1379,6 +1422,55 @@ def test_revision_none_author_or_committer():
         Revision.from_dict(rev_dict)
 
 
+def test_revision_directory_swhid():
+    revision = Revision.from_dict(revision_example)
+    assert revision.directory_swhid() == CoreSWHID.from_string(
+        "swh:1:dir:85a74718d377195e1efd0843ba4f3260bad4fe07"
+    )
+
+
+def test_revision_parent_swhids():
+    revision_d = revision_example.copy()
+    revision_d["parents"].append(
+        hash_to_bytes("b2a7e1260492e344fab3cbf91bc13c91e05426fd")
+    )
+    revision = Revision.from_dict(revision_d)
+    assert revision.parent_swhids() == [
+        CoreSWHID.from_string("swh:1:rev:01e2d0627a9a6edb24c37db45db5ecb31e9de808"),
+        CoreSWHID.from_string("swh:1:rev:b2a7e1260492e344fab3cbf91bc13c91e05426fd"),
+    ]
+
+
+@pytest.fixture
+def snapshot_with_all_types():
+    return Snapshot.from_dict(snapshot_example)
+
+
+def test_snapshot_branch_swhids(snapshot_with_all_types):
+    assert {
+        name: branch and branch.swhid()
+        for (name, branch) in snapshot_with_all_types.branches.items()
+    } == {
+        b"directory": CoreSWHID.from_string(
+            "swh:1:dir:1bd0e65f7d2ff14ae994de17a1e7fe65111dcad8"
+        ),
+        b"content": CoreSWHID.from_string(
+            "swh:1:cnt:fe95a46679d128ff167b7c55df5d02356c5a1ae1"
+        ),
+        b"alias": None,
+        b"revision": CoreSWHID.from_string(
+            "swh:1:rev:aafb16d69fd30ff58afdd69036a26047f3aebdc6"
+        ),
+        b"release": CoreSWHID.from_string(
+            "swh:1:rel:7045404f3d1c54e6473c71bbb716529fbad4be24"
+        ),
+        b"snapshot": CoreSWHID.from_string(
+            "swh:1:snp:1a8893e6a86f444e8be8e7bda6cb34fb1735a00e"
+        ),
+        b"dangling": None,
+    }
+
+
 @given(strategies.objects(split_content=True))
 def test_object_type(objtype_and_obj):
     obj_type, obj = objtype_and_obj