feat(Horizon): add lazy migration

Author: emptybutton

src/tgdb/entities/horizon/horizon.py

@@ -22,6 +22,7 @@
     Transaction,
     start_transaction,
 )
+from tgdb.entities.relation.tuple import Tuple, VersionedTuple
 from tgdb.entities.time.logic_time import LogicTime
 from tgdb.entities.tools.assert_ import assert_
 from tgdb.entities.tools.map import first_map_value
@@ -99,8 +100,8 @@ def view_tuple(
         self,
         time: LogicTime,
         xid: XID,
-        effect: ViewedTuple,
-    ) -> None:
+        tuple: VersionedTuple,
+    ) -> Tuple:
         """
         :raises tgdb.entities.horizon.horizon.NotMonotonicTimeError:
         :raises tgdb.entities.horizon.horizon.NoTransactionError:
@@ -112,7 +113,20 @@ def view_tuple(
         transaction = self._transaction(
             xid, SerializableTransactionState.active
         )
-        transaction.include(effect)
+
+        if isinstance(transaction, NonSerializableReadTransaction):
+            transaction.include(ViewedTuple(tuple.latest_version().id))
+            return tuple.latest_version()
+
+        latest_version = tuple.latest_version()
+        old_versions = tuple.old_versions()
+
+        if old_versions:
+            transaction.include(MutatedTuple(latest_version))
+        else:
+            transaction.include(ViewedTuple(tuple.latest_version().id))
+
+        return latest_version
 
     def rollback_transaction(self, time: LogicTime, xid: XID) -> None:
         """

src/tgdb/entities/relation/tuple.py

@@ -1,8 +1,9 @@
-from collections.abc import Iterator
+from collections.abc import Iterable, Iterator, Mapping, Sequence
 from dataclasses import dataclass
 
 from tgdb.entities.numeration.number import Number
 from tgdb.entities.relation.scalar import Scalar
+from tgdb.entities.tools.assert_ import assert_
 
 
 @dataclass(frozen=True)
@@ -17,7 +18,39 @@ class Tuple:
     scalars: tuple[Scalar, ...]
 
     def __iter__(self) -> Iterator[Scalar]:
-        return iter(self.scalars)
+        yield self.id.scalar
+        yield from iter(self.scalars)
+
+    def __len__(self) -> int:
+        return len(self.scalars) + 1
+
+
+class HeterogeneousVersionedTupleError(Exception): ...
+
+
+@dataclass(frozen=True)
+class VersionedTuple:
+    map: Mapping[Number, Tuple]
+
+    def latest_version(self) -> Tuple:
+        return self.map[max(self.map)]
+
+    def old_versions(self) -> Sequence[Tuple]:
+        latest_version_number = max(self.map)
+
+        return tuple(
+            self.map[latest_version_number]
+            for number in self.map
+            if number != latest_version_number
+        )
+
+    def __post_init__(self) -> None:
+        """
+        :raises tgdb.entities.relation.tuple.HeterogeneousVersionedTupleError:
+        """
+
+        id_set = frozenset(version.id for version in self.map.values())
+        assert_(len(id_set) == 1, else_=HeterogeneousVersionedTupleError)
 
 
 def tuple_(*scalars: Scalar, relation_id: Number = Number(0)) -> "Tuple":  # noqa: B008