Use an efficient representation for merged components of operations

cirq-core/cirq/transformers/connected_component.py

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+# Copyright 2025 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Defines a connected component of operations, to be used in merge transformers."""
+
+from __future__ import annotations
+
+from typing import Callable, cast, Sequence, TYPE_CHECKING
+
+from cirq import ops, protocols
+
+if TYPE_CHECKING:
+    import cirq
+
+class Component:
+    """Internal representation for a connected component of operations.
+
+    It uses the disjoint-set data structure to implement merge efficiently.
+    Additional merge conditions can be added by deriving from the Component
+    class and overriding the merge function (see ComponentWithOps and
+    ComponentWithCircuitOp) below.
+    """
+
+    # Properties for the disjoint set data structure
+    parent: Component|None = None
+    rank: int = 0
+
+    # True if the component can be merged
+    is_mergeable: bool
+
+    # Circuit moment containing the component
+    moment: int
+    # Union of all op qubits in the component
+    qubits: frozenset[cirq.Qid]
+    # Union of all measurement keys in the component
+    mkeys: frozenset[cirq.MeasurementKey]
+    # Union of all control keys in the component
+    ckeys: frozenset[cirq.MeasurementKey]
+    # Initial operation in the component
+    op: cirq.Operation
+
+    def __init__(self, op: cirq.Operation, moment: int, is_mergeable = True):
+        """Initializes a singleton component."""
+        self.is_mergeable = is_mergeable
+        self.moment = moment
+        self.qubits = frozenset(op.qubits)
+        self.mkeys = protocols.measurement_key_objs(op)
+        self.ckeys = protocols.control_keys(op)
+        self.op = op
+
+    def find(self) -> Component:
+        """Finds the component representative."""
+
+        root = self
+        while root.parent != None:
+            root = root.parent
+        x = self
+        while x != root:
+            parent = x.parent
+            x.parent = root
+            x = cast(Component, parent)
+        return root
+
+    def merge(self, c: Component, merge_left = True) -> Component|None:
+        """Attempts to merge two components.
+
+        We assume the following is true whenever merge is called:
+            - if merge_left = True then c.qubits are a subset of self.qubits
+            - if merge_left = False then self.qubits are a subset of c.qubits
+
+        If merge_left is True, c is merged into this component, and the representative
+        will keep this moment and qubits. If merge_left is False, this component is
+        merged into c, and the representative will keep c's moment and qubits.
+
+        Args:
+            c: other component to merge
+            merge_left: True to keep self's data for the merged component, False to
+                keep c's data for the merged component.
+
+        Returns:
+            None, if the components can't be merged.
+            Otherwise the new component representative.
+        """
+        x = self.find()
+        y = c.find()
+
+        if not x.is_mergeable or not y.is_mergeable:
+            return None
+
+        if x == y:
+            return x
+
+        if x.rank < y.rank:
+            if merge_left:
+                # As y will be the new representative, copy moment and qubits from x
+                y.moment = x.moment
+                y.qubits = x.qubits
+            x, y = y, x
+        elif not merge_left:
+            # As x will be the new representative, copy moment and qubits from y
+            x.moment = y.moment
+            x.qubits = y.qubits
+
+        y.parent = x
+        if x.rank == y.rank:
+            x.rank += 1
+
+        x.mkeys = x.mkeys.union(y.mkeys)
+        x.ckeys = x.ckeys.union(y.ckeys)
+        return x
+
+
+class ComponentWithOps(Component):
+    """Component that keeps track of operations.
+
+    Encapsulates a method can_merge that is used to decide if two components
+    can be merged.
+    """
+
+    # List of all operations in the component
+    ops: list[cirq.Operation]
+
+    # Method to decide if two components can be merged based on their operations
+    can_merge: Callable[[Sequence[cirq.Operation], Sequence[cirq.Operation]], bool]
+
+    def __init__(self, op: cirq.Operation, moment: int,
+                 can_merge: Callable[[Sequence[cirq.Operation], Sequence[cirq.Operation]], bool],
+                 is_mergeable = True):
+        super().__init__(op, moment, is_mergeable)
+        self.ops = [op]
+        self.can_merge = can_merge
+
+    def merge(self, c: Component, merge_left = True) -> Component|None:
+        """Attempts to merge two components.
+
+        Returns:
+            None if can_merge is False, otherwise the new representative.
+                The representative will have ops = a.ops + b.ops.
+        """
+        x = cast(ComponentWithOps, self.find())
+        y = cast(ComponentWithOps, c.find())
+
+        if x == y:
+            return x
+
+        if not x.is_mergeable or not y.is_mergeable or not x.can_merge(x.ops, y.ops):
+            return None
+
+        root = cast(ComponentWithOps, super(ComponentWithOps, x).merge(y, merge_left))
+        if not root:
+            return None
+        root.ops = x.ops + y.ops
+        # Clear the ops list in the non-representative set to avoid memory consumption
+        if x != root:
+            x.ops = []
+        else:
+            y.ops = []
+        return root
+
+
+class ComponentWithCircuitOp(Component):
+    """Component that keeps track of operations as a CircuitOperation.
+
+    Encapsulates a method merge_func that is used to merge two components.
+    """
+
+    # CircuitOperation containing all the operations in the component,
+    # or a single Operation if the component is a singleton
+    circuit_op: cirq.Operation
+
+    merge_func: Callable[[ops.Operation, ops.Operation], ops.Operation | None]
+
+    def __init__(self, op: cirq.Operation, moment: int,
+                 merge_func: Callable[[ops.Operation, ops.Operation], ops.Operation | None],
+                 is_mergeable = True):
+        super().__init__(op, moment, is_mergeable)
+        self.circuit_op = op
+        self.merge_func = merge_func
+
+    def merge(self, c: Component, merge_left = True) -> Component|None:
+        """Attempts to merge two components.
+
+        Returns:
+            None if merge_func returns None, otherwise the new representative.
+        """
+        x = cast(ComponentWithCircuitOp, self.find())
+        y = cast(ComponentWithCircuitOp, c.find())
+
+        if x == y:
+            return x
+
+        if not x.is_mergeable or not y.is_mergeable:
+            return None
+
+        new_op = x.merge_func(x.circuit_op, y.circuit_op)
+        if not new_op:
+            return None
+
+        root = cast(ComponentWithCircuitOp, super(ComponentWithCircuitOp, x).merge(y, merge_left))
+        if not root:
+            return None
+
+        root.circuit_op = new_op
+        # The merge_func can be arbitrary, so we need to recompute the component properties
+        root.qubits = frozenset(new_op.qubits)
+        root.mkeys = protocols.measurement_key_objs(new_op)
+        root.ckeys = protocols.control_keys(new_op)
+
+        # Clear the circuit op in the non-representative set to avoid memory consumption
+        if x != root:
+            del x.circuit_op
+        else:
+            del y.circuit_op
+        return root
+
+
+class ComponentFactory:
+    """Factory for components."""
+
+    is_mergeable: Callable[[cirq.Operation], bool]
+
+    def __init__(self,
+                 is_mergeable: Callable[[cirq.Operation], bool]):
+        self.is_mergeable = is_mergeable
+
+    def new_component(self, op: cirq.Operation, moment: int, is_mergeable = True) -> Component:
+        return Component(op, moment, self.is_mergeable(op) and is_mergeable)
+
+
+class ComponentWithOpsFactory(ComponentFactory):
+    """Factory for components with operations."""
+
+    can_merge: Callable[[Sequence[cirq.Operation], Sequence[cirq.Operation]], bool]
+
+    def __init__(self,
+                 is_mergeable: Callable[[cirq.Operation], bool],
+                 can_merge: Callable[[Sequence[cirq.Operation], Sequence[cirq.Operation]], bool]):
+        super().__init__(is_mergeable)
+        self.can_merge = can_merge
+
+    def new_component(self, op: cirq.Operation, moment: int, is_mergeable = True) -> Component:
+        return ComponentWithOps(op, moment, self.can_merge, self.is_mergeable(op) and is_mergeable)
+
+
+class ComponentWithCircuitOpFactory(ComponentFactory):
+    """Factory for components with operations as CircuitOperation."""
+
+    merge_func: Callable[[ops.Operation, ops.Operation], ops.Operation | None]
+
+    def __init__(self,
+                 is_mergeable: Callable[[cirq.Operation], bool],
+                 merge_func: Callable[[ops.Operation, ops.Operation], ops.Operation | None]):
+        super().__init__(is_mergeable)
+        self.merge_func = merge_func
+
+    def new_component(self, op: cirq.Operation, moment: int, is_mergeable=True) -> Component:
+        return ComponentWithCircuitOp(op, moment, self.merge_func, self.is_mergeable(op) and is_mergeable)
+
+
+
+