Improve initial qubit mapping algorithm (#144)

* Improve initial qubit mapping algorithm.

* Include qubits from 1-qubit gates in the initial mapping.

* Add better test cases.

* Refactor the initial mapping logic into a utils file.

* Address comments; integrate with SwapUpdater.

* Fix type annotation and comment.

Author: wingers

recirq/quantum_chess/circuit_transformer.py

@@ -17,6 +17,8 @@
 import cirq
 
 import recirq.quantum_chess.controlled_iswap as controlled_iswap
+import recirq.quantum_chess.initial_mapping_utils as imu
+import recirq.quantum_chess.swap_updater as su
 
 ADJACENCY = [(0, 1), (0, -1), (1, 0), (-1, 0)]
 
@@ -305,6 +307,49 @@ def transform(self, circuit: cirq.Circuit) -> cirq.Circuit:
         return circuit.transform_qubits(lambda q: self.mapping[q])
 
 
+class DynamicLookAheadHeuristicCircuitTransformer(CircuitTransformer):
+    """Optimizer that transforms a circuit to satify a device's constraints.
+
+    This implements the initial mapping algorithm and the SWAP-based update
+    algorithm proposed by the paper "A Dynamic Look-Ahead Heuristic for the
+    Qubit Mapping Problem of NISQ Computer":
+    https://ieeexplore.ieee.org/abstract/document/8976109.
+
+    The initial mapping algorithm first maps the center of the logical qubits
+    graph to the center of the physical qubits graph. It then traverses the
+    logical qubits in a breadth-first traversal order starting from the center
+    of the logical qubits graph. For each logical qubit, it finds the physical
+    qubit that minimizes the nearest neighbor distance for the leftmost gates.
+
+    The SWAP-based update algorithm uses a heuristic cost function of a SWAP
+    operation called maximum consecutive positive effect (MCPE) to greedily
+    look ahead in each moment for SWAP operations that will reduce the nearest
+    neighbor distance for the largest number of gates in the current look-ahead
+    window.
+
+    Reference:
+    P. Zhu, Z. Guan and X. Cheng, "A Dynamic Look-Ahead Heuristic for the
+    Qubit Mapping Problem of NISQ Computers," in IEEE Transactions on Computer-
+    Aided Design of Integrated Circuits and Systems, vol. 39, no. 12, pp. 4721-
+    4735, Dec. 2020, doi: 10.1109/TCAD.2020.2970594.
+    """
+    def __init__(self, device: cirq.Device):
+        super().__init__()
+        self.device = device
+
+    def transform(self, circuit: cirq.Circuit) -> cirq.Circuit:
+        """Returns a transformed circuit.
+
+        The transformed circuit satisfies all physical adjacency constraints
+        of the device.
+
+        Args:
+          circuit: The circuit to transform.
+        """
+        initial_mapping = imu.calculate_initial_mapping(self.device, circuit)
+        updater = su.SwapUpdater(circuit, self.device.qubit_set(), initial_mapping)
+        return cirq.Circuit(updater.add_swaps())
+
 class SycamoreDecomposer(cirq.PointOptimizer):
     """Optimizer that decomposes all three qubit operations into
     sqrt-ISWAPs.

recirq/quantum_chess/circuit_transformer_test.py

@@ -30,62 +30,82 @@
 d1 = cirq.NamedQubit('d1')
 
 
+@pytest.mark.parametrize('transformer',
+                         [ct.ConnectivityHeuristicCircuitTransformer,
+                          ct.DynamicLookAheadHeuristicCircuitTransformer])
 @pytest.mark.parametrize('device',
-                         (cirq.google.Sycamore23, cirq.google.Sycamore))
-def test_single_qubit_ops(device):
-    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_single_qubit_ops(transformer, device):
     c = cirq.Circuit(cirq.X(a1), cirq.X(a2), cirq.X(a3))
-    transformer.qubit_mapping(c)
-    c = transformer.transform(c)
-    device.validate_circuit(c)
+    t = transformer(device)
+    device.validate_circuit(t.transform(c))
 
 
+@pytest.mark.parametrize('transformer',
+                         [ct.ConnectivityHeuristicCircuitTransformer,
+                          ct.DynamicLookAheadHeuristicCircuitTransformer])
 @pytest.mark.parametrize('device',
-                         (cirq.google.Sycamore23, cirq.google.Sycamore))
-def test_single_qubit_with_two_qubits(device):
-    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_single_qubit_and_two_qubits_ops(transformer, device):
     c = cirq.Circuit(cirq.X(a1), cirq.X(a2), cirq.X(a3),
                      cirq.ISWAP(a3, a4) ** 0.5)
-    transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.transform(c))
+    t = transformer(device)
+    device.validate_circuit(t.transform(c))
 
 
+@pytest.mark.parametrize('transformer',
+                         [ct.ConnectivityHeuristicCircuitTransformer,
+                          ct.DynamicLookAheadHeuristicCircuitTransformer])
 @pytest.mark.parametrize('device',
-                         (cirq.google.Sycamore23, cirq.google.Sycamore))
-def test_three_split_moves(device):
-    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_three_split_moves(transformer, device):
     c = cirq.Circuit(qm.split_move(a1, a2, b1), qm.split_move(a2, a3, b3),
                      qm.split_move(b1, c1, c2))
-    transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.transform(c))
+    t = transformer(device)
+    device.validate_circuit(t.transform(c))
 
 
+@pytest.mark.parametrize('transformer',
+                         [ct.ConnectivityHeuristicCircuitTransformer,
+                          ct.DynamicLookAheadHeuristicCircuitTransformer])
 @pytest.mark.parametrize('device',
-                         (cirq.google.Sycamore23, cirq.google.Sycamore))
-def test_disconnected(device):
-    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_disconnected(transformer, device):
     c = cirq.Circuit(qm.split_move(a1, a2, a3), qm.split_move(a3, a4, d1),
                      qm.split_move(b1, b2, b3), qm.split_move(c1, c2, c3))
-    transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.transform(c))
+    t = transformer(device)
+    device.validate_circuit(t.transform(c))
 
 
+@pytest.mark.parametrize('transformer',
+                         [ct.ConnectivityHeuristicCircuitTransformer,
+                          ct.DynamicLookAheadHeuristicCircuitTransformer])
 @pytest.mark.parametrize('device',
-                         (cirq.google.Sycamore23, cirq.google.Sycamore))
-def test_move_around_square(device):
-    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_move_around_square(transformer, device):
     c = cirq.Circuit(qm.normal_move(a1, a2), qm.normal_move(a2, b2),
                      qm.normal_move(b2, b1), qm.normal_move(b1, a1))
-    transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.transform(c))
+    t = transformer(device)
+    device.validate_circuit(t.transform(c))
 
 
+@pytest.mark.parametrize('transformer',
+                         [ct.ConnectivityHeuristicCircuitTransformer,
+                          ct.DynamicLookAheadHeuristicCircuitTransformer])
 @pytest.mark.parametrize('device',
-                         (cirq.google.Sycamore23, cirq.google.Sycamore))
-def test_split_then_merge(device):
-    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_split_then_merge(transformer, device):
     c = cirq.Circuit(qm.split_move(a1, a2, b1), qm.split_move(a2, a3, b3),
                      qm.split_move(b1, c1, c2), qm.normal_move(c1, d1),
                      qm.normal_move(a3, a4), qm.merge_move(a4, d1, a1))
-    transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.transform(c))
+    t = transformer(device)
+    device.validate_circuit(t.transform(c))
+
+
+@pytest.mark.parametrize('device',
+                         [cirq.google.Sycamore23, cirq.google.Sycamore])
+def test_split_then_merge_trapezoid(device):
+    c = cirq.Circuit(qm.split_move(a1, a2, b1), qm.normal_move(a2, a3),
+                     qm.merge_move(a3, b1, b3))
+    t = ct.DynamicLookAheadHeuristicCircuitTransformer(device)
+    device.validate_circuit(t.transform(c))