Add StandardPass for basic qubit relabelling (#247)

* add NaivePlacement Class and naive_placement_mapping_pass

* clang format

* PR Requested Changes

pytket/binders/passes.cpp

@@ -496,6 +496,13 @@ PYBIND11_MODULE(passes, m) {
       ":py:class:`Architecture` Nodes",
       py::arg("placer"));
 
+  m.def(
+      "NaivePlacementPass", &gen_naive_placement_pass,
+      ":param architecture: The Architecture used for relabelling."
+      "\n:return: a pass to relabel :py:class:`Circuit` Qubits to "
+      ":py:class:`Architecture` Nodes",
+      py::arg("arc"));
+
   m.def(
       "RenameQubitsPass", &gen_rename_qubits_pass, "Rename some or all qubits.",
       "\n\n:param qubit_map: map from old to new qubit names",

pytket/docs/changelog.rst

@@ -31,6 +31,8 @@ Minor new features:
 * New ``pytket.passes.auto_rebase_pass`` and ``pytket.passes.auto_squash_pass``
   which attempt to construct rebase and squash passess given a target gate set from known
   decompositions.
+* New ``pytket.passes.NaivePlacementPass`` which completes a basic relabelling of all Circuit Qubit
+  not labelled as some Architecture Node to any available Architecture Node
 
 0.19.2 (February 2022)
 ----------------------

pytket/tests/predicates_test.py

@@ -32,6 +32,7 @@
     RoutingPass,
     CXMappingPass,
     PlacementPass,
+    NaivePlacementPass,
     RenameQubitsPass,
     FullMappingPass,
     DefaultMappingPass,
@@ -202,14 +203,29 @@ def test_routing_and_placement_pass() -> None:
     pl = Placement(arc)
     routing = RoutingPass(arc)
     placement = PlacementPass(pl)
+    nplacement = NaivePlacementPass(arc)
     cu = CompilationUnit(circ.copy())
     assert placement.apply(cu)
     assert routing.apply(cu)
+    assert nplacement.apply(cu)
     expected_map = {q[0]: n1, q[1]: n0, q[2]: n2, q[3]: n5, q[4]: n3}
     assert cu.initial_map == expected_map
 
+    cu1 = CompilationUnit(circ.copy())
+    assert nplacement.apply(cu1)
+    arcnodes = arc.nodes
+    expected_nmap = {
+        q[0]: arcnodes[0],
+        q[1]: arcnodes[1],
+        q[2]: arcnodes[2],
+        q[3]: arcnodes[3],
+        q[4]: arcnodes[4],
+    }
+    assert cu1.initial_map == expected_nmap
     # check composition works ok
-    seq_pass = SequencePass([SynthesiseTket(), placement, routing, SynthesiseUMD()])
+    seq_pass = SequencePass(
+        [SynthesiseTket(), placement, routing, nplacement, SynthesiseUMD()]
+    )
     cu2 = CompilationUnit(circ.copy())
     assert seq_pass.apply(cu2)
     assert cu2.initial_map == expected_map
@@ -223,7 +239,7 @@ def test_routing_and_placement_pass() -> None:
 
 def test_default_mapping_pass() -> None:
     circ = Circuit()
-    q = circ.add_q_register("q", 5)
+    q = circ.add_q_register("q", 6)
     circ.CX(0, 1)
     circ.H(0)
     circ.Z(1)
@@ -233,14 +249,17 @@ def test_default_mapping_pass() -> None:
     circ.X(2)
     circ.CX(1, 4)
     circ.CX(0, 4)
+    circ.H(5)
     n0 = Node("b", 0)
     n1 = Node("b", 1)
     n2 = Node("b", 2)
     n3 = Node("a", 0)
     n4 = Node("f", 0)
-    arc = Architecture([[n0, n1], [n1, n2], [n2, n3], [n3, n4]])
+    n5 = Node("g", 7)
+    arc = Architecture([[n0, n1], [n1, n2], [n2, n3], [n3, n4], [n4, n5]])
     pl = GraphPlacement(arc)
 
+    nplacement = NaivePlacementPass(arc)
     routing = RoutingPass(arc)
     placement = PlacementPass(pl)
     default = DefaultMappingPass(arc)
@@ -249,6 +268,7 @@ def test_default_mapping_pass() -> None:
 
     assert placement.apply(cu_rp)
     assert routing.apply(cu_rp)
+    assert nplacement.apply(cu_rp)
     assert default.apply(cu_def)
     assert cu_rp.circuit == cu_def.circuit
 
@@ -650,6 +670,10 @@ def sq(a: float, b: float, c: float) -> Circuit:
     assert p_pass.to_dict()["StandardPass"]["name"] == "PlacementPass"
     assert p_pass.to_dict()["StandardPass"]["placement"]["type"] == "GraphPlacement"
     assert p_pass.to_dict()["StandardPass"]["placement"]["config"]["depth_limit"] == 5
+    # NaivePlacementPass
+    np_pass = NaivePlacementPass(arc)
+    assert np_pass.to_dict()["StandardPass"]["name"] == "NaivePlacementPass"
+    assert check_arc_dict(arc, np_pass.to_dict()["StandardPass"]["architecture"])
     # RenameQubitsPass
     qm = {Qubit("a", 0): Qubit("b", 1), Qubit("a", 1): Qubit("b", 0)}
     rn_pass = RenameQubitsPass(qm)
@@ -662,29 +686,35 @@ def sq(a: float, b: float, c: float) -> Circuit:
     assert fm_pass.to_dict()["pass_class"] == "SequencePass"
     p_pass = fm_pass.get_sequence()[0]
     r_pass = fm_pass.get_sequence()[1]
-    assert p_pass.to_dict()["StandardPass"]["name"] == "PlacementPass"
+    np_pass = fm_pass.get_sequence()[2]
+    assert np_pass.to_dict()["StandardPass"]["name"] == "NaivePlacementPass"
     assert r_pass.to_dict()["StandardPass"]["name"] == "RoutingPass"
+    assert p_pass.to_dict()["StandardPass"]["name"] == "PlacementPass"
     assert check_arc_dict(arc, r_pass.to_dict()["StandardPass"]["architecture"])
     assert p_pass.to_dict()["StandardPass"]["placement"]["type"] == "GraphPlacement"
     # DefaultMappingPass
     dm_pass = DefaultMappingPass(arc)
     assert dm_pass.to_dict()["pass_class"] == "SequencePass"
     p_pass = dm_pass.get_sequence()[0].get_sequence()[0]
     r_pass = dm_pass.get_sequence()[0].get_sequence()[1]
+    np_pass = dm_pass.get_sequence()[0].get_sequence()[2]
     d_pass = dm_pass.get_sequence()[1]
     assert d_pass.to_dict()["StandardPass"]["name"] == "DelayMeasures"
     assert p_pass.to_dict()["StandardPass"]["name"] == "PlacementPass"
+    assert np_pass.to_dict()["StandardPass"]["name"] == "NaivePlacementPass"
     assert r_pass.to_dict()["StandardPass"]["name"] == "RoutingPass"
     assert check_arc_dict(arc, r_pass.to_dict()["StandardPass"]["architecture"])
     assert p_pass.to_dict()["StandardPass"]["placement"]["type"] == "GraphPlacement"
     # DefaultMappingPass with delay_measures=False
     dm_pass = DefaultMappingPass(arc, False)
     assert dm_pass.to_dict()["pass_class"] == "SequencePass"
-    assert len(dm_pass.get_sequence()) == 2
+    assert len(dm_pass.get_sequence()) == 3
     p_pass = dm_pass.get_sequence()[0]
     r_pass = dm_pass.get_sequence()[1]
+    np_pass = dm_pass.get_sequence()[2]
     assert p_pass.to_dict()["StandardPass"]["name"] == "PlacementPass"
     assert r_pass.to_dict()["StandardPass"]["name"] == "RoutingPass"
+    assert np_pass.to_dict()["StandardPass"]["name"] == "NaivePlacementPass"
     assert check_arc_dict(arc, r_pass.to_dict()["StandardPass"]["architecture"])
     assert p_pass.to_dict()["StandardPass"]["placement"]["type"] == "GraphPlacement"
     # AASRouting

schemas/compiler_pass_v1.json

@@ -403,6 +403,20 @@
             ]
           }
         },
+        {
+          "if": {
+            "properties": {
+              "name": {
+                "const": "NaivePlacementPass"
+              }
+            }
+          },
+          "then": {
+            "required": [
+              "architecture"
+            ]
+          }
+        },
         {
           "if": {
             "properties": {

tket/src/Placement/Placement.cpp

@@ -154,6 +154,45 @@ std::vector<qubit_mapping_t> Placement::get_all_placement_maps(
   return {get_placement_map(circ_)};
 }
 
+qubit_mapping_t NaivePlacement::get_placement_map(const Circuit &circ_) const {
+  return get_all_placement_maps(circ_).at(0);
+}
+
+std::vector<qubit_mapping_t> NaivePlacement::get_all_placement_maps(
+    const Circuit &circ_) const {
+  qubit_mapping_t placement;
+  qubit_vector_t to_place;
+  std::vector<Node> placed;
+
+  // Find which/if any qubits need placing
+  for (const Qubit &q : circ_.all_qubits()) {
+    Node n(q);
+    if (!this->arc_.node_exists(n)) {
+      to_place.push_back(n);
+    } else {
+      placed.push_back(n);
+      // if already placed, make sure qubit retains placement
+      placement.insert({n, n});
+    }
+  }
+  // avoid doing std::set_difference unless qubits need to be placed
+  unsigned n_placed = to_place.size();
+  if (n_placed > 0) {
+    std::vector<Node> difference,
+        architecture_nodes = this->arc_.get_all_nodes_vec();
+    std::set_difference(
+        architecture_nodes.begin(), architecture_nodes.end(), placed.begin(),
+        placed.end(), std::inserter(difference, difference.begin()));
+    // should always be enough remaining qubits to assign unplaced qubits to
+    TKET_ASSERT(difference.size() >= n_placed);
+    for (unsigned i = 0; i < n_placed; i++) {
+      // naively assign each qubit to some free node
+      placement.insert({to_place[i], difference[i]});
+    }
+  }
+  return {placement};
+}
+
 qubit_mapping_t LinePlacement::get_placement_map(const Circuit &circ_) const {
   return get_all_placement_maps(circ_).at(0);
 }

tket/src/Placement/include/Placement/Placement.hpp

@@ -269,6 +269,42 @@ class Placement {
   Architecture arc_;
 };
 
+/**
+ * NaivePlacement class provides methods for relabelling any
+ * Qubit objects in some Circuit to Node objects in some Architecture
+ * given the constraint that only Qubit that are not already labelled
+ * as some Node can be relabelled, and only to Architecture Node
+ * that are not already in the Circuit.
+ */
+class NaivePlacement : public Placement {
+ public:
+  /**
+   * @param _arc Architecture object later relabellings are produced for
+   */
+  explicit NaivePlacement(const Architecture& _arc) { arc_ = _arc; }
+  /**
+   * Given some circuit, returns a map between Qubit which defines some
+   * relabelling of some Circuit qubits to Architecture qubits
+   *
+   * @param circ_ Circuit map relabelling is defined for
+   *
+   * @return Map defining relabelling for circuit Qubit objects
+   */
+  qubit_mapping_t get_placement_map(const Circuit& circ_) const override;
+
+  /**
+   * Given some circuit, returns a single map for relabelling
+   * in a vector.
+   *
+   * @param circ_ Circuit map relabelling is defined for
+   *
+   * @return Vector of a single Map defining relabelling for Circuit
+   * Qubit objects.
+   */
+  std::vector<qubit_mapping_t> get_all_placement_maps(
+      const Circuit& circ_) const override;
+};
+
 class LinePlacement : public Placement {
  public:
   explicit LinePlacement(const Architecture& _arc) { arc_ = _arc; }

tket/src/Predicates/CompilerPass.cpp

@@ -438,6 +438,9 @@ void from_json(const nlohmann::json& j, PassPtr& pp) {
 
     } else if (passname == "PlacementPass") {
       pp = gen_placement_pass(content.at("placement").get<PlacementPtr>());
+    } else if (passname == "NaivePlacementPass") {
+      pp = gen_naive_placement_pass(
+          content.at("architecture").get<Architecture>());
     } else if (passname == "RenameQubitsPass") {
       pp = gen_rename_qubits_pass(
           content.at("qubit_map").get<std::map<Qubit, Qubit>>());

tket/src/Predicates/PassGenerators.cpp

@@ -182,10 +182,34 @@ PassPtr gen_placement_pass(const PlacementPtr& placement_ptr) {
   return std::make_shared<StandardPass>(precons, t, pc, j);
 }
 
+PassPtr gen_naive_placement_pass(const Architecture& arc) {
+  Transform::Transformation trans = [=](Circuit& circ,
+                                        std::shared_ptr<unit_bimaps_t> maps) {
+    NaivePlacement np(arc);
+    return np.place(circ, maps);
+  };
+  Transform t = Transform(trans);
+  PredicatePtr n_qubit_pred =
+      std::make_shared<MaxNQubitsPredicate>(arc.n_nodes());
+
+  PredicatePtrMap precons{CompilationUnit::make_type_pair(n_qubit_pred)};
+  PredicatePtr placement_pred = std::make_shared<PlacementPredicate>(arc);
+  PredicatePtrMap s_postcons{CompilationUnit::make_type_pair(placement_pred)};
+  PostConditions pc{s_postcons, {}, Guarantee::Preserve};
+  // record pass config
+  nlohmann::json j;
+  j["name"] = "NaivePlacementPass";
+  j["architecture"] = arc;
+  return std::make_shared<StandardPass>(precons, t, pc, j);
+}
+
 PassPtr gen_full_mapping_pass(
     const Architecture& arc, const PlacementPtr& placement_ptr,
     const std::vector<RoutingMethodPtr>& config) {
-  return gen_placement_pass(placement_ptr) >> gen_routing_pass(arc, config);
+  std::vector<PassPtr> vpp = {
+      gen_placement_pass(placement_ptr), gen_routing_pass(arc, config),
+      gen_naive_placement_pass(arc)};
+  return std::make_shared<SequencePass>(vpp);
 }
 
 PassPtr gen_default_mapping_pass(const Architecture& arc, bool delay_measures) {

tket/src/Predicates/include/Predicates/PassGenerators.hpp

@@ -45,6 +45,8 @@ PassPtr gen_clifford_simp_pass(bool allow_swaps = true);
 PassPtr gen_rename_qubits_pass(const std::map<Qubit, Qubit>& qm);
 
 PassPtr gen_placement_pass(const PlacementPtr& placement_ptr);
+
+PassPtr gen_naive_placement_pass(const Architecture& arc);
 /* This higher order function generates a Routing pass using the
 std::vector<RoutingMethodPtr> object */
 PassPtr gen_full_mapping_pass(

tket/tests/test_CompilerPass.cpp

@@ -234,12 +234,13 @@ SCENARIO("Test making (mostly routing) passes using PassGenerators") {
     }
   }
   GIVEN("Synthesise Passes in a row then routing") {
-    Circuit circ(4);
+    Circuit circ(5);
     circ.add_op<unsigned>(OpType::H, {0});
     circ.add_op<unsigned>(OpType::CZ, {0, 1});
     circ.add_op<unsigned>(OpType::CH, {0, 2});
     circ.add_op<unsigned>(OpType::CnX, {0, 1, 2, 3});
     circ.add_op<unsigned>(OpType::CZ, {0, 1});
+    circ.add_op<unsigned>(OpType::X, {4});
     OpTypeSet ots = {OpType::CX, OpType::TK1, OpType::SWAP};
     PredicatePtr gsp = std::make_shared<GateSetPredicate>(ots);
     SquareGrid grid(2, 3);