fix wire swap handling in the creation of a phase poly box [TKET-1653]

tket/src/Circuit/Circuit.hpp

@@ -1272,6 +1272,17 @@ class Circuit {
    */
   void replace_SWAPs();
 
+  /**
+   * this function replaces an implicit wire swap between the two given qubits
+   * with three CX operations
+   *
+   * @param first qubits to add the wireswap on
+   * @param second qubits to add the wireswap on
+   *
+   * O(c)
+   */
+  void replace_implicit_wire_swap(const Qubit first, const Qubit second);
+
   // O(E+V+q)
   Circuit dagger() const;
 

tket/src/Circuit/macro_manipulation.cpp

@@ -381,6 +381,18 @@ void Circuit::replace_SWAPs() {
   remove_vertices(bin, GraphRewiring::No, VertexDeletion::Yes);
 }
 
+void Circuit::replace_implicit_wire_swap(
+    const Qubit first, const Qubit second) {
+  add_op<UnitID>(OpType::CX, {first, second});
+  add_op<UnitID>(OpType::CX, {second, first});
+  Vertex cxvertex = add_op<UnitID>(OpType::CX, {first, second});
+  EdgeVec outs = get_all_out_edges(cxvertex);
+  Edge out1 = outs[0];
+  dag[out1].ports.first = 1;
+  Edge out2 = outs[1];
+  dag[out2].ports.first = 0;
+}
+
 // helper functions for the dagger and transpose
 void Circuit::_handle_boundaries(Circuit& circ, vertex_map_t& vmap) const {
   // Handle boundaries

tket/src/Converters/PhasePoly.cpp

@@ -147,18 +147,55 @@ Circuit gray_synth(
 
 PhasePolyBox::PhasePolyBox(const Circuit& circ)
     : Box(OpType::PhasePolyBox), n_qubits_(circ.n_qubits()) {
-  if (circ.n_bits() != 0)
+  Circuit newcirc(circ);
+
+  // check for classical bits
+  if (newcirc.n_bits() != 0)
     throw NotValid(
         "Cannot construct phase polynomial from classical controlled "
         "gates");
+
+  // check the gateset of the circuit
+  for (const Command& com : newcirc) {
+    OpType ot = com.get_op_ptr()->get_type();
+
+    switch (ot) {
+      case OpType::CX: {
+        break;
+      }
+      case OpType::Rz: {
+        break;
+      }
+      default: {
+        throw NotValid("Only CXs and Rzs allowed in Phase Polynomials");
+      }
+    }
+  }
+
+  // replace wireswaps with three CX
+  while (newcirc.has_implicit_wireswaps()) {
+    bool foundswap = false;
+
+    qubit_map_t perm = newcirc.implicit_qubit_permutation();
+    for (const std::pair<const Qubit, Qubit>& pair : perm) {
+      if (pair.first != pair.second) {
+        if (!foundswap) {
+          newcirc.replace_implicit_wire_swap(pair.first, pair.second);
+          foundswap = true;
+        }
+      }
+    }
+  }
+
+  // generate box
   signature_ = op_signature_t(n_qubits_, EdgeType::Quantum);
   unsigned i = 0;
-  for (const Qubit& qb : circ.all_qubits()) {
+  for (const Qubit& qb : newcirc.all_qubits()) {
     qubit_indices_.insert({qb, i});
     ++i;
   }
   linear_transformation_ = MatrixXb::Identity(n_qubits_, n_qubits_);
-  for (const Command& com : circ) {
+  for (const Command& com : newcirc) {
     OpType ot = com.get_op_ptr()->get_type();
     unit_vector_t qbs = com.get_args();
     if (ot == OpType::CX) {
@@ -180,7 +217,7 @@ PhasePolyBox::PhasePolyBox(const Circuit& circ)
       else
         pp_it->second += com.get_op_ptr()->get_params().at(0);
     } else
-      throw NotImplemented("Only CXs and Rzs allowed in Phase Polynomials");
+      TKET_ASSERT(!"Only CXs and Rzs allowed in Phase Polynomials");
   }
 }
 

tket/tests/test_PhasePolynomials.cpp

@@ -713,6 +713,40 @@ SCENARIO("Test conversion of circuit to circuit with phase poly boxes") {
     REQUIRE(test_statevector_comparison(circ, result));
   }
 }
+SCENARIO("Phase polynomial synthesis without architecture") {
+  GIVEN("single SWAP circuit") {
+    Circuit circ(2);
+    circ.add_op<unsigned>(OpType::SWAP, {0, 1});
+    circ.replace_SWAPs();
+    PhasePolyBox ppbox(circ);
+    std::shared_ptr<Circuit> circptr = ppbox.to_circuit();
+    Circuit circ2 = *circptr;
+    REQUIRE(test_unitary_comparison(circ, circ2));
+  }
+  GIVEN("more SWAP circuit") {
+    Circuit circ(5);
+    circ.add_op<unsigned>(OpType::SWAP, {0, 1});
+    circ.add_op<unsigned>(OpType::SWAP, {0, 2});
+    circ.add_op<unsigned>(OpType::SWAP, {0, 3});
+    circ.replace_SWAPs();
+    PhasePolyBox ppbox(circ);
+    std::shared_ptr<Circuit> circptr = ppbox.to_circuit();
+    Circuit circ2 = *circptr;
+    REQUIRE(test_unitary_comparison(circ, circ2));
+  }
+  GIVEN("more SWAP circuit II") {
+    Circuit circ(5);
+    circ.add_op<unsigned>(OpType::SWAP, {0, 1});
+    circ.add_op<unsigned>(OpType::SWAP, {1, 2});
+    circ.add_op<unsigned>(OpType::SWAP, {2, 3});
+    circ.add_op<unsigned>(OpType::SWAP, {3, 4});
+    circ.replace_SWAPs();
+    PhasePolyBox ppbox(circ);
+    std::shared_ptr<Circuit> circptr = ppbox.to_circuit();
+    Circuit circ2 = *circptr;
+    REQUIRE(test_unitary_comparison(circ, circ2));
+  }
+}
 
 }  // namespace test_PhasePolynomials
 }  // namespace tket