Output specified classical register names to QASM3

src/circuit/quantumcircuit_impl.hpp

@@ -1506,16 +1506,29 @@ std::string QuantumCircuit::to_qasm3(void)
     }
     qk_opcounts_clear(&opcounts);
 
-    // registers
-    std::string creg_name = "c";
-    std::string qreg_name = "q";
-    qasm3 << "bit[" << num_clbits() << "] " << creg_name << ";" << std::endl;
-    qasm3 << "qubit[" << num_qubits() << "] " << qreg_name << ";" << std::endl;
-
     // save ops
     uint_t nops;
     nops = qk_circuit_num_instructions(rust_circuit_.get());
 
+    // Declare registers
+    // After transpilation, qubit registers will be mapped to physical registers,
+    // so we need to combined them in a single quantum register "q";
+    const std::string qreg_name = "q";
+    qasm3 << "qubit[" << num_qubits() << "] " << qreg_name << ";" << std::endl;
+    for(const auto& creg : cregs_)
+    {
+        qasm3 << "bit[" << creg.size() << "] " << creg.name() << ";" << std::endl;
+    }
+
+    auto recover_reg_data = [this](uint_t index) -> std::pair<std::string, uint_t>
+    {
+        auto it = std::upper_bound(cregs_.begin(), cregs_.end(), index,
+                [](uint_t v, const ClassicalRegister& reg) { return v < reg.base_index(); });
+        assert(it != cregs_.begin());
+        it = std::prev(it);
+        return std::make_pair(it->name(), index - it->base_index());
+    };
+
     for (uint_t i = 0; i < nops; i++)
     {
         QkCircuitInstruction *op = new QkCircuitInstruction;
@@ -1526,7 +1539,8 @@ std::string QuantumCircuit::to_qasm3(void)
             {
                 for (uint_t j = 0; j < op->num_qubits; j++)
                 {
-                    qasm3 << creg_name << "[" << op->clbits[j] << "] = " << op->name << " " << qreg_name << "[" << op->qubits[j] << "];" << std::endl;
+                    const auto creg_data = recover_reg_data(op->clbits[j]);
+                    qasm3 << creg_data.first << "[" << creg_data.second << "] = " << op->name << " " << qreg_name << "[" << op->qubits[j] << "];" << std::endl;
                 }
             }
         }

src/circuit/register.hpp

@@ -146,7 +146,7 @@ class Register
 
     /// @brief Return the size of this register
     /// @return the size of this register
-    uint_t size(void) { return size_; }
+    uint_t size(void) const { return size_; }
 
     /// @brief Return the name of this register
     /// @return the name of this register
@@ -157,6 +157,10 @@ class Register
         base_index_ = base;
     }
 
+    /// @brief Return the base index of this register
+    /// @return the base index of this register
+    uint_t base_index() const { return base_index_; }
+
     Bit &operator[](const uint_t i) { return bits_[i]; }
 
 protected:

test/test_circuit.cpp

@@ -614,13 +614,43 @@ static int test_compose(void) {
     return Ok;
 }
 
+static int test_to_qasm3_multi_regs(void) {
+    auto qreg1 = QuantumRegister(2, std::string("q1"));
+    auto qreg2 = QuantumRegister(1, std::string("q2"));
+    auto creg1 = ClassicalRegister(2, std::string("c1"));
+    auto creg2 = ClassicalRegister(1, std::string("c2"));
+    QuantumCircuit circ(std::vector<QuantumRegister>({qreg1, qreg2}), std::vector<ClassicalRegister>({creg1, creg2}));
+
+    circ.measure(qreg2, creg2);
+    circ.measure(qreg1, creg1);
+
+    const auto actual = circ.to_qasm3();
+    const std::string expected =
+        "OPENQASM 3.0;\n"
+        "include \"stdgates.inc\";\n"
+        "qubit[3] q;\n"
+        "bit[2] c1;\n"
+        "bit[1] c2;\n"
+        "c2[0] = measure q[2];\n"
+        "c1[0] = measure q[0];\n"
+        "c1[1] = measure q[1];\n";
+    if (actual != expected) {
+        std::cerr << "  to_qasm3_multi_regs test : \n    expected:\n" << expected
+            << "\n    actual:\n" << actual << std::endl;
+        return EqualityError;
+    }
+
+    return Ok;
+}
+
 
 extern "C" int test_circuit(void) {
     int num_failed = 0;
     num_failed += RUN_TEST(test_standard_gates);
     num_failed += RUN_TEST(test_measure);
     num_failed += RUN_TEST(test_append);
     num_failed += RUN_TEST(test_compose);
+    num_failed += RUN_TEST(test_to_qasm3_multi_regs);
 
     std::cerr << "=== Number of failed subtests: " << num_failed << std::endl;
     return num_failed;