🐛 Fix stripping of idle qubits

src/ir/QuantumComputation.cpp

@@ -721,23 +721,27 @@ void QuantumComputation::dump(const std::string& filename) const {
 
 void QuantumComputation::dumpOpenQASM(std::ostream& of, bool openQASM3) const {
   // dump initial layout and output permutation
+  // since it might happen that the physical qubit indices are not consecutive,
+  // due to qubit removals, we need to adjust them accordingly.
   Permutation inverseInitialLayout{};
-  for (const auto& q : initialLayout) {
-    inverseInitialLayout.insert({q.second, q.first});
+  std::size_t idx = 0;
+  for (const auto& [physical, logical] : initialLayout) {
+    inverseInitialLayout.emplace(logical, idx++);
   }
   of << "// i";
-  for (const auto& q : inverseInitialLayout) {
-    of << " " << static_cast<std::size_t>(q.second);
+  for (const auto& [logical, physical] : inverseInitialLayout) {
+    of << " " << static_cast<std::size_t>(physical);
   }
   of << "\n";
 
   Permutation inverseOutputPermutation{};
-  for (const auto& q : outputPermutation) {
-    inverseOutputPermutation.insert({q.second, q.first});
+  idx = 0;
+  for (const auto& [physical, logical] : outputPermutation) {
+    inverseOutputPermutation.emplace(logical, idx++);
   }
   of << "// o";
-  for (const auto& q : inverseOutputPermutation) {
-    of << " " << q.second;
+  for (const auto& [logical, physical] : inverseOutputPermutation) {
+    of << " " << physical;
   }
   of << "\n";
 

test/ir/test_qfr_functionality.cpp

@@ -1492,4 +1492,28 @@ TEST_F(QFRFunctionality, TryGettingRegisterForQubitNotInRegister) {
   EXPECT_THROW(std::ignore = qc.getQubitRegister(2), QFRException);
 }
 
+TEST_F(QFRFunctionality, stripIdleQubits) {
+  QuantumComputation qc(3, 2);
+  qc.x(0);
+  qc.x(2);
+  qc.measure(0, 0);
+  qc.measure(2, 1);
+  qc.stripIdleQubits(true);
+
+  const auto* const expected = "// i 0 1\n"
+                               "// o 0 1\n"
+                               "OPENQASM 3.0;\n"
+                               "include \"stdgates.inc\";\n"
+                               "qubit[1] q_l;\n"
+                               "qubit[1] q_h;\n"
+                               "bit[2] c;\n"
+                               "x q_l[0];\n"
+                               "x q_h[0];\n"
+                               "c[0] = measure q_l[0];\n"
+                               "c[1] = measure q_h[0];\n";
+
+  EXPECT_EQ(qc.toQASM(), expected);
+  const auto qc2 = QuantumComputation::fromQASM(expected);
+  EXPECT_EQ(qc2.toQASM(), expected);
+}
 } // namespace qc