Function getNonTracedQubitOrder

quest/src/core/localiser.cpp:1721–1758

auto getNonTracedQubitOrder(Qureg qureg, vector<int> originalTargs, vector<int> revisedTargs) {

    // prepare a list of all the qureg's qubits when treated as a statevector
    vector<int> allQubits(2*qureg.numQubits);
    for (size_t q=0; q<allQubits.size(); q++)
        allQubits[q] = q;
    
    // determine the ordering of all the Qureg's qubits after swaps
    for (size_t i=0; i<originalTargs.size(); i++) {
        int qb1 = originalTargs[i];
        int qb2 = revisedTargs[i];
        if (qb1 != qb2)
            std::swap(allQubits[qb1], allQubits[qb2]);
    }

    // use a mask to avoid quadratic nested iteration below
    qindex revisedMask = util_getBitMask(revisedTargs);

    // retain only non-targeted qubits
    vector<int> remainingQubits;
    remainingQubits.reserve(allQubits.size() - originalTargs.size());
    for (size_t q=0; q<allQubits.size(); q++)
        if (!getBit(revisedMask, q))
            remainingQubits.push_back(allQubits[q]);

    // shift down remaining qubits to be contiguous...
    qindex remainingMask = util_getBitMask(remainingQubits);
    for (int &qubit : remainingQubits) {
        int bound = qubit;

        // by subtracting the number of smaller un-targeted qubits from each qubit index
        for (int i=0; i<bound; i++)
            qubit -= ! getBit(remainingMask, i);
    }

    // return the ordering, i.e. a list [0, #final-qubits)
    return remainingQubits;
}


void reorderReducedQureg(Qureg inQureg, Qureg outQureg, vector<int> allTargs, vector<int> suffixTargs) {