Function getQubitsSwappedToMaxSuffix

quest/src/core/localiser.cpp:203–238

auto getQubitsSwappedToMaxSuffix(Qureg qureg, vector<int> qubits) {

    // this function is called by any-targ partial trace, and is used to find
    // targets in the prefix substate and where they can be swapped into the suffix
    // to enable subsequent embarrassingly parallel simulation. Note we seek the MAX
    // available indices in the suffix, since this heuristically reduces the
    // disordering of the surviving qubits after the trace, reducing the number of
    // subsequent order-restoring SWAPs

    // nothing to do if all qubits are already in suffix
    if (!doesGateRequireComm(qureg, qubits))
        return qubits;

    // prepare mask to avoid quadratic nested looping
    qindex qubitMask = getBitMask(qubits.data(), qubits.size());
    int maxFreeSuffixQubit = getIndOfNextLeftmostZeroBit(qubitMask, qureg.logNumAmpsPerNode);

    // enumerate qubits backward, modifying our copy of qubits as we go
    for (size_t i=qubits.size(); i-- != 0; ) {
        int qubit = qubits[i];

        // consider only qubits in the prefix substate
        if (util_isQubitInSuffix(qubit, qureg))
            continue;

        // swap the prefix qubit into the largest available suffix position
        qubits[i] = maxFreeSuffixQubit;

        // update trackers
        qubitMask = flipTwoBits(qubitMask, qubit, maxFreeSuffixQubit);
        maxFreeSuffixQubit = getIndOfNextLeftmostZeroBit(qubitMask, maxFreeSuffixQubit);
    }

    // return our modified copy
    return qubits;
}


auto getNonSwappedCtrlsAndStates(vector<int> oldCtrls, vector<int> oldStates, vector<int> newCtrls) {