Function anyCtrlMultiSwapBetweenPrefixAndSuffix

quest/src/core/localiser.cpp:912–938

void anyCtrlMultiSwapBetweenPrefixAndSuffix(Qureg qureg, vector<int> ctrls, vector<int> ctrlStates, vector<int> targsA, vector<int> targsB) {

    // this is an internal function called by the below routines which require
    // performing a sequence of SWAPs to reorder qubits, or move them into suffix.
    // the SWAPs act on unique qubit pairs and so commute.

    /// @todo
    ///   - the sequence of pair-wise full-swaps should be more efficient as a
    ///     "single" sequence of smaller messages sending amps directly to their
    ///     final destination node. This could use a new "multiSwap" function.
    ///   - if the user has compiled cuQuantum, and Qureg is GPU-accelerated, the
    ///     multiSwap function should use custatevecSwapIndexBits() if local,
    ///     or custatevecDistIndexBitSwapSchedulerSetIndexBitSwaps() if distributed,
    ///     although the latter requires substantially more work like setting up
    ///     a communicator which may be inelegant alongside our own distribution scheme.

    // perform necessary swaps to move all targets into suffix, each of which invokes communication
    for (size_t i=0; i<targsA.size(); i++) {

        if (targsA[i] == targsB[i])
            continue;

        int suffixTarg = std::min(targsA[i], targsB[i]);
        int prefixTarg = std::max(targsA[i], targsB[i]);
        anyCtrlSwapBetweenPrefixAndSuffix(qureg, ctrls, ctrlStates, suffixTarg, prefixTarg);
    }
}