Function reorderReducedQureg

quest/src/core/localiser.cpp:1761–1788

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

    /// @todo 
    /// this function performs a sequence of SWAPs which are NOT necessarily upon disjoint qubits,
    /// and ergo do not commute. We still however may be able to effect this more efficiently in
    /// a single communicating operation rather than this sequence of SWAP gates, and might still
    /// even be able to use cuQuantum's distributed bit index swaps API. Check this!

    // determine the relative ordering of outQureg's remaining qubits
    auto remainingQubits = getNonTracedQubitOrder(inQureg, allTargs, suffixTargs);

   // perform additional swaps to re-order the remaining qubits (heuristically starting from back)
    for (int qubit=(int)remainingQubits.size(); qubit-- != 0; ) {

        // locate the next qubit which is out of its sorted position
        if (remainingQubits[qubit] == qubit)
            continue;

        // qubit is misplaced; locate its position among the remaining qubits
        int pair = 0;
        while (remainingQubits[pair] != qubit)
            pair++;
        
        // and swap it directly to its required position, triggering any communication scenario (I think)
        localiser_statevec_anyCtrlSwap(outQureg, {}, {}, qubit, pair);
        std::swap(remainingQubits[qubit], remainingQubits[pair]);
    }
}


void partialTraceOnSuffix(Qureg inQureg, Qureg outQureg, vector<int> ketTargs) {