Function anyCtrlTwoOrAnyTargDenseMatr

quest/src/core/localiser.cpp:1009–1052

// T can be CompMatr2 or CompMatr
template <typename T>
void anyCtrlTwoOrAnyTargDenseMatr(Qureg qureg, vector<int> ctrls, vector<int> ctrlStates, vector<int> targs, T matr, bool conj, bool transp) {

    // node has nothing to do if all local amps violate control condition
    if (!doAnyLocalStatesHaveQubitValues(qureg, ctrls, ctrlStates))
        return;

    // skip straight to embarrasingly parallel simulation if possible
    if (!doesGateRequireComm(qureg, targs)) {

        // using only the suffix ctrls
        removePrefixQubitsAndStates(qureg, ctrls, ctrlStates);
        anyCtrlTwoOrAnyTargDenseMatrOnSuffix(qureg, ctrls, ctrlStates, targs, matr, conj, transp);
        return;
    }

    // find suffix positions for all prefix targs, moving colliding ctrls out of the way
    auto [newCtrls, newTargs] = getCtrlsAndTargsSwappedToMinSuffix(qureg, ctrls, targs);

    // only unmoved ctrls can be applied to the swaps, to accelerate them
    auto [unmovedCtrls, unmovedCtrlStates] = getNonSwappedCtrlsAndStates(ctrls, ctrlStates, newCtrls); 

    /// @todo
    /// above, we track which control qubits are un-targeted by the SWAPs; such controls can be
    /// seen as 'meta' to the entire operation, and so SHOULD be passable to the SWAPs below in
    /// order to accelerate them (since more ctrls = fewer comm). However, this is strangely not
    /// working; controlling the SWAPs upon these 'meta' control qubits is breaking the unit tests!
    /// Until we better understand this, we disable this optimisation by removing all SWAP controls.
    unmovedCtrls = {};
    unmovedCtrlStates = {};

    // perform necessary swaps to move all targets into suffix, invoking communication (swaps are real, so no need to conj)
    anyCtrlMultiSwapBetweenPrefixAndSuffix(qureg, unmovedCtrls, unmovedCtrlStates, targs, newTargs);

    // if the moved ctrls do not eliminate this node's need for local simulation...
    if (doAnyLocalStatesHaveQubitValues(qureg, newCtrls, ctrlStates)) {

        // perform embarrassingly parallel simulation using only the new suffix ctrls
        removePrefixQubitsAndStates(qureg, newCtrls, ctrlStates);
        anyCtrlTwoOrAnyTargDenseMatrOnSuffix(qureg, newCtrls, ctrlStates, newTargs, matr, conj, transp);
    }

    // undo swaps, again invoking communication
    anyCtrlMultiSwapBetweenPrefixAndSuffix(qureg, unmovedCtrls, unmovedCtrlStates, targs, newTargs);
}


void localiser_statevec_anyCtrlTwoTargDenseMatr(Qureg qureg, vector<int> ctrls, vector<int> ctrlStates, int targ1, int targ2, CompMatr2 matr, bool conj, bool transp) {