Function getFullStateOperator

tests/utils/evolve.cpp:117–147

qmatrix getFullStateOperator(vector<int> ctrls, vector<int> ctrlStates, vector<int> targs, qmatrix matrix, size_t numQubits) {
    DEMAND( numQubits >= ctrls.size() + targs.size() );
    DEMAND( getPow2(targs.size()) == (qindex) matrix.size() );
    DEMAND( (ctrlStates.empty() || ctrlStates.size() == ctrls.size()) );

    // construct controlled-(matrix) upon lowest order qubits
    qmatrix full = getControlledMatrix(matrix, ctrls.size());

    // left-pad 'full' to be numQubits large
    if (numQubits > ctrls.size() + targs.size()) {
        size_t pad = getPow2(numQubits - ctrls.size() - targs.size());
        full = getKroneckerProduct(getIdentityMatrix(pad), full);
    }
    
    // apply swaps to retarget 'full' to given ctrls and targs
    auto [swaps, unswaps] = getSwapAndUnswapMatrices(ctrls, targs, numQubits);
    qmatrix out = unswaps * full * swaps;

    // apply NOT to all zero-controlled qubits (recurses just once)
    qmatrix matrX = {{0,1},{1,0}};
    for (size_t i=0; i<ctrlStates.size(); i++) {
        
        if (ctrlStates[i] == 1)
            continue;

        qmatrix fullX = getFullStateOperator({}, {}, {ctrls[i]}, matrX, numQubits);
        out = fullX * out * fullX;
    }

    return out;
}