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   /**
     * @brief Update state with a new data point
     */
    template <class OtherHandle>
    AvgVarTransitionState &operator+=(const double x){
        double diff = (x - avg);
        double normalizer = static_cast<double>(numRows + 1);
        // online update mean
        this->avg += diff / normalizer;
        // online update variance
        double new_diff = (x - avg);
        double a = static_cast<double>(numRows) / normalizer;
        this->var = (var * a) + (diff * new_diff) / normalizer;
    }
 
/**
 * @brief Merge with another State object
 *
 * We update mean and variance in a online fashion
 * to avoid intermediate large sum. 
 */
template <class OtherHandle>
AvgVarTransitionState &operator+=(
    const AvgVarTransitionState<OtherHandle> &inOtherState) {

    if (mStorage.size() != inOtherState.mStorage.size())
        throw std::logic_error("Internal error: Incompatible transition "
                               "states");
    double avg_ = inOtherState.avg;
    double var_ = inOtherState.var;
    uint16uint64_t numRows_ = static_cast<uint16cast<uint64_t>(inOtherState.numRows);
    double totalNumRows = static_cast<double>(numRows + numRows_);
    double p = static_cast<double>(numRows) / totalNumRows;
    double p_ = static_cast<double>(numRows_) / totalNumRows;
    double totalAvg = avg * p + avg_ * p_;
    double a = avg - totalAvg;
    double a_ = avg_ - totalAvg;

    numRows += numRows_;
    var = p * var + p_ * var_ + p * a * a + p_ * a_ * a_;
    avg = totalAvg;
    return *this;
}