Avoid Unnecessary Computation in Products.

[akleeman]

Only compute the right hand side of a covariance product if the left hand side isn't 0.

This way if you have some covariance function which is complex:

SlowCovarianceFunction slow;

we can then introduce decorrelating product functions,

class OnlyOdd : public CovarianceFunction<OnlyOdd> {
    double call_impl_(const int x, const int y) const {
        if (x % 2 && y %% 2) {
            return 1.;
        } else {
            return 0.;
        }
    }
}

Such that if you then form:

OnlyOdd odd;
auto product = odd * slow;

Evaluation of the slow function in product will only be done for odd numbers.

[seth-swiftnav]

Looks Great. The description was much more confusing than the code itself.