Optimally selecting the top $k$ values from $X+Y$ with layer-ordered heaps

Selection and sorting the Cartesian sum, $X+Y$, are classic and important problems. Here, a new algorithm is presented, which generates the top $k$ values of the form $X_i+Y_j$. The algorithm relies only on median-of-medians and is simple to implement. Furthermore, it uses data structures contiguous in memory, and is fast in practice. The presented algorithm is demonstrated to be theoretically optimal.