Necklaces, convolutions, and X+Y

Written with David Bremner, Timothy M. Chan, Erik D. Demaine, Ferran Hurtado, John Iacono, Stefan Langerman, Mihai Pătraşcu, and Perouz Taslakian*.

Algorithmica 69(2):294–314, 2014.

Preliminary version (without Mihai's contributions) in Proceedings of the 12th Annual European Symposium on Algorithms, 160-171, 2006.

We give subquadratic algorithms that, given two necklaces each with n beads at arbitrary positions, compute the optimal rotation of the necklaces to best align the beads. Here alignment is measured according to the Lp norm of the vector of distances between pairs of beads from opposite necklaces in the best perfect matching. We show surprisingly different results for p=1, p=2, and p=∞. For p=2, we reduce the problem to standard convolution, while for p=∞ and p=1, we reduce the problem to (min,+) convolution and (median,+) convolution. Then we solve the latter two convolution problems in subquadratic time, which are interesting results in their own right. These results shed some light on the classic sorting X+Y problem, because the convolutions can be viewed as computing order statistics on the antidiagonals of the X+Y matrix. All of our algorithms run in o(n2) time, whereas the obvious algorithms for these problems run in Θ(n2) time.

Publications - Jeff Erickson ( 14 Octo 2014