Linear transformation distance for bichromatic matchings

Let P=B∪R be a set of 2n points in general position in the plane, where B is a set of n blue points and R a set of n red points. A BR-matching is a plane geometric perfect matching on P such that each edge has one red endpoint and one blue endpoint. Two BR-matchings are compatible if their union is also plane. The transformation graph of BR-matchings contains one node for each BR-matching and an edge joining two such nodes if and only if the corresponding two BR-matchings are compatible. At SoCG 2013 it has been shown by Aloupis, Barba, Langerman, and Souvaine that this transformation graph is always connected, but its diameter remained an open question. In this paper we provide an alternative proof for the connectivity of the transformation graph and prove an upper bound of 2n for its diameter, which is asymptotically tight. Moreover, we present an O(n²log⁡n) time algorithm for constructing a transformation of length O(n) between two given BR-matchings.