Department of Computer Science, Helsinki Institute for Information Technology HIIT, University of Helsinki, Helsinki, Finland.|Department of Computer and Information Science and Engineering, University of Florida, Gainesville, FL, USA.|Department of Computer Science, Helsinki Institute for Information Technology HIIT, University of Helsinki, Helsinki, Finland.|Department of Computer Science, Colorado State University, Fort Collins, CO, USA.|Department of Computer and Information Science and Engineering, University of Florida, Gainesville, FL, USA.