School of Computing, National University of Singapore, Singapore and Institute for Infocomm Research, Singapore. liugm@comp.nus.edu.sg|School of Computing, National University of Singapore, Singapore and Institute for Infocomm Research, Singapore. liugm@comp.nus.edu.sg|School of Computing, National University of Singapore, Singapore and Institute for Infocomm Research, Singapore. liugm@comp.nus.edu.sg