1. Department of Mathematics and Computer Science, Chulalongkorn University, Bangkok 10330, Thailand
2. Department of Computer Science, COMSATS University Islamabad, Islamabad 45550, Pakistan
3. School of Computing and Engineering, College of Science and Engineering, University of Derby, Derby DE22 1GB, UK
4. Institute of Information Technology, Quaid i Azam University, Islamabad 15320, Pakistan
5. Institute of Computer Technology, Technische Universitat Wien (TU Wien), 1040 Wien, Austria