Supported by the National Natural Science Foundation of China under Grant No.60403031(国家自然科学基金);the National HighTech Research and Development Plan of China under Grant No.2005AA121560(国家高技术研究发展计划(863));the Fundamental Research Project of the Institute of Computing Technology; the Chinese Academy of Sciences (中国科学院计算技术研究所基础研究项目);the Youth Innovation Foundation of the Institute of Computing Technology; the Chinese Academy of Sciences(中国科学院计算技术研究所青年创新基金)