Efficient variable ordering heuristics for shared ROBDD

Several ordering heuristics for shared, reduced and ordered binary decision diagrams (ROBDDs) are described. These heuristics are tested on ISCAS and MCNC benchmark circuits. In all examples, the ordering is accomplished in a few seconds and generates smaller shared ROBDDs than other previously proposed heuristics. The objective is to provide a fast way to generate shared ROBDDs of reasonable sizes. The results could be used as a good initial solution to any semi-exhaustive ordering method to further reduce the sizes