To perform HFNS, recursively makes clusters of closest sinks, less than
30 sinks and with a control of the half-perimeter at all the clusters
levels. Clearly needs lots of improvements but the backbone of the
feature works. Make use of the pool buffers as do the clock tree.
Clustering is done with a degenerated Kruskal algorithm.
This is rougly based on the article:
Buffered Steiner Trees for Difficult Instances
Charles J. Alpert, Member, IEEE
IEEE TCAD, Vol. 21, No. 1, January 2002
0278–0070/02$17.00 (c) 2002 IEEE
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