Ancestor-controlled submodule inclusion in design databases

A paradigm is proposed for representing hierarchically specified design data in CAD database systems where there are to be alternate expansions of hierarchical modules. The paradigm is based on an ancestor-based expansion scheme to control which instances of submodules are to be placed within each instance of a given module, and is formalized in a model called a VDAG. The approach is aimed at reducing storage space in engineering design database systems, and providing a means for designers to specify alternate expansions of a module. The VDAG model is defined, and a mechanism whereby a VDAG generates an exploded forest of design trees is described. Algorithms are provided which, given a VDAG or VDAGs, generate design forests, determine whether one module is contained by a larger module, extract a version from a VDAG, test whether two VDAGs are equivalent, and try to reduce the size of a VDAG. Problems such as module containment and VDAG inequivalence are shown to be NP-complete, and the problems of finding a minimum-sized VDAG equivalent to a given VDAG NP-hard.<<ETX>>