Gary Lindstrom

Two new algorithms are presented for list structure copying using bounded workspace. The first, of primarily theoretical interest, shows that without cell tag bits the task can be performed in time n2. The second algorithm, assuming one tag bit in each cell, delivers attractive practical speed. Any noncyclic structure is copied in linear speed, while cyclic structures are copied in average time less than n log n. No foreknowledge of cycle absence is necessary to achieve linear speed. A variation of the second algorithm solves an open problem concerning list structure marking. That result demonstrates that marking can be done in average time n log n without the aid of supplemental tag bits or stacks.