Destructive Reordering of CDR-Coded Lists

Abstract

Linked list structures can be compactly represented by encoding the CDR ("next") pointer in a two-bit field and linearizing list structures as much as possible. This "CDR-coding" technique can save up to 50% on storage for linked lists. The RPLACD (alter CDR pointer) operation can be accommodated under such a scheme by using indirect pointers. Standard destructive reordering algorithms, such as REVERSE and SORT, use RPLACD quite heavily. If these algorithms are used on CDR-coded lists, the result is a proliferation of indirect pointers. We present here algorithms for destructive reversal and sorting of CDR-coded lists which avoid creation of indirect pointers. The essential idea is to note that a general list can be viewed as a linked list of array-like "chunks". The algorithm applied to such "chunky lists" is a fusion of separate array- and list-specific algorithms; intuitively, the array-specific algorithm is applied to each chunk, and the list algorithm to the list with each chunk considered as a single element.