Persistent Data Structures

Candidate: Sarnak,Neil Ivor


This dissertation introduces the concept of persistence in data structures. Classical algorithms operate on data structures is such a manner that modifications to the structure do not preserve its state as it appeared before the modification. A persistent data structure is one in which multiple versions of the structure as it varies through time are maintained. Data structures that do not maintain the history of states of the structure are called ephemeral. A differentiation between two types of persistence, partial persistence and full persistence, is made. A partially persistent data structure allows the modification only of the most recent version of the structure. This makes partial persistence useful in cases where the history of update operations is required for query purposes but no changes of prior versions are desired. Under certain constraints, any ephemeral data structure may be made persistent without a major blow-up of the space and time complexity measures. Full persistence allows modification of any version of the data structure. This dissertation presents algorithms that support persistent search trees, with applications in computational geometry. In particular, the planar point location problem will be solved using persistent binary search trees with an O(log n) query time and O(n) space. Persistent lists are described, with applications in applicative programming languages. In particular, persistent deques are presented that have constant space overhead per deque operation, while still maintaining O(1) update times. Persistent finger search trees are also presented, with applications in text editing. Persistent finger search trees are implemented with an O(log d) space overhead per update, and an O(log d) time bound, where d is the distance between the finger and the affected position. A general result is shown that allows making arbitrary ephemeral data structures partially persistent with an O(1) space overhead per update operation.