Gray means version is read only and blue means version is read-write.
Confluent persistence presents new challenges. Firstly, we again need to find a new representation of versions. Our tree traversal technique is does not extend to DAGs. Also, it is possible to have paths in version history after after u confluent updates. For example by concatenating a string with itself repeatedly we get a version diagram like that in figure [exponential-paths].
Deques (double ended queues allowing stack and queue operations) with concatenation can be done in constant time per operation (Kaplan, Okasaki, and Tarjan @kot). Like with a string, we can create implicitly exponential deques in polynomial time by recursively concatenating a deque with itself.
The general transformation due to Fiat and Kaplan @fiat is as follows:
. This measure is called the ‘effective depth’ of the version DAG: if we unravel the tree via a DFS (by making a copy of each path as if they didn’t overlap) and rebalanced that tree this is the best we could hope to achieve.
d(v) = depth of node v in version DAG
This results reflects poor performance when where u is the number of updates. This is still exponentially better than the complete copy.
An example of e(v) being linear on the number of updates.
A lower bound also by Fiat and Kaplan is for update if queries are free. Their construction makes e(v) queries per update.
OPEN: O(1) or even O(log* n)* space overhead per operation.
Collette, Iacono and Langerman consider the special case of ‘disjoint operations’: confluent operations performed only between versions with no shared data nodes. From there they show O(log* n)* overhead is possible for that case.
If we only allow disjoint operations, then each data node’s version history is a tree. When evaluating read(node, field, version) there are tree cases: when node modified at version, we simply read the new version. Where node along path between modifications, we first need to find the last modification. This problem can be solved with use of ‘link-cut trees’ (see lecture 19). Finally, when version is below a leaf the problem is more complicated. The proof makes use of techniques such as fractional cascading which will be covered in lecture 3. The full construction is explained in @collette.
Functional persistence and data structures are explored in @okasaki. Simple examples of existing techniques include the following.
Functional balanced BSTs – to persist BST’s functionally, the main idea (a.k.a. ‘Path copying’) is to duplicate the modified node and propagate pointer changes by duplicating all ancestors. If there are no parent pointers, work top down. This technique has an overhead of O(log* n)* per operation, assuming the tree is balanced. Demaine, Langerman, Price show this for link-cut trees as well @dlp.
Deques – (double ended queues allowing stack and queue operations) with concatenation can be done in constant time per operation (Kaplan, Okasaki, and Tarjan @kot). Furthermore, Brodal, Makris and Tsichlas show in @bmt it can be done with concatenation in constant time and update and search in O(log* n)*
Tries – with local navigation and subtree copy and delete. Demaine, Langerman, Price show how to persist this structure optimally in @dlp.
Pippenger shows at most log() cost separation of the functional version from the regular data structure in @pippenger.
OPEN: (for both functional and confluent) bigger separation? more general structure transformations?
OPEN: Lists with split and concatenate? General pointer machine?
OPEN: Array with cut and paste? Special DAGs?
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