Fast persistent immutable data structures

Fast persistent immutable data structures.

Bagwellian hash array-mapped trie, with:

  • Persistence via implicit path-copying / reference-sharing, à la Clojure
  • Lazy sequential operations via standard ES6 Iterator protocol
  • Deep value-equality checking via equals
  • Unrestricted key types
  • Near constant time O(log32n) retrieve and accrete operations
  • Familiar higher-order functions, e.g. filter, map, reduce

Preliminary benchmarks indicate performance gains of 2–7X for accrete-based operations (e.g. create, associate) compared to equivalent ClojureScript operations (via mori.js).

Preliminary V8 heap profiling indicates space efficiency gains for HashMap over a range of approximately 0.5–8.0X, depending on collection size and hash distribution, compared to an identical hash map in cljs/mori.

  • HashMap

Compared to cljs/mori, heap allocation efficiency for HashMap may be expected to regress by some factor less than 2X for certain generational cases, namely those yielded over the course of many association/dissociation operations.

  • The regressive effect described is due to the exclusive definition of a single HashMapNode type which bears a pairwise array of maximum length 64, as opposed to the definition of two distinct classes, BitmapIndexedNode for internal nodes, which bears a max-32 array of nodes, and ArrayNode for terminal nodes, which bears a pairwise max-64 array of key-value pairs.

  • From this distinction it follows that replication of any given HashMapNode then redundantly duplicates the bitmap integers associated with any child nodes. As density increases, node replication may thus incur up to twice the memory cost compared to cljs/mori.

  • It is possible that such regression may be mitigated overall, in part or in whole, as a consequence of the generally shorter trie height that results from storing pairs directly within HashMapNode.

  • This regressive effect is expected, but as yet neither confirmed nor tested.

  • Removal / dissociate
  • Derivative structures Vector, Set