Skew-binary random access lists

1 unstable release

0.1.0 Mar 21, 2020

MIT license

754 lines


An implementation of skew-binary random access lists.

Skew-binary random access lists are a persistent list data structure. They allow logarithmic time random access. Most notably, online lowest common ancestors can be implemented in logarithmic time in the length of the path.

These lists are persistent, i.e. they allow preserving the old version of itself when mutated Eg. consider a simple list like this:

A - B - C - D    List: ABCD

If, after B, we clone the list and append E and F, we will get the resulting structure:

A - B - C - D    First list: ABCD
        E - F    Second list: ABEF

Here we can see how A and B will be shared among the two lists. Thus the "lists" in skew- binary random access lists can really be seen as paths in a tree instead. To emphasize this, this implmentations refers to skew-binary random access lists as paths, or more specifically as the SkewPath type.

Since we want to be able to share nodes, the paths themselves do not own the nodes. Instead, the paths are indexes into a structure that does own the nodes. This structure, the SkewForest, encapsulates the shared graph of the paths.


An additional wrinkle is that the SkewForest and SkewPath in this implementation does not store any actual values. They only store the path topology, i.e. the sequence of node indexes that forms the nodes of a path. When push operation is called on a SkewPath the index of the node is returned.

To actually associate a value with a node, a SkewMap can be constructed to map these indices to values.


The example below demonstrates creating two lists as shown above.

use skew_forest::{SkewForest, SkewPath, SkewPathNode, SkewMap};

let mut forest = SkewForest::default();
let mut path_a = SkewPath::<[SkewPathNode; 8]>::default();

// Push A and B onto `path_a`
let node_a = forest.push(&mut path_a);
let node_b = forest.push(&mut path_a);

// Clone A to B
let mut path_b = path_a.clone();

// Push C and D onto `path_a`
let node_c = forest.push(&mut path_a);
let node_d = forest.push(&mut path_a);

// Push E and F onto `path_b`
let node_e = forest.push(&mut path_b);
let node_f = forest.push(&mut path_b);

// Check that `path_a` matches ABCD
    vec![node_a, node_b, node_c, node_d],

// Check that `path_b` matches ABCD
    vec![node_a, node_b, node_e, node_f],

License: MIT