okasaki-rb-tree

An implementation of the Okasaki red-black tree in JavaScript

Overview

In 1999, Chris Okasaki wrote a research paper called functional pearls: red-black trees in a functional setting. In it, he demonstrated a functional, immutable way to implement a red-black tree in Haskell.

This is an implementation of Okasaki’s work—specifically his rotations—in JavaScript. The names of the rotations in the library are based on an in-order traversal of the the imbalanced subtrees described in figure 1 on page 3.

All the data structures generated in this library are done so with Object.create(). Although JavaScript will let you add your own properties to them, its own are immutable.

Example

import createTree from 'okasaki-rb-tree';
const emptyTree = createTree();
const threeTree = emptyTree.insert(3);
emptyTree.root; // null
threeTree.root; // {color: "black", value: 3, left: null, right: null}

API

createTree(options)

• options.root: the root node of the tree (default: null).
• options.aEqualsB: the equality function used for searches and insertions, e.g. for case-insensitivity (default: (a, b) => a === b).
• options.aIsLessThanB: the comparator function used for searches and insertions (default: (a, b) => a < b).

tree

• tree.root: node or null.
• tree.aEqualsB: see createTree(options).
• tree.aIsLessThanB: see createTree(options).
• tree.search(value): returns the value if found, null if not.
• tree.insert(value): returns a new, rebalanced tree with the new value. NB: tree instances are immutable; the original tree will remain unchanged.
• tree.insertBatch(values): pass an array and it returns a new, rebalanced tree with its values.
• tree.make(properties): returns a new, rebalanced tree with properties merged with the original.

node

• node.color: "red" or "black".
• node.value: an arbitrary insert-ed value.
• node.leftChild: node or null.
• node.rightChild: node or null.
• node.make(properties): returns a new, rebalanced subtree with properties merged with the original.

Todo

• [ ] Write unit tests for node.
• [ ] Write unit tests for rotations.
• [ ] Switch extensions from .js to .mjs to support node’s experimental module mode once Jest is ready.
• [ ] Plan how to handle falsey insertion values. (Disallow or configure?)
• [ ] Add tree.remove(). (Okasaki’s original paper didn’t include one.)
• [ ] Add Rollup to convert to CommonJS?