BitArray

Written by Redu, with contributions by Atriace due to the question of “How do I create a bit array in JavaScript”.

The BitArray class, starting with version 2.0, is implemented in TypeScript and built using AssemblyScript. However it’s JS compiled version is also available for the web.

BitArray utilizes bit masks to access the bits stored in groups of 8 bytes, known as <u64>, within an ArrayBufferView structure. In addition, this class offers convenient standard boolean operations, as described on Wikipedia.

Benefits of BitArray

• BitArray is developed with a focus on performance, and thanks to AssemblyScript WASM, it’s very fast. See the benchmarks below for more information.

• BitArray has a much smaller memory footprint compared to other types of arrays that hold the same number of boolean elements.

Constructor: BitArray(length)

• length: A positive integer Number. To ensure optimal performance, most properties and methods in BitArray, such as .popcnt(), .all(), .wipe(number), and .and(), use 64-bit access to the underlying ArrayBuffer. As a result, the buffer is always set to the minimum multiple of 8 bytes (64 bits) that is equal to or greater than the requested integer size. In theory, BitArray should support sizes up to 34,359,738,336 (0x07ffffffe0).

Installation

With Deno you can directly import the BitArray package from the Third Party Modules like

import { BA } from "https://deno.land/x/bit_array@v2.0.3/mod.ts"

For JS environments like Web browsers you can directly import the JS compiled version from the Third Party Modules like

import { BA } from "https://deno.land/x/bit_array@v2.0.3/mod.js"

Working with Node or Bun you can clone the BitArray Repo in your project folder and do like

import { BA } from "./JSBitArray/mod.ts"

Syntax

new BA(length : number);

Examples

Starting with something simple…

var a = new BA(10);
a.set(0);
a.set(3);

console.log(`${a}`); // 1001000000
a.length;            // 10
a.popcnt();          // 2

Properties

• length : number: An immutable property returning the actual length of the BitArray.
• dataView : DataView: The dataView property of BitArray is an instance of a class that extends the standard DataView object. However, for safety reasons, this class does not expose the underlying ArrayBuffer through the buffer property. Despite this, it is fully suitable for accessing and manipulating the BitArray using DataView prototype methods such as setUint8(), getInt32(), etc.

Methods

Tests

• .all() : boolean: Returns true if all bits in the BitArray are set.

var a = new BA(10);
a.wipe(1); // 1111111111
a.all();   // true
a.reset(7);// 1111111011
a.all();   // false 

• .any() : boolean: Returns true if any of the bits in the BitArray are set. If returns false then all bits are 0.

var a = new BA(10);
a.any();   // false
a.set(7);  // 0000000100
a.any();   // true 

• .isEqual() : boolean: Returns true if tested BitArrays have the same bits set.

var a = new BA(10),
    b;
a.wipe(2);     // 1000111100 (randomly set bits)
b = a.slice(); // 1000111100
a.isEqual(b);  // true 

Logical Operators

• .and(bar : BitArray): And of this and bar. Example: 1100 & 1001 = 1000 returned as a new BitArray.

var a = new BA(10),
    b = new BA(37),
    c;

a.length;  // 10
b.length;  // 37

a.set(7);  // 0000000100
a.set(8);  // 0000000110
b.set(8);  // 0000000010000000000000000000000000000

a.and(b);  // 0000000010
b.and(a);  // 0000000010

• .not() : BitArray: Returns a new BitArray with all bits flipped. Example: 1100 = 0011.
• .or(bar : BitArray) : BitArray: Or of this and bar. Example: 1100 & 1001 = 1101 returned as a new BitArray.
• .xor(bar : BitArray) : BitArray: Xor of this and bar. Example: 1100 & 1001 = 0101 returned as a new BitArray.

In Place Modifiers

• .reset(i : number) : BitArray: Resets the value at given index i.
• .set(i : number) : BitArray: Sets the value at given index i.
• .toggle(i : number) : BitArray: Flips the value at given index i.
• .wipe(n : number = 0) : BitArray:
  • .wipe() or .wipe(0) fills the BitArray with 0.
  • .wipe(1) fills the BitArray with 1.
  • .wipe(2+) fills the BitArray with random bits.

Others

• .slice(a = 0, b = this.length) : BitArray: Slices BitArray and returns a new BitArray. When invoked with no arguments, the default argument values instantiate a clone. No negative values are allowed.
• .toString() : string: Returns the string representation of the BitArray.

Benchmarks

We demonstrate a meaningful use case for the BitArray class in an optimized segmented Sieve of Sundaram algorithm for finding the number of prime numbers up to a given number n. While this single threaded PI function is still in pure JS it is tested against using 4 different array structures for n values ranging from 0 to 1000000 (1e6). Array, BitArray (without WASM), BitArrayWASM, and Uint8Array. The benchmarking was performed using Deno’s built-in benchmarking tool.

So just run

/path-to-project$ deno bench --unstable

at the root of the project to see it for yourself that this BitArray WASM implementation is around 20% faster by using only 12.5% of the memory footprint compared to native JS array structures.