bitstream-io

This library provides a decoder and encoder for you to manipulate bits in nanoseconds.

Features

• Decoder allows you to pop bits from the k-bit encoded string.
• Encoder allows you to push bits to form k-bit encoded string.
• Highly optimized, bits manipulation can be done in nanoseconds.
• No dependencies.
• TypeScript supported.
• Tree-shakeable.
• Browser compatability: IE6+.

Usage

CommonJS

const { Decoder, Encoder } = require('bitstream-io');

ES Module

import { Decoder, Encoder } from 'bitstream-io';

Browser

<script src="https://cdn.jsdelivr.net/npm/bitstream-io/dist/index.min.js"></script> // From CDN
<script src="/path/to/node_modules/bitstream-io/dist/index.min.js"></script> // From node_modules
const decoder = BitStream.Decoder(6, cb);
const encoder = BitStream.Encoder(6, cb);

APIs

Decoder(k, cb)

Creates a decoder that can decode k-bit encoded strings. You need to provide a callback to map a symbol to a number. You can find some common decoders here. The following example uses a base64 decoder.

// 010101_000110_100001_100111_011100_110010_111000
decoder.from('VGhncy4');

decoder.pop(8); //Extracts the first 8 bits, returns 84 (01010100).

decoder.pop(32); // Extracts the next 32 bits, returns 1751610158 (01101000 01100111 01110011 00101110).

decoder.pop(2); // Extracts the last 2 bits, returns 0.

decoder.pop(1); // null as no more available bits.

// Resets the bit position and pops the next 6 bits (000110).
decoder.offset(6);
decoder.pop(6); // 6

decoder.from(input)

Loads an encoded string to the decoder, and replaces the existing one.

Remark: Since the input is k-bit encoded, if the number of bits encoded is not a multiple of k, 0s are padded at the end of the stream. It is very likely that the total bits you pushed is less than input.length * k. Therefore, you are recommended to push a preserved bits to indicate the end of the stream, otherwise you may pop unnecessary bits.

decoder.pop(k = 8)

Extracts the next k bits and converts it to decimal, k should be at most 32.

decoder.offset(k = 0)

Resets the bit position by skipping the first k bits.

Encoder(k, cb)

Creates an encoder that can convert bits to k-bit encoded string. You need to provide a callback to map a number to a symbol. You can find some common encoders here. The following example uses a base64 encoder.

encoder.push(46, 8); // Appends 00101110.
encoder.push(46, 16); // Appends 00000000_00101110.
encoder.push(1, 3); // Appends 001.
encoder.push(0, 1); // Appends 0.
encoder.push(-1, 8); // Error thrown, only unsigned binary is allowed.
encoder.push(255, 4); // Error thrown, as 255 cannot be represented by 4 bits.

// Encodes the bitstream (001011_100000_000000_101110_0010) to base64 format,
const encoded = encoder.flush(); // LgAuI

encoder.push(binary, k = 8)

Converts an integer to k-bit unsigned binary, and appends it to the end of the bit stream. k should be at most 32.

encoder.flush()

Encodes the bitstream, and resets the encoder.

Examples

Base64

const symbols = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'.split('');
const dictionary = {};
symbols.forEach((symbol, i) => dictionary[symbol] = i);

const decoder = Decoder(6, (symbol) => dictionary[symbol]);
const encoder = Encoder(6, (i) => symbols[i]);

Base16 (Hex)

const symbols = '0123456789ABCDEF'.split('');
const dictionary = {};
symbols.forEach((symbol, i) => dictionary[symbol] = i);

const decoder = Decoder(4, (symbol) => dictionary[symbol]);
const encoder = Encoder(4, (i) => symbols[i]);

UTF-16

const decoder = Decoder(16, (symbol) => symbol.charCodeAt(0));
const encoder = Encoder(16, (i) => String.fromCharCode(i));

Performance

npm run benchmark

Operation	Benchmark (ops/sec)	Time (ns/ops)
Push 1 bit	147,115,982	6.80
Pop 1 bit	84,702,022	11.81
Push 8 bits	106,736,041	9.37
Pop 8 bits	51,118,344	19.56
Push 16 bits	53,374,346	18.74
Pop 16 bits	36,004,238	27.77
Push 32 bits	20,496,482	48.79
Pop 32 bits	19,224,379	52.02

Base64 encoder/decoder, M1 Macbook Air 8GB

Build

The following command creates index.cjs, index.mjs and index.min.js in ./dist directory.

npm run build

Test

The following command runs all tests in ./tests directory.

npm run test