vlq.js

Convert integers to a Base64-encoded VLQ string, and vice versa. No dependencies, works in node.js or browsers, supports AMD.

Why would you want to do that?

Sourcemaps are the most likely use case. Mappings from original source to generated content are encoded as a sequence of VLQ strings.

What is a VLQ string?

A variable-length quantity is a compact way of encoding large integers in text (i.e. in situations where you can't transmit raw binary data). An integer represented as digits will always take up more space than the equivalent VLQ representation:

Integer	VLQ
0	A
1	C
-1	D
123	2H
123456789	qxmvrH
123456789123456789	gxvh6sB

Installation

npm install vlq

...or...

bower install vlq

...or grab the vlq.js file and include it with a <script src='vlq.js'> tag.

Usage

Encoding

vlq.encode accepts an integer, or an array of integers, and returns a string:

vlq.encode( 123 ); // '2H';
vlq.encode([ 123, 456, 789 ]); // '2HwcqxB'

Decoding

vlq.decode accepts a string and always returns an array:

vlq.decode( '2H' ); // [ 123 ]
vlq.decode( '2HwcqxB' ); // [ 123, 456, 789 ]

Encoding in 8-bit

vlq.encode8Bit accepts an integer and returns a Buffer

vlq.encode8Bit( 0x7f ); // <Buffer 7f>
vlq.encode8Bit( 0x80 ); // <Buffer 81 00>
vlq.encode8Bit( 0x2000 ); // <Buffer c0 00>

Decoding in 8-bit

vlq.decode8Bit accepts a buffer or an array of octets and returns an integer

vlq.decode8Bit( [0x7f] ); // 0x7f
vlq.decode8Bit( [0x81, 0] ); // 0x80
vlq.decode8Bit( [0xc0, 0] ); // 0x2000

Using vlq.js with sourcemaps

See here for an example of using vlq.js with sourcemaps.

Credits

Adapted from murzwin.com/base64vlq.html by Alexander Pavlov.

License

MIT.

jsrmath/vlq