axy-sourcemap-base64vlq

Codec for VLQ (variable-length quantity) Base64 algorithm (Node.js).

• GitHub: axyjs/axy-sourcemap-base64vlq
• NPM: axy-sourcemap-base64vlq
• LICENSE: MIT
• Author: Oleg Grigoriev

VQL + Base64

Base64 allows us to represent a sequence of numbers in a text string that can be stored and transmit in text formats (JSON, XML, etc).

VLQ allows us to represent an integer in a sequence numbers with little digit capacity. For example 6 bit is the limit for Base64. The resulting numbers are called "VLQ digits". Small input number is represented by fewer VLQ-digits than big number. Thus VLQ is most effective if the input sequence is contains mainly the small numbers.

VLQ+Base64 allows us effectively represented a sequence of integers (dominated by a small number of) in the text format.

For example, it used in JavaScript/CSS source map.

The Algorithm

For example, we have a block of numbers: [12345, -12345, 0].

(1). VLQ only works with unsigned integers. Transfer the sign bit to the end of the integer.

12345 in binary is 11000000111001. Added 0 (positive) to the end: 110000001110010.

For -12345 take a positive form and add 1 (negative) to the end: 110000001110011.

Result is [110000001110010, 110000001110011, 0].

(2). Transform to the VLQ-sequence. For Base64 we need a block of 6-bit numbers. Most significant bit is reserved - it is "continuation".

Split numbers to groups of 5 bits: [11000 00011 10010, 11000 00011 10011, 00000]. Output starting from the least significant bits. If the group is not the last in the current number then set the continuation bit.

Result: [110010 100011 011000 110011 100011 011000 000000]. Or decimal [50, 35, 24, 51, 35, 24, 0]. These are VLQ digits.

(3). Replace the numbers on the letters of the Base64-alphabet. The standard alphabet is A..Za..z0..9+/.

Result is yjYzjYA.

How to use

npm install axy-sourcemap-base64vlq

var encoder = require("axy-sourcemap-base64vlq");
 
encoder.encode([12345, -12345, 0]); // yjYzjYA
encoder.decode("yjYzjYA"); //  [12345, 012345, 9]
 
encoder.decode("Variable+Length+QuantitY"); // [-10, 13, -13349, -13 ... -12797139]