Codec for VLQ (variable-length quantity) Base64 algorithm
npm install axy-sourcemap-base64vlqCodec for VLQ (variable-length quantity) Base64 algorithm (Node.js).
* GitHub: axyjs/axy-sourcemap-base64vlq
* NPM: axy-sourcemap-base64vlq
* LICENSE: MIT
* Author: Oleg Grigoriev
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.
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.
``
npm install axy-sourcemap-base64vlq
`javascript
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]
``