# axy-sourcemap-base64vlq

> Codec for VLQ (variable-length quantity) Base64 algorithm

Latest version **0.1.1** (published 2015-04-10) · MIT license · 0 weekly downloads

## Install

```sh
npm install axy-sourcemap-base64vlq
pnpm add axy-sourcemap-base64vlq
yarn add axy-sourcemap-base64vlq
bun add axy-sourcemap-base64vlq
```

## Health

**Score 15/100 (F)** — status: abandoned.

Positive: no vulnerabilities.

Warnings: low downloads; no types; no esm support; pre 1.0.

Negative: abandoned; low maintenance score.

## Facts

| | |
|---|---|
| Version | 0.1.1 |
| Published | 2015-04-10 |
| First published | 2015-04-10 |
| Weekly downloads | 0 |
| License | MIT |
| TypeScript types | none |
| Module format | CommonJS |
| Dependencies | 0 |
| Known vulnerabilities | 0 |
| Install scripts | no |
| GitHub stars | 0 |
| Author | Oleg Grigoriev |
| Maintainers | vasa-c |
| Keywords | Base64, VLQ, Source Map |

## Links

- npm: https://www.npmjs.com/package/axy-sourcemap-base64vlq
- Repository: https://github.com/axyjs/axy-sourcemap-base64vlq
- Issues: https://github.com/axyjs/axy-sourcemap-base64vlq/issues
- npm.io page: https://npm.io/package/axy-sourcemap-base64vlq

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## Recent versions

- 0.1.1 (latest) — 2015-04-10

## README

# axy-sourcemap-base64vlq

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

* GitHub: [axyjs/axy-sourcemap-base64vlq](https://github.com/axyjs/axy-sourcemap-base64vlq)
* NPM: [axy-sourcemap-base64vlq](https://www.npmjs.com/package/axy-sourcemap-base64vlq)
* LICENSE: [MIT](LICENSE)
* 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](https://www.google.ru/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&sqi=2&ved=0CBwQFjAA&url=https%3A%2F%2Fdocs.google.com%2Fdocument%2Fd%2F1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k%2Fedit&ei=abnpVKTZKeaHygPs0oK4BA&usg=AFQjCNFpOFA_dC_8cB50KJ1dXbOH7pvJnA&bvm=bv.86475890,d.bGQ).

## 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
```

```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]
```

---
_Source: https://npm.io/package/axy-sourcemap-base64vlq · Machine-readable twin of the npm.io package page. Health data is recomputed on every publish._