fast-source-map

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fast-source-map

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# Fast Source Map [![Build Status](https://travis-ci.org/krisselden/fast-source-map.svg?branch=master)](https://travis-ci.org/krisselden/fast-source-map)

Keeping the JIT gods happy since circa 2015

When working with sourcemaps, VLQ encoding and decoding typically dominiates
cost. To address this, this library provides a Fast VLQ encoder & decoder for
JavaScript.  By reducing allocations, minimizing transformations, and writing
JIT friendly code we are able to drammatically improve performance.

How much faster? At the time this README was written, decoding scala.js.map took:

```
v8: 4.6.85.28
node: 5.0.0
os: 10.10.3
hw: 2.5 GHz Intel Core i7, 16gb
```

* source-map: ~5,000ms
* fast-source-map ~ 500ms

# Usage

First we will need to have our `sourceMap.mappings` stored as a buffer. We have
future plans to work directly on a sourceMap buffer, as that will allow for
more ideal performance. Even without, the performance improvements are still
well worth it.

```js
var buffer = /* try to leave the map inert, and in buffer form. Otherwise convert to buffer */
```

Typically, the following steps are required:

Setup the reader

The reader can read mappings from any array of ascii values.
We will use Uint8Array in this example.

```js
let map = JSON.parse(fs.readFileSync('path/to/source/to/decode.js.map', 'utf8'));

let byteArray = new Uint8Array(map.mappings.length);
let buffer = Buffer.from(byteArray.buffer);
buffer.write(map.mappings, 0, 'ascii');

const SM = require("fast-source-map");
let reader = new SM.IntBufferReader(byteArray, 0, byteArray.length);
```

Setup the decoder

```js
let decoder = new SM.Decoder();
let mappingsDecoder = new SM.MappingsDecoder(decoder);
```

Now for some actual decoding

```js
mappingsDecoder.decode(reader);
decoder.mappings // => is the quickly decoded
```

To concatenate multiple source maps

```js
const decodeFile = SM.decodeFile;

let concatenator = new SM.SourceMapConcatenator();
concatenator.push(decodeFile('path/to/file-1.js.map'));
concatenator.push(decodeFile('path/to/file-2.js.map'));
concatenator.push(decodeFile('path/to/file-3.js.map'));

concatenator.toJSON(); // => the concatenated source maps
```

Now to reconcatenate the source maps after a source file is removed.

```js
// file-2.js.map is removed
concatenator.splice(1, 1);
concatenator.toJSON(); // => the concatenated source maps
```

And reconcatenate again after adding back other files.

```js
// an updated file 2 and a new file 4 are added
concatenator.splice(1, 0, decodeFile('path/to/file-2-updated.js.map'));
concatenator.push(decodeFile('path/to/file-4.js.map'));
concatenator.toJSON(); // => the concatenated source maps
```

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