@elsassph/fast-source-map
v0.3.0
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fast source mapping decode/encode
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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.
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.
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
let decoder = new SM.Decoder();
let mappingsDecoder = new SM.MappingsDecoder(decoder);
Now for some actual decoding
mappingsDecoder.decode(reader);
decoder.mappings // => is the quickly decoded
To concatenate multiple source maps
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.
// file-2.js.map is removed
concatenator.splice(1, 1);
concatenator.toJSON(); // => the concatenated source maps
And reconcatenate again after adding back other files.
// 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