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# @jridgewell/sourcemap-codec
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Encode/decode the `mappings` property of a [sourcemap](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit).
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## Why?
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Sourcemaps are difficult to generate and manipulate, because the `mappings` property – the part that actually links the generated code back to the original source – is encoded using an obscure method called [Variable-length quantity](https://en.wikipedia.org/wiki/Variable-length_quantity). On top of that, each segment in the mapping contains offsets rather than absolute indices, which means that you can't look at a segment in isolation – you have to understand the whole sourcemap.
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This package makes the process slightly easier.
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## Installation
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```bash
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npm install @jridgewell/sourcemap-codec
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```
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## Usage
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```js
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import { encode, decode } from '@jridgewell/sourcemap-codec';
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var decoded = decode( ';EAEEA,EAAE,EAAC,CAAE;ECQY,UACC' );
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assert.deepEqual( decoded, [
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// the first line (of the generated code) has no mappings,
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// as shown by the starting semi-colon (which separates lines)
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[],
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// the second line contains four (comma-separated) segments
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[
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// segments are encoded as you'd expect:
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// [ generatedCodeColumn, sourceIndex, sourceCodeLine, sourceCodeColumn, nameIndex ]
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// i.e. the first segment begins at column 2, and maps back to the second column
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// of the second line (both zero-based) of the 0th source, and uses the 0th
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// name in the `map.names` array
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[ 2, 0, 2, 2, 0 ],
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// the remaining segments are 4-length rather than 5-length,
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// because they don't map a name
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[ 4, 0, 2, 4 ],
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[ 6, 0, 2, 5 ],
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[ 7, 0, 2, 7 ]
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],
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// the final line contains two segments
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[
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[ 2, 1, 10, 19 ],
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[ 12, 1, 11, 20 ]
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]
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]);
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var encoded = encode( decoded );
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assert.equal( encoded, ';EAEEA,EAAE,EAAC,CAAE;ECQY,UACC' );
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```
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## Benchmarks
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```
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node v18.0.0
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amp.js.map - 45120 segments
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Decode Memory Usage:
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@jridgewell/sourcemap-codec 5479160 bytes
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sourcemap-codec 5659336 bytes
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source-map-0.6.1 17144440 bytes
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source-map-0.8.0 6867424 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Decode speed:
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decode: @jridgewell/sourcemap-codec x 502 ops/sec ±1.03% (90 runs sampled)
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decode: sourcemap-codec x 445 ops/sec ±0.97% (92 runs sampled)
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decode: source-map-0.6.1 x 36.01 ops/sec ±1.64% (49 runs sampled)
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decode: source-map-0.8.0 x 367 ops/sec ±0.04% (95 runs sampled)
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Fastest is decode: @jridgewell/sourcemap-codec
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Encode Memory Usage:
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@jridgewell/sourcemap-codec 1261620 bytes
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sourcemap-codec 9119248 bytes
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source-map-0.6.1 8968560 bytes
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source-map-0.8.0 8952952 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Encode speed:
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encode: @jridgewell/sourcemap-codec x 738 ops/sec ±0.42% (98 runs sampled)
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encode: sourcemap-codec x 238 ops/sec ±0.73% (88 runs sampled)
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encode: source-map-0.6.1 x 162 ops/sec ±0.43% (84 runs sampled)
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encode: source-map-0.8.0 x 191 ops/sec ±0.34% (90 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec
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***
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babel.min.js.map - 347793 segments
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Decode Memory Usage:
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@jridgewell/sourcemap-codec 35338184 bytes
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sourcemap-codec 35922736 bytes
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source-map-0.6.1 62366360 bytes
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source-map-0.8.0 44337416 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Decode speed:
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decode: @jridgewell/sourcemap-codec x 40.35 ops/sec ±4.47% (54 runs sampled)
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decode: sourcemap-codec x 36.76 ops/sec ±3.67% (51 runs sampled)
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decode: source-map-0.6.1 x 4.44 ops/sec ±2.15% (16 runs sampled)
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decode: source-map-0.8.0 x 59.35 ops/sec ±0.05% (78 runs sampled)
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Fastest is decode: source-map-0.8.0
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Encode Memory Usage:
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@jridgewell/sourcemap-codec 7212604 bytes
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sourcemap-codec 21421456 bytes
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source-map-0.6.1 25286888 bytes
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source-map-0.8.0 25498744 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Encode speed:
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encode: @jridgewell/sourcemap-codec x 112 ops/sec ±0.13% (84 runs sampled)
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encode: sourcemap-codec x 30.23 ops/sec ±2.76% (53 runs sampled)
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encode: source-map-0.6.1 x 19.43 ops/sec ±3.70% (37 runs sampled)
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encode: source-map-0.8.0 x 19.40 ops/sec ±3.26% (37 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec
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***
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preact.js.map - 1992 segments
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Decode Memory Usage:
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@jridgewell/sourcemap-codec 500272 bytes
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sourcemap-codec 516864 bytes
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source-map-0.6.1 1596672 bytes
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source-map-0.8.0 517272 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Decode speed:
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decode: @jridgewell/sourcemap-codec x 16,137 ops/sec ±0.17% (99 runs sampled)
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decode: sourcemap-codec x 12,139 ops/sec ±0.13% (99 runs sampled)
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decode: source-map-0.6.1 x 1,264 ops/sec ±0.12% (100 runs sampled)
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decode: source-map-0.8.0 x 9,894 ops/sec ±0.08% (101 runs sampled)
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Fastest is decode: @jridgewell/sourcemap-codec
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Encode Memory Usage:
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@jridgewell/sourcemap-codec 321026 bytes
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sourcemap-codec 830832 bytes
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source-map-0.6.1 586608 bytes
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source-map-0.8.0 586680 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Encode speed:
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encode: @jridgewell/sourcemap-codec x 19,876 ops/sec ±0.78% (95 runs sampled)
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encode: sourcemap-codec x 6,983 ops/sec ±0.15% (100 runs sampled)
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encode: source-map-0.6.1 x 5,070 ops/sec ±0.12% (102 runs sampled)
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encode: source-map-0.8.0 x 5,641 ops/sec ±0.17% (100 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec
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***
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react.js.map - 5726 segments
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Decode Memory Usage:
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@jridgewell/sourcemap-codec 734848 bytes
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sourcemap-codec 954200 bytes
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source-map-0.6.1 2276432 bytes
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source-map-0.8.0 955488 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Decode speed:
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decode: @jridgewell/sourcemap-codec x 5,723 ops/sec ±0.12% (98 runs sampled)
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decode: sourcemap-codec x 4,555 ops/sec ±0.09% (101 runs sampled)
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decode: source-map-0.6.1 x 437 ops/sec ±0.11% (93 runs sampled)
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decode: source-map-0.8.0 x 3,441 ops/sec ±0.15% (100 runs sampled)
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Fastest is decode: @jridgewell/sourcemap-codec
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Encode Memory Usage:
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@jridgewell/sourcemap-codec 638672 bytes
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sourcemap-codec 1109840 bytes
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source-map-0.6.1 1321224 bytes
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source-map-0.8.0 1324448 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec
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Encode speed:
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encode: @jridgewell/sourcemap-codec x 6,801 ops/sec ±0.48% (98 runs sampled)
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encode: sourcemap-codec x 2,533 ops/sec ±0.13% (101 runs sampled)
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encode: source-map-0.6.1 x 2,248 ops/sec ±0.08% (100 runs sampled)
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encode: source-map-0.8.0 x 2,303 ops/sec ±0.15% (100 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec
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```
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# License
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MIT
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