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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 v20.10.0
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amp.js.map - 45120 segments
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Decode Memory Usage:
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local code 5815135 bytes
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@jridgewell/sourcemap-codec 1.4.15 5868160 bytes
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sourcemap-codec 5492584 bytes
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source-map-0.6.1 13569984 bytes
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source-map-0.8.0 6390584 bytes
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chrome dev tools 8011136 bytes
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Smallest memory usage is sourcemap-codec
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Decode speed:
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decode: local code x 492 ops/sec ±1.22% (90 runs sampled)
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decode: @jridgewell/sourcemap-codec 1.4.15 x 499 ops/sec ±1.16% (89 runs sampled)
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decode: sourcemap-codec x 376 ops/sec ±1.66% (89 runs sampled)
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decode: source-map-0.6.1 x 34.99 ops/sec ±0.94% (48 runs sampled)
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decode: source-map-0.8.0 x 351 ops/sec ±0.07% (95 runs sampled)
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chrome dev tools x 165 ops/sec ±0.91% (86 runs sampled)
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Fastest is decode: @jridgewell/sourcemap-codec 1.4.15
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Encode Memory Usage:
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local code 444248 bytes
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@jridgewell/sourcemap-codec 1.4.15 623024 bytes
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sourcemap-codec 8696280 bytes
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source-map-0.6.1 8745176 bytes
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source-map-0.8.0 8736624 bytes
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Smallest memory usage is local code
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Encode speed:
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encode: local code x 796 ops/sec ±0.11% (97 runs sampled)
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encode: @jridgewell/sourcemap-codec 1.4.15 x 795 ops/sec ±0.25% (98 runs sampled)
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encode: sourcemap-codec x 231 ops/sec ±0.83% (86 runs sampled)
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encode: source-map-0.6.1 x 166 ops/sec ±0.57% (86 runs sampled)
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encode: source-map-0.8.0 x 203 ops/sec ±0.45% (88 runs sampled)
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Fastest is encode: local code,encode: @jridgewell/sourcemap-codec 1.4.15
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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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local code 35424960 bytes
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@jridgewell/sourcemap-codec 1.4.15 35424696 bytes
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sourcemap-codec 36033464 bytes
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source-map-0.6.1 62253704 bytes
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source-map-0.8.0 43843920 bytes
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chrome dev tools 45111400 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec 1.4.15
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Decode speed:
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decode: local code x 38.18 ops/sec ±5.44% (52 runs sampled)
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decode: @jridgewell/sourcemap-codec 1.4.15 x 38.36 ops/sec ±5.02% (52 runs sampled)
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decode: sourcemap-codec x 34.05 ops/sec ±4.45% (47 runs sampled)
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decode: source-map-0.6.1 x 4.31 ops/sec ±2.76% (15 runs sampled)
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decode: source-map-0.8.0 x 55.60 ops/sec ±0.13% (73 runs sampled)
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chrome dev tools x 16.94 ops/sec ±3.78% (46 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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local code 2606016 bytes
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@jridgewell/sourcemap-codec 1.4.15 2626440 bytes
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sourcemap-codec 21152576 bytes
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source-map-0.6.1 25023928 bytes
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source-map-0.8.0 25256448 bytes
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Smallest memory usage is local code
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Encode speed:
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encode: local code x 127 ops/sec ±0.18% (83 runs sampled)
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encode: @jridgewell/sourcemap-codec 1.4.15 x 128 ops/sec ±0.26% (83 runs sampled)
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encode: sourcemap-codec x 29.31 ops/sec ±2.55% (53 runs sampled)
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encode: source-map-0.6.1 x 18.85 ops/sec ±3.19% (36 runs sampled)
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encode: source-map-0.8.0 x 19.34 ops/sec ±1.97% (36 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec 1.4.15
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***
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preact.js.map - 1992 segments
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Decode Memory Usage:
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local code 261696 bytes
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@jridgewell/sourcemap-codec 1.4.15 244296 bytes
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sourcemap-codec 302816 bytes
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source-map-0.6.1 939176 bytes
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source-map-0.8.0 336 bytes
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chrome dev tools 587368 bytes
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Smallest memory usage is source-map-0.8.0
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Decode speed:
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decode: local code x 17,782 ops/sec ±0.32% (97 runs sampled)
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decode: @jridgewell/sourcemap-codec 1.4.15 x 17,863 ops/sec ±0.40% (100 runs sampled)
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decode: sourcemap-codec x 12,453 ops/sec ±0.27% (101 runs sampled)
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decode: source-map-0.6.1 x 1,288 ops/sec ±1.05% (96 runs sampled)
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decode: source-map-0.8.0 x 9,289 ops/sec ±0.27% (101 runs sampled)
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chrome dev tools x 4,769 ops/sec ±0.18% (100 runs sampled)
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Fastest is decode: @jridgewell/sourcemap-codec 1.4.15
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Encode Memory Usage:
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local code 262944 bytes
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@jridgewell/sourcemap-codec 1.4.15 25544 bytes
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sourcemap-codec 323048 bytes
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source-map-0.6.1 507808 bytes
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source-map-0.8.0 507480 bytes
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Smallest memory usage is @jridgewell/sourcemap-codec 1.4.15
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Encode speed:
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encode: local code x 24,207 ops/sec ±0.79% (95 runs sampled)
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encode: @jridgewell/sourcemap-codec 1.4.15 x 24,288 ops/sec ±0.48% (96 runs sampled)
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encode: sourcemap-codec x 6,761 ops/sec ±0.21% (100 runs sampled)
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encode: source-map-0.6.1 x 5,374 ops/sec ±0.17% (99 runs sampled)
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encode: source-map-0.8.0 x 5,633 ops/sec ±0.32% (99 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec 1.4.15,encode: local code
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***
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react.js.map - 5726 segments
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Decode Memory Usage:
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local code 678816 bytes
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@jridgewell/sourcemap-codec 1.4.15 678816 bytes
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sourcemap-codec 816400 bytes
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source-map-0.6.1 2288864 bytes
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source-map-0.8.0 721360 bytes
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chrome dev tools 1012512 bytes
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Smallest memory usage is local code
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Decode speed:
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decode: local code x 6,178 ops/sec ±0.19% (98 runs sampled)
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decode: @jridgewell/sourcemap-codec 1.4.15 x 6,261 ops/sec ±0.22% (100 runs sampled)
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decode: sourcemap-codec x 4,472 ops/sec ±0.90% (99 runs sampled)
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decode: source-map-0.6.1 x 449 ops/sec ±0.31% (95 runs sampled)
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decode: source-map-0.8.0 x 3,219 ops/sec ±0.13% (100 runs sampled)
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chrome dev tools x 1,743 ops/sec ±0.20% (99 runs sampled)
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Fastest is decode: @jridgewell/sourcemap-codec 1.4.15
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Encode Memory Usage:
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local code 140960 bytes
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@jridgewell/sourcemap-codec 1.4.15 159808 bytes
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sourcemap-codec 969304 bytes
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source-map-0.6.1 930520 bytes
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source-map-0.8.0 930248 bytes
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Smallest memory usage is local code
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Encode speed:
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encode: local code x 8,013 ops/sec ±0.19% (100 runs sampled)
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encode: @jridgewell/sourcemap-codec 1.4.15 x 7,989 ops/sec ±0.20% (101 runs sampled)
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encode: sourcemap-codec x 2,472 ops/sec ±0.21% (99 runs sampled)
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encode: source-map-0.6.1 x 2,200 ops/sec ±0.17% (99 runs sampled)
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encode: source-map-0.8.0 x 2,220 ops/sec ±0.37% (99 runs sampled)
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Fastest is encode: local code
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***
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vscode.map - 2141001 segments
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Decode Memory Usage:
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local code 198955264 bytes
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@jridgewell/sourcemap-codec 1.4.15 199175352 bytes
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sourcemap-codec 199102688 bytes
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source-map-0.6.1 386323432 bytes
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source-map-0.8.0 244116432 bytes
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chrome dev tools 293734280 bytes
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Smallest memory usage is local code
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Decode speed:
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decode: local code x 3.90 ops/sec ±22.21% (15 runs sampled)
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decode: @jridgewell/sourcemap-codec 1.4.15 x 3.95 ops/sec ±23.53% (15 runs sampled)
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decode: sourcemap-codec x 3.82 ops/sec ±17.94% (14 runs sampled)
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decode: source-map-0.6.1 x 0.61 ops/sec ±7.81% (6 runs sampled)
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decode: source-map-0.8.0 x 9.54 ops/sec ±0.28% (28 runs sampled)
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chrome dev tools x 2.18 ops/sec ±10.58% (10 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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local code 13509880 bytes
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@jridgewell/sourcemap-codec 1.4.15 13537648 bytes
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sourcemap-codec 32540104 bytes
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source-map-0.6.1 127531040 bytes
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source-map-0.8.0 127535312 bytes
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Smallest memory usage is local code
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Encode speed:
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encode: local code x 20.10 ops/sec ±0.19% (38 runs sampled)
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encode: @jridgewell/sourcemap-codec 1.4.15 x 20.26 ops/sec ±0.32% (38 runs sampled)
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encode: sourcemap-codec x 5.44 ops/sec ±1.64% (18 runs sampled)
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encode: source-map-0.6.1 x 2.30 ops/sec ±4.79% (10 runs sampled)
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encode: source-map-0.8.0 x 2.46 ops/sec ±6.53% (10 runs sampled)
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Fastest is encode: @jridgewell/sourcemap-codec 1.4.15
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```
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# License
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MIT
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