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robhogan wants to merge 2 commits into from
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Source map consumer: Decode VLQ mappings in a single pass #1979

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de75d07
Consumer: Decode VLQ mappings in a single pass
robhogan Sep 26, 2026
bd8a725
Change right shift operator to zero-fill right shift
robhogan Sep 28, 2026
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3 changes: 1 addition & 2 deletions packages/metro-source-map/package.json
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Original file line number Diff line number Diff line change
Expand Up @@ -25,8 +25,7 @@
"metro-symbolicate": "0.87.1",
"nullthrows": "^1.1.1",
"ob1": "0.87.1",
"source-map": "^0.5.6",
"vlq": "^1.0.0"

@robhogan robhogan Sep 28, 2026 •
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This is safe to merge without Meta import because even though we're dropping a dependency here, metro-symbolicate has the same vlq dependency, so no lockfile change.

(This code snippet makes it look like we're dropping two deps - we're not, source-map just loses the trailing comma)

"source-map": "^0.5.6"
},
"license": "MIT",
"devDependencies": {
Expand Down
152 changes: 86 additions & 66 deletions packages/metro-source-map/src/Consumer/MappingsConsumer.js
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Expand Up @@ -30,7 +30,23 @@ import normalizeSourcePath from './normalizeSourcePath';
import {greatestLowerBound} from './search';
import invariant from 'invariant';
import {add, add0, get0, inc, sub} from 'ob1';
import {decode as decodeVlq} from 'vlq';

/* eslint-disable no-bitwise */

const COMMA = 44; // ','
const SEMICOLON = 59; // ';'
const VLQ_BASE_SHIFT = 5;
const VLQ_BASE_MASK = (1 << VLQ_BASE_SHIFT) - 1;
const VLQ_CONTINUATION_BIT = 1 << VLQ_BASE_SHIFT;
const BASE64_DECODE: Int8Array = (() => {
const table = new Int8Array(128).fill(-1);
const chars =
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
for (let i = 0; i < chars.length; i++) {
table[chars.charCodeAt(i)] = i;
}
return table;
})();

/**
* A source map consumer that supports "basic" source maps (that have a
Expand Down Expand Up @@ -93,85 +109,89 @@ export default class MappingsConsumer
return {...EMPTY_POSITION};
}

*_decodeMappings(): Generator<Mapping, void, void> {
_decodeMappings(): Array<Mapping> {
const normalizedSources = this._normalizeAndCacheSources();
const {mappings: mappingsRaw, names} = this._sourceMap;
const result: Array<Mapping> = [];

let generatedLine = FIRST_LINE;
let generatedColumn = FIRST_COLUMN;
let originalLine = FIRST_LINE;
let originalColumn = FIRST_COLUMN;
let nameIndex = add0(0);
let sourceIndex = add0(0);

const normalizedSources = this._normalizeAndCacheSources();

const {mappings: mappingsRaw, names} = this._sourceMap;
let next;
const vlqCache = new Map<string, any>();
for (let i = 0; i < mappingsRaw.length; i = next) {
switch (mappingsRaw[i]) {
case ';':
// The VLQ fields of the segment being decoded, decoded in place from
// character codes rather than by slicing out each segment.
const fields = [0, 0, 0, 0, 0];
let fieldCount = 0;
let value = 0;
let shift = 0;

const length = mappingsRaw.length;
for (let i = 0; i <= length; i++) {
const charCode = i < length ? mappingsRaw.charCodeAt(i) : SEMICOLON;
if (charCode === COMMA || charCode === SEMICOLON) {
// A trailing field cut off mid-VLQ is dropped, as `vlq` does.
invariant(
fieldCount > 0 || shift === 0,
'Invalid generated column delta',
);
value = 0;
shift = 0;
if (fieldCount > 0) {
invariant(fieldCount !== 2, 'Invalid original line delta');
invariant(fieldCount !== 3, 'Invalid original column delta');
generatedColumn = add(generatedColumn, fields[0]);
let source = null;
let name = null;
let mappingOriginalLine = null;
let mappingOriginalColumn = null;
if (fieldCount >= 4) {
sourceIndex = add(sourceIndex, fields[1]);
source = normalizedSources[get0(sourceIndex)];
originalLine = add(originalLine, fields[2]);
originalColumn = add(originalColumn, fields[3]);
mappingOriginalLine = originalLine;
mappingOriginalColumn = originalColumn;
if (fieldCount >= 5) {
nameIndex = add(nameIndex, fields[4]);
name = names[get0(nameIndex)];
}
}
result.push({
generatedLine,
generatedColumn,
source,
name,
originalLine: mappingOriginalLine,
originalColumn: mappingOriginalColumn,
});
fieldCount = 0;
}
if (charCode === SEMICOLON && i < length) {
generatedLine = inc(generatedLine);
generatedColumn = FIRST_COLUMN;
/* falls through */
case ',':
next = i + 1;
continue;
}
findNext: for (next = i + 1; next < mappingsRaw.length; ++next) {
switch (mappingsRaw[next]) {
case ';':
/* falls through */
case ',':
break findNext;
}
continue;
}
const mappingRaw = mappingsRaw.slice(i, next);
let decodedVlqValues;
if (vlqCache.has(mappingRaw)) {
decodedVlqValues = vlqCache.get(mappingRaw);
const digit = charCode < 128 ? BASE64_DECODE[charCode] : -1;
invariant(digit !== -1, 'Invalid character in source map mappings');
value = value + ((digit & VLQ_BASE_MASK) << shift);
if (digit & VLQ_CONTINUATION_BIT) {
shift = shift + VLQ_BASE_SHIFT;
} else {
decodedVlqValues = decodeVlq(mappingRaw);
vlqCache.set(mappingRaw, decodedVlqValues);
}
invariant(Array.isArray(decodedVlqValues), 'Decoding VLQ tuple failed');
const [
generatedColumnDelta,
sourceIndexDelta,
originalLineDelta,
originalColumnDelta,
nameIndexDelta,
] = decodedVlqValues;
invariant(generatedColumnDelta != null, 'Invalid generated column delta');
generatedColumn = add(generatedColumn, generatedColumnDelta);
const mapping: {...Mapping, ...} = {
generatedLine,
generatedColumn,
source: null,
name: null,
originalLine: null,
originalColumn: null,
};

if (sourceIndexDelta != null) {
sourceIndex = add(sourceIndex, sourceIndexDelta);
mapping.source = normalizedSources[get0(sourceIndex)];

invariant(originalLineDelta != null, 'Invalid original line delta');
invariant(originalColumnDelta != null, 'Invalid original column delta');

originalLine = add(originalLine, originalLineDelta);
originalColumn = add(originalColumn, originalColumnDelta);

mapping.originalLine = originalLine;
mapping.originalColumn = originalColumn;

if (nameIndexDelta != null) {
nameIndex = add(nameIndex, nameIndexDelta);
mapping.name = names[get0(nameIndex)];
const negate = value & 1;
value = value >>> 1;
if (fieldCount < 5) {
fields[fieldCount] = negate ? -value : value;
}
fieldCount++;
value = 0;
shift = 0;
}

yield mapping;
}
return result;
}

_normalizeAndCacheSources(): ReadonlyArray<string> {
Expand All @@ -185,7 +205,7 @@ export default class MappingsConsumer

_decodeAndCacheMappings(): ReadonlyArray<Mapping> {
if (!this._decodedMappings) {
this._decodedMappings = [...this._decodeMappings()];
this._decodedMappings = this._decodeMappings();
}
return this._decodedMappings;
}
Expand Down
58 changes: 58 additions & 0 deletions packages/metro-source-map/src/__tests__/Consumer-test.js
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Expand Up @@ -217,6 +217,64 @@ describe('basic maps', () => {
]
`);
});

test('multi-digit and negative deltas, empty segments and lines', () => {
expect([
...new Consumer({
version: 3,
mappings: 'AAAU,gBCoBHC,,w+B;;KDjBg9D',
names: ['name0', 'name1'],
sources: ['source0', 'source1'],
}).generatedMappings(),
]).toEqual([
{
generatedLine: 1,
generatedColumn: 0,
source: 'source0',
name: null,
originalLine: 1,
originalColumn: 10,
},
{
generatedLine: 1,
generatedColumn: 16,
source: 'source1',
name: 'name1',
originalLine: 21,
originalColumn: 7,
},
{
generatedLine: 1,
generatedColumn: 1016,
source: null,
name: null,
originalLine: null,
originalColumn: null,
},
{
generatedLine: 3,
generatedColumn: 5,
source: 'source0',
name: null,
originalLine: 4,
originalColumn: 2007,
},
]);
});

test.each([
['AA', 'Invalid original line delta'],
['AAAA,AAA', 'Invalid original column delta'],
['AAAA,A!AA', undefined],
])('throws on malformed mappings %s', (mappings, message) => {
const consumer = new Consumer({
version: 3,
mappings,
names: [],
sources: ['source0'],
});
expect(() => [...consumer.generatedMappings()]).toThrow(message);
});
});

describe('sourceContentFor', () => {
Expand Down
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