chrome-devtools-frontend/front_end/core/sdk/SourceMapRangeMappings.ts

Chrome DevTools UI

// Copyright 2026 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import {TokenIterator} from './SourceMap.js';

/**
 * Implements decoding of the `rangeMappings` field of the "range mappings" proposal.
 *
 * The field holds one entry per line of the generated code, separated by `;`. Each line is
 * a bare sequence of unsigned Base64 VLQs (no separators in between) denoting which of the
 * mappings on that line are range mappings: the first VLQ is an absolute index into the
 * line's mappings, every subsequent one an offset from the previous index. An offset of zero
 * repeats the previous index, which is tolerated rather than treated as an error.
 *
 * @returns for every line of the generated code, the sorted indices of the mappings on
 *          that line which are range mappings.
 * @throws if the field is not a well-formed sequence of unsigned VLQs and `;` separators.
 * @see https://github.com/tc39/source-map/blob/main/proposals/range-mappings.md
 */
export function decodeRangeMappings(encodedRangeMappings: string): number[][] {
  const rangeMappings: number[][] = [];
  const tokenIter = new TokenIterator(encodedRangeMappings);

  let indices: number[] = [];
  let currentIndex = 0;
  while (tokenIter.hasNext()) {
    if (tokenIter.peek() === ';') {
      tokenIter.next();
      rangeMappings.push(indices);
      indices = [];
      currentIndex = 0;
      continue;
    }

    // The first index of a line is absolute, i.e. relative to 0.
    currentIndex += tokenIter.nextUnsignedVLQ();
    indices.push(currentIndex);
  }
  rangeMappings.push(indices);

  return rangeMappings;
}