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[markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer - #2748

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markdown_unique_heading_id_suffix
Oct 29, 2026
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[markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer#2748
jonasfj merged 4 commits into
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markdown_unique_heading_id_suffix

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@jonasfj jonasfj commented Oct 29, 2026

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Cache the next candidate suffix for each duplicate heading base ID in
HtmlRenderer._lastSuffix (and clear _lastSuffix at the start of
HtmlRenderer.render).

Why this change is necessary

When HeaderWithIdSyntax is enabled (such as in ExtensionSet.gitHubWeb),
each heading Element has a generatedId derived from its text, and
HtmlRenderer.visitElementBefore calls uniquifyId(generatedId) so duplicate
headings receive IDs id, id-1, id-2, etc.

Previously, HtmlRenderer.uniquifyId always initialized var suffix = 1; and
incremented suffix in a while (uniqueIds.contains('$id-$suffix')) loop
starting from 1. On a document with n identical headings (such as
'# a\n' * n):

  • the 2nd heading probed a-1 (1 lookup);
  • the 3rd heading probed a-1, a-2 (2 lookups);
  • the k-th heading probed a-1, a-2, ..., a-(k-1) (k - 1 lookups).
    Across n duplicate headings, this performed 1 + 2 + ... + (n - 1) = O(n^2)
    string allocations and hash-set lookups.

How this change works

  1. HtmlRenderer maintains final _lastSuffix = <String, int>{}; mapping each
    base id to the next suffix integer to try after the most recently assigned
    suffix for id.
  2. In uniquifyId(String id):
    • If id itself is not in uniqueIds, it is added and returned immediately.
    • Otherwise suffix starts at _lastSuffix[id] ?? 1 and probes
      $id-$suffix using ++suffix on collision (which also avoids probing the
      initial $id-$suffix twice when the first candidate collides and keeps
      suffix equal to the assigned suffix when the loop exits).
    • Because every suffix 1 <= j < (_lastSuffix[id] ?? 1) was already present
      in uniqueIds when _lastSuffix[id] was recorded, starting at
      _lastSuffix[id] ?? 1 never skips an available smaller suffix (even if
      explicit headings like # a-2 appeared earlier).
    • Once an unused $id-$suffix is found, _lastSuffix[id] = suffix + 1
      records the next candidate suffix so each suffix for id is tested at most
      once, making uniquifyId O(1) amortized per heading.
  3. HtmlRenderer.render clears _lastSuffix.clear() alongside
    uniqueIds.clear() so reusing an HtmlRenderer instance across multiple
    render() calls starts fresh on each call.

@jonasfj
[markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer
aff7c1f 
Cache the next candidate suffix for each duplicate heading base ID in
`HtmlRenderer._lastSuffix` (and clear `_lastSuffix` at the start of
`HtmlRenderer.render`).

----

### Why this change is necessary

When `HeaderWithIdSyntax` is enabled (such as in `ExtensionSet.gitHubWeb`),
each heading `Element` has a `generatedId` derived from its text, and
`HtmlRenderer.visitElementBefore` calls `uniquifyId(generatedId)` so duplicate
headings receive IDs `id`, `id-1`, `id-2`, etc.

Previously, `HtmlRenderer.uniquifyId` always initialized `var suffix = 1;` and
incremented `suffix` in a `while (uniqueIds.contains('$id-$suffix'))` loop
starting from `1`. On a document with n identical headings (such as
`'# a\n' * n`):
- the 2nd heading probed `a-1` (1 lookup);
- the 3rd heading probed `a-1`, `a-2` (2 lookups);
- the k-th heading probed `a-1`, `a-2`, ..., `a-(k-1)` (`k - 1` lookups).
Across n duplicate headings, this performed 1 + 2 + ... + (n - 1) = O(n^2)
string allocations and hash-set lookups.

### How this change works

1. `HtmlRenderer` maintains `final _lastSuffix = <String, int>{};` mapping each
   base `id` to the next suffix integer to try after the most recently assigned
   suffix for `id`.
2. In `uniquifyId(String id)`:
   - If `id` itself is not in `uniqueIds`, it is added and returned immediately.
   - Otherwise `suffix` starts at `_lastSuffix[id] ?? 1` and probes
     `$id-$suffix` using `++suffix` on collision (which also avoids probing the
     initial `$id-$suffix` twice when the first candidate collides and keeps
     `suffix` equal to the assigned suffix when the loop exits).
   - Because every suffix `1 <= j < (_lastSuffix[id] ?? 1)` was already present
     in `uniqueIds` when `_lastSuffix[id]` was recorded, starting at
     `_lastSuffix[id] ?? 1` never skips an available smaller suffix (even if
     explicit headings like `# a-2` appeared earlier).
   - Once an unused `$id-$suffix` is found, `_lastSuffix[id] = suffix + 1`
     records the next candidate suffix so each suffix for `id` is tested at most
     once, making `uniquifyId` O(1) amortized per heading.
3. `HtmlRenderer.render` clears `_lastSuffix.clear()` alongside
   `uniqueIds.clear()` so reusing an `HtmlRenderer` instance across multiple
   `render()` calls starts fresh on each call.
@jonasfj
jonasfj requested a review from srawlins as a code owner October 29, 2026 12:58
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Code Review

This pull request optimizes the HtmlRenderer.uniquifyId method in pkgs/markdown/lib/src/html_renderer.dart by introducing a _lastSuffix map to track the last used suffix for duplicate heading IDs, avoiding redundant probes starting from 2. The _lastSuffix map is cleared in the render method, and a corresponding entry has been added to the changelog. There are no review comments to address, and the changes look correct.

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Breaking changes ✔️
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markdown Non-Breaking 7.3.1 7.4.1-wip 7.4.1-wip ✔️

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pkgs/markdown/lib/src/html_renderer.dart 💚 99 %

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
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pkgs/html/test/differential_test.dart
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@jonasfj
jonasfj merged commit cac1421 into main Oct 29, 2026
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@jonasfj
jonasfj deleted the markdown_unique_heading_id_suffix branch October 29, 2026 13:17
copybara-service Bot pushed a commit to dart-lang/sdk that referenced this pull request Nov 3, 2026
@natebosch dart-scoped@luci-project-accounts.iam.gserviceaccount.com
[deps] rev core, ecosystem, http, i18n, shelf, test, tools
c48ebb8 
Revisions updated by `dart tools/rev_sdk_deps.dart`.

core (https://github.com/dart-lang/core/compare/d7bd45c..24e09c4):
  24e09c4c  Sun Nov 1 21:12:19 2026 +0000  dependabot[bot]  Bump the github-actions group with 4 updates (dart-lang/core#1037)
  87ac1b4c  Fri Oct 30 16:02:43 2026 +0100  Michael Thomsen  [os_detect] Deprecate package:os_detect in favor of package:platform (dart-lang/core#1003)
  8debf335  Fri Oct 30 01:57:36 2026 +0530  Sahaja Pallapothula  Clarify Characters equality and Unicode normalization (dart-lang/core#1031)

ecosystem (https://github.com/dart-lang/ecosystem/compare/24b9ba8..f8faabf):
  f8faabf  Sun Nov 1 18:16:20 2026 +0000  dependabot[bot]  Bump the github-actions group with 2 updates (dart-lang/ecosystem#481)
  27c2fc8  Sat Oct 31 12:36:53 2026 +0100  Moritz  Fix publish workflow pins, add tag_prefix to health, and update docs (dart-lang/ecosystem#480)
  05641a9  Sat Oct 31 09:56:42 2026 +0100  Moritz  Restore deprecated publish.yaml workflow and firehose CLI entrypoint (dart-lang/ecosystem#479)
  78abf72  Fri Oct 30 08:24:07 2026 -0700  Kevin Moore  fix(repo_manage): use pub_semver for changelog -wip transitions (dart-lang/ecosystem#475)
  0bfa654  Fri Oct 30 09:33:57 2026 +0100  Moritz  Move publish check to health workflow and use individual package publishing workflows from `setup-dart` (dart-lang/ecosystem#454)
  adf7979  Thu Oct 29 23:46:49 2026 -0700  Kevin Moore  test(firehose): use hasLength, containsPair, and isFalse (dart-lang/ecosystem#477)

http (https://github.com/dart-lang/http/compare/c76dfb3..8c0d66a):
  8c0d66a  Fri Oct 30 09:54:41 2026 -0700  Brian Quinlan  chore(cupertino_http): check ffigen generation (dart-lang/http#2044)
  c8737d1  Fri Oct 30 09:11:46 2026 +0100  Moritz  chore(http): run Firefox tests with -headless instead of xvfb-run (dart-lang/http#2033)
  05d0b18  Fri Oct 30 07:08:19 2026 +0100  Moritz  fix(http2): make StreamException a StreamTransportException and forward ALPN errors to onError (dart-lang/http#2009)

i18n (https://github.com/dart-lang/i18n/compare/4c7b4ad..7475ce6):
  7475ce64  Sat Oct 31 02:49:28 2026 +0100  Moritz  [intl4x] Fix NumberFormat.percent ignoring digits (dart-lang/i18n#1115)

shelf (https://github.com/dart-lang/shelf/compare/62d3e4e..9946de5):
  9946de5  Sun Nov 1 12:27:59 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 3 updates (dart-lang/shelf#554)

test (https://github.com/dart-lang/test/compare/6d74720..2a9e017):
  2a9e0175  Sun Nov 1 03:47:24 2026 +0000  dependabot[bot]  Bump the github-actions group with 6 updates (dart-lang/test#2813)
  55cd5cd5  Fri Oct 30 13:43:47 2026 -0700  Nate Bosch  Migrate publishing workflows from firehose to setup-dart (dart-lang/test#2812)
  10dc78c0  Fri Oct 30 11:21:53 2026 -0700  Nate Bosch  [checks] Describe what the stream did in failures (dart-lang/test#2811)
  c11ad06d  Fri Oct 30 08:59:57 2026 -0700  Nate Bosch  [checks] Add CheckFailure.describeNested (dart-lang/test#2793)
  0137b43e  Fri Oct 30 05:40:43 2026 +0530  Shriraj Jadhav  [checks] Use the Iterable.last member for the last expectation (dart-lang/test#2807)
  50c9f454  Thu Oct 29 15:11:39 2026 -0700  Nate Bosch  [checks] Collapse clauses for described conditions (dart-lang/test#2794)

tools (https://github.com/dart-lang/tools/compare/3980dca..1579f8d):
  1579f8d69  Mon Nov 2 02:44:58 2026 +0530  Joshua  fix(coverage): reject package: and dart: URIs outside known roots in Resolver (dart-lang/tools#2764)
  f869fdca4  Sun Nov 1 10:26:34 2026 +0100  dart-ecosystem-bot[bot]  [boring] Update BoringSSL to ed9209cf (dart-lang/tools#2774)
  d10e2b87f  Sun Nov 1 10:26:11 2026 +0100  dart-ecosystem-bot[bot]  [boring] Pin the prebuilt binaries of boring-binaries-20261101-33e9e69722af (dart-lang/tools#2777)
  33e9e6972  Sun Nov 1 05:56:30 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 4 updates (dart-lang/tools#2775)
  f84d0d0d2  Sun Nov 1 05:36:12 2026 +0000  dependabot[bot]  build(deps): bump ffigen from 22.0.0 to 23.0.0 in /pkgs/boring (dart-lang/tools#2776)
  14ad5e515  Fri Oct 30 10:48:58 2026 -0700  Kevin Moore  [mime] Harden BoundMultipartStream error routing, headers, and epilogue (dart-lang/tools#2629)
  7a780d48c  Fri Oct 30 12:57:03 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed plain-text runs in InlineParser (dart-lang/tools#2757)
  3671d33a9  Fri Oct 30 12:56:48 2026 +0100  Jonas Finnemann Jensen  [markdown] Index footnote definitions and labels on Document for O(1) lookup (dart-lang/tools#2760)
  6f38e9fda  Fri Oct 30 12:56:09 2026 +0100  Jonas Finnemann Jensen  [markdown] Bound the domain of extended URL autolinks to DNS limits (dart-lang/tools#2759)
  aa0c38544  Fri Oct 30 12:20:40 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic removeRange in _combineAdjacentText (dart-lang/tools#2758)
  f425baf46  Fri Oct 30 11:16:28 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed space runs in LineBreakSyntax (dart-lang/tools#2756)
  c7eafbec3  Fri Oct 30 11:09:45 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject '<' in AutolinkSyntax (dart-lang/tools#2755)
  86ed0f902  Fri Oct 30 11:08:25 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic backtracking in headerPattern (dart-lang/tools#2754)
  8ea07517f  Fri Oct 30 07:45:42 2026 +0100  Moritz  [boring] Regenerate the x509-limbo expected failures in the refresh job (dart-lang/tools#2712)
  40235c365  Thu Oct 29 14:46:17 2026 +0100  Jonas Finnemann Jensen  [markdown] Make HtmlBlockSyntax.parseChildLines iterative (dart-lang/tools#2749)
  0ff03f643  Thu Oct 29 14:41:33 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid re-escaping nested image alt text in ImageSyntax.createNode (dart-lang/tools#2752)
  784654d01  Thu Oct 29 14:37:11 2026 +0100  Jonas Finnemann Jensen  [markdown] Add test cases covering parser and renderer edge cases (dart-lang/tools#2753)
  4f2718ad8  Thu Oct 29 14:36:52 2026 +0100  Jonas Finnemann Jensen  [markdown] Make Document._parseInlineContent iterative (dart-lang/tools#2750)
  cac1421d5  Thu Oct 29 14:17:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer (dart-lang/tools#2748)
  66880bcd3  Thu Oct 29 14:11:27 2026 +0100  Jonas Finnemann Jensen  [markdown] Check footnote label and empty map before scanning footnote keys (dart-lang/tools#2751)
  7f8696523  Thu Oct 29 14:07:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic blank-line lookahead in CodeBlockSyntax (dart-lang/tools#2747)
  9da6bb291  Thu Oct 29 14:03:04 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid exponential and quadratic backtracking in tablePattern (dart-lang/tools#2746)

dartdoc is held back at db94437, before dart-lang/dartdoc#4312, because its
mock SDK version changes conflict in the google3 3-way merge with the hotfix
in cl/1005963642.

native is held back at e0ca7c6, before dart-lang/native#3742, because
deprecating `MacOSCodeConfig(targetVersion: ...)` causes a fatal
`deprecated_member_use` diagnostic in `pkg/dartdev/lib/src/native_assets.dart`.

web is held back at 4e60e61, before dart-lang/web#596, because it changes
`package:web` signatures (e.g. `innerHTML=`, `parseFromString`, `Worker()`)
from `JSAny` to `String` in ways that break callers in google3.

Change-Id: I53d2cbde741c45b8fd33bca42742549819dd332b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/572780
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Nate Bosch <[email protected]>
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@szakarias szakarias szakarias approved these changes
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