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[markdown] Avoid quadratic removeRange in _combineAdjacentText - #2758

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[markdown] Avoid quadratic removeRange in _combineAdjacentText#2758
jonasfj merged 2 commits into
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markdown_combine_adjacent_text_compaction

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@jonasfj jonasfj commented Oct 29, 2026 •
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Compact each List<Node> in InlineParser._combineAdjacentText using a single
in-place read/write pass (writeIndex) followed by a single
nodes.length = writeIndex truncation instead of calling nodes.removeRange
inside the loop.

Why this change is necessary

At the end of InlineParser.parse(), _combineAdjacentText walks the parsed
inline AST and merges adjacent Text siblings into a single Text node.
Previously, whenever a run of adjacent Text nodes from index i to j was
merged, _combineAdjacentText called nodes.removeRange(i + 1, j) inside the
for loop:

  1. Quadratic list shifting (O(n^2)): In Dart, List.removeRange shifts
    all elements after j left by j - i - 1 positions (O(nodes.length)).
    When a line produces thousands of runs of adjacent Text nodes alternating
    with inline Elements (for example '[a *b* ' * n or 'a_b *c* ' * n,
    where each unmatched [ or _ leaves adjacent Text nodes before an <em>
    element), calling removeRange O(n) times shifted O(n) elements each time,
    taking O(n^2) time.
  2. Skipped last/single child Elements: Because the old loop ran
    for (var i = 0; i < nodes.length - 1; i++), it never visited
    nodes[nodes.length - 1], failing to recurse into node.children whenever
    an Element was the last (or only) child in nodes (such as
    <strong><strong>...</strong></strong>).

How this change works

We iterate for (var i = 0; i < nodes.length; i++) with a write pointer
var writeIndex = 0;:

  • When node is an Element with children, we recurse into node.children!
    (including when node is the last or only element in nodes).
  • When node is Text && i + 1 < nodes.length && nodes[i + 1] is Text, a
    StringBuffer consumes all contiguous Text nodes starting at i in a
    single inner do/while loop and replaces node with
    Text(buffer.toString()).
  • Each surviving node is written to nodes[writeIndex] (when
    writeIndex != i) before incrementing writeIndex, and at the end of the
    loop nodes.length = writeIndex truncates the list once in O(1) time.

Differential testing

Rendered every input with origin/main and with this branch (GFM extension
set, 30 s per-file timeout) and compared the HTML. Sources:

  • pub.dev: 367,926 Markdown files from 93,452 packages
  • Real-world docs: 116,231 files from mdn/content, github/docs,
    kubernetes/website, freeCodeCamp, tldr, flutter/website,
    dart-lang/site-www, microsoft/vscode-docs, withastro/docs,
    rust-lang/book, GitHub READMEs, and Wikipedia
  • Parser test suites: 6,458 cases from cmark, cmark-gfm, commonmark.js,
    pulldown-cmark, md4c, goldmark, markdown-it, comrak, marked,
    pandoc, and pathological generators
  • OSS-Fuzz: 57,716 inputs from the cmark, md4c, and markdown-it-py fuzzers

OSS-Fuzz inputs that sit at the 30 s timeout or the stack limit flip between
runs; every one reported as a new failure was re-run standalone on both trees.

Results:

  • Rendered HTML differs for 3 inputs: 2 real-world files (notion_dart_kit
    doc/uploading-small-files.md, @rasgo/text-tree README.md) and 1
    OSS-Fuzz input. The old loop never visited the last node of a list, so Text
    nodes inside a trailing <strong>/<em> were never merged; merging them
    changes which lines HtmlRenderer.visitText left-trims after <p>/<li>/
    <br>. Example from @rasgo/text-tree:
    _</strong></strong>______/ → _</strong></strong>______/.
    This is leading whitespace on a continuation line inside <p>, which
    browsers collapse, and it makes those lines consistent with lines outside the
    element, which were already trimmed.
  • 4 inputs were reported as new Stack Overflow and one as timeout →
    Stack Overflow. All are 65–524 KB of nested *; run standalone on main
    they overflow as well (three immediately, one after 29 s), so these are
    pre-existing and not caused by this change (the inline nesting cap in [markdown] WIP: Fix quadratic, exponential, and deep-recursion #2737
    addresses them).
  • 1 OSS-Fuzz timeout resolved; cumulative parse time −1.4%.

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Code Review

This pull request optimizes the _combineAdjacentText method in inline_parser.dart to merge adjacent Text nodes in a single in-place pass instead of using repeated List.removeRange calls. It also ensures that merging covers the last child of an element. A corresponding unit test has been added to document_test.dart to verify this behavior, and the CHANGELOG.md has been updated. There are no review comments, and I have no feedback to provide.

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Breaking changes ✔️
Package Change Current Version New Version Needed Version Looking good?
markdown Non-Breaking 7.3.1 7.4.1-wip 7.4.1-wip ✔️

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pkgs/markdown/lib/src/inline_parser.dart 💚 98 %

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License Headers ✔️ 
// 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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@jonasfj
jonasfj force-pushed the markdown_combine_adjacent_text_compaction branch from 211d975 to 80dceb7 Compare October 30, 2026 09:22
@jonasfj
[markdown] Avoid quadratic removeRange in _combineAdjacentText
44f35a9 
Compact each `List<Node>` in `InlineParser._combineAdjacentText` using a single
in-place read/write pass (`writeIndex`) followed by a single
`nodes.length = writeIndex` truncation instead of calling `nodes.removeRange`
inside the loop.

----

### Why this change is necessary

At the end of `InlineParser.parse()`, `_combineAdjacentText` walks the parsed
inline AST and merges adjacent `Text` siblings into a single `Text` node.
Previously, whenever a run of adjacent `Text` nodes from index `i` to `j` was
merged, `_combineAdjacentText` called `nodes.removeRange(i + 1, j)` inside the
`for` loop:
1. **Quadratic list shifting (`O(n^2)`)**: In Dart, `List.removeRange` shifts
   all elements after `j` left by `j - i - 1` positions (`O(nodes.length)`).
   When a line produces thousands of runs of adjacent `Text` nodes alternating
   with inline `Element`s (for example `'[a *b* ' * n` or `'a_b *c* ' * n`,
   where each unmatched `[` or `_` leaves adjacent `Text` nodes before an `<em>`
   element), calling `removeRange` O(n) times shifted O(n) elements each time,
   taking O(n^2) time.
2. **Skipped last/single child `Element`s**: Because the old loop ran
   `for (var i = 0; i < nodes.length - 1; i++)`, it never visited
   `nodes[nodes.length - 1]`, failing to recurse into `node.children` whenever
   an `Element` was the last (or only) child in `nodes` (such as
   `<strong><strong>...</strong></strong>`).

### How this change works

We iterate `for (var i = 0; i < nodes.length; i++)` with a write pointer
`var writeIndex = 0;`:
- When `node` is an `Element` with children, we recurse into `node.children!`
  (including when `node` is the last or only element in `nodes`).
- When `node is Text && i + 1 < nodes.length && nodes[i + 1] is Text`, a
  `StringBuffer` consumes all contiguous `Text` nodes starting at `i` in a
  single inner `do/while` loop and replaces `node` with
  `Text(buffer.toString())`.
- Each surviving `node` is written to `nodes[writeIndex]` (when
  `writeIndex != i`) before incrementing `writeIndex`, and at the end of the
  loop `nodes.length = writeIndex` truncates the list once in O(1) time.
@jonasfj
jonasfj force-pushed the markdown_combine_adjacent_text_compaction branch from 80dceb7 to 44f35a9 Compare October 30, 2026 10:00
@jonasfj
jonasfj marked this pull request as ready for review October 30, 2026 10:07
@jonasfj
jonasfj requested a review from srawlins as a code owner October 30, 2026 10:07
@jonasfj
jonasfj requested a review from szakarias October 30, 2026 10:07
szakarias
szakarias approved these changes Oct 30, 2026
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drop WIP here too.

@jonasfj jonasfj changed the title [WIP] [markdown] Avoid quadratic removeRange in _combineAdjacentText [markdown] Avoid quadratic removeRange in _combineAdjacentText Oct 30, 2026
@jonasfj
jonasfj merged commit aa0c385 into main Oct 30, 2026
30 of 32 checks passed
@jonasfj
jonasfj deleted the markdown_combine_adjacent_text_compaction branch October 30, 2026 11:20
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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