Memoize text width by MichaReiser · Pull Request #6552 · astral-sh/ruff

MichaReiser · 2023-08-14T08:21:53Z

Summary

The formatter processes strings a couple of times:

propagate_expands: Test if the string contains a newline and, if so, set mode: Propagated on all enclosing groups
print_text: Measures the width of the text to compute the line_width
fits_text (only if the text is inside of a group): Measures the width of the text to determine if the content fits. Runs n times where n is the number of parent groups.

This PR adds a TextWidth field to all text FormatElements that either stores the computed width or that it is a multiline string.
This reduces the traversal for non-multiline strings to exactly once. Multiline strings still get traversed multiple times and you could argue this PR makes it worse because the implementation now computes the text width only to throw it away when realizing it is a multiline string. However, multiline strings are rare.

Memoizing the text width improves performance by 2-12%.

The downsides of this change are:

It will make it harder to add more fields to Text FormatElements because we are now very close to the 24 bytes
The same logic is now spread in multiple places, increasing the risk of one-off bugs

Test Plan

The tests are currently failing because I need to update them to pass TextWidth

MichaReiser · 2023-08-14T08:22:00Z

Current dependencies on/for this PR:

main
- PR Introduce Token element #7048
  - PR Memoize text width #6552 👈
    - PR BestFitting: Reduce allocations #7037

This comment was auto-generated by Graphite.

crates/ruff_formatter/src/format_element.rs

konstin · 2023-09-05T10:45:01Z

crates/ruff_formatter/src/format_element.rs

+                '\t' => tab_width.value(),
+                '\n' => return TextWidth::Multiline,
+                #[allow(clippy::cast_possible_truncation)]
+                c => c.width().unwrap_or(0) as u32,


When does c not have a width and why would we go with 0 instead, is this about control characters?

From the unicode width documentation

Returns the character's displayed width in columns, or None if the character is a control character other than '\x00'.

So yes, this is about control characters and using 0 seems reasonable to me (this is the same logic as applied by the printer today)

crates/ruff_formatter/src/printer/mod.rs

konstin · 2023-09-05T10:50:47Z

crates/ruff_formatter/src/printer/mod.rs

-                                    self.state.line_width = 0;
-                                    continue;
+            Text::Text { text, width } => {
+                if let Some(width) = width.width() {


nit: i think i'd use a match here

Me too but our pedantic clippy rule doesn't allow me to

crates/ruff_python_formatter/src/comments/format.rs

konstin · 2023-09-05T10:53:22Z

Sorry i had missed this PR.

I'll try to trigger codspeed for perf numbers.

MichaReiser · 2023-09-05T10:56:13Z

Sorry i had missed this PR.

I'll try to trigger codspeed for perf numbers.

Code speed only comments on regressions but you can see the results in the run summary https://github.com/astral-sh/ruff/pull/6552/checks?check_run_id=16438240046

konstin · 2023-09-05T10:56:28Z

3-12%, that's huge!

MichaReiser · 2023-09-06T07:02:48Z

crates/ruff_formatter/src/format_element.rs

+/// This imprecision shouldn't matter in practice because either text are longer than any configured line width
+/// and thus, the text should break.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct Width(NonZeroU32);


I added this newtype wrapper to lock down the access to the inner NonZeroU32. Not that someone uses it and then gets values that are off by one.

MichaReiser · 2023-09-06T07:05:58Z

3-12%, that's huge!

The win is bigger for formats that

a) Use more groups or best fitting elements because the printer has to measure fits for every group (there's some optimisation but the worst case scenario is that it computes it for every group)
b) Code that has many lines exceeding the line width (with nested groups), because the printer than has to measure fits for every inner group

Back port from astral-sh/ruff#6552

MichaReiser requested a review from charliermarsh August 14, 2023 08:24

MichaReiser force-pushed the printer-reserve-buffer-upfront branch from a97a59a to 3076d97 Compare August 14, 2023 08:26

MichaReiser force-pushed the memoize-text-width branch from dae8c98 to 8a932fb Compare August 14, 2023 08:26

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MichaReiser force-pushed the printer-reserve-buffer-upfront branch 2 times, most recently from 2db43a5 to 493a3b6 Compare August 14, 2023 12:03

MichaReiser force-pushed the memoize-text-width branch from 8a932fb to 9b9db1b Compare August 14, 2023 12:03

Base automatically changed from printer-reserve-buffer-upfront to main August 14, 2023 12:15

MichaReiser force-pushed the memoize-text-width branch from 9b9db1b to 4ed9a28 Compare August 14, 2023 12:32

konstin added the formatter Related to the formatter label Aug 14, 2023

MichaReiser force-pushed the memoize-text-width branch from 4ed9a28 to c878190 Compare August 16, 2023 06:37

MichaReiser changed the base branch from main to token-element September 1, 2023 19:16

MichaReiser force-pushed the memoize-text-width branch from c878190 to 2aa6815 Compare September 1, 2023 19:16

This was referenced Sep 1, 2023

Introduce Token element #7048

Merged

BestFitting: Reduce allocations #7037

Closed

MichaReiser force-pushed the memoize-text-width branch 5 times, most recently from eb8d51f to fb43ed9 Compare September 1, 2023 22:08

MichaReiser force-pushed the token-element branch from fc0d6aa to 409f8f9 Compare September 2, 2023 07:47

MichaReiser force-pushed the memoize-text-width branch from fb43ed9 to 81036a0 Compare September 2, 2023 07:48

MichaReiser marked this pull request as ready for review September 2, 2023 07:49

Base automatically changed from token-element to main September 2, 2023 08:05

MichaReiser force-pushed the memoize-text-width branch from 81036a0 to 6a31ef4 Compare September 2, 2023 08:06

MichaReiser added the performance Potential performance improvement label Sep 2, 2023

MichaReiser requested a review from konstin September 5, 2023 08:21

konstin approved these changes Sep 5, 2023

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konstin closed this Sep 5, 2023

konstin reopened this Sep 5, 2023

MichaReiser added 2 commits September 6, 2023 08:37

Memoize text width

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Code review feedback

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MichaReiser force-pushed the memoize-text-width branch from 6a31ef4 to e09159a Compare September 6, 2023 06:59

MichaReiser commented Sep 6, 2023

View reviewed changes

MichaReiser enabled auto-merge (squash) September 6, 2023 07:03

MichaReiser merged commit 5f59101 into main Sep 6, 2023

MichaReiser deleted the memoize-text-width branch September 6, 2023 07:10

zanieb mentioned this pull request Sep 6, 2023

Add warnings for nursery and preview rule selection #7210

Merged

zanieb pushed a commit that referenced this pull request Sep 6, 2023

Memoize text width (#6552)

9ce5e4c

MichaReiser mentioned this pull request Sep 19, 2023

refactor(formatter): Use OnceCell in Memoized biomejs/biome#338

Merged

This was referenced Nov 3, 2025

refactor(formatter): introduce Token element biomejs/biome#7968

Merged

refactor(formatter): memoize text width biomejs/biome#7983

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Dunqing mentioned this pull request Nov 6, 2025

perf(formatter): memorize text width for FormatElement::Text oxc-project/oxc#15372

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graphite-app bot pushed a commit to oxc-project/oxc that referenced this pull request Nov 7, 2025

perf(formatter): memorize text width for FormatElement::Text (#15372)

8f25a0e

Back port from astral-sh/ruff#6552

Dunqing mentioned this pull request Nov 24, 2025

formatter: backport FormatElement performance improvements from Ruff oxc-project/oxc#16022

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