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perf(formatter): pre-allocate enough space for the FormatElement buffer #15422

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Merged
graphite-app merged 1 commit into from
Nov 10, 2025
Merged

perf(formatter): pre-allocate enough space for the FormatElement buffer #15422

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8 changes: 7 additions & 1 deletion crates/oxc_formatter/src/formatter/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -354,8 +354,14 @@ pub fn format<'ast>(
context: FormatContext<'ast>,
arguments: Arguments<'_, 'ast>,
) -> FormatResult<Formatted<'ast>> {
// Pre-allocate buffer at 40% of source length (source_len * 2 / 5).
// Analysis of 4,891 VSCode files shows FormatElement buffer length is typically 19% of source (median),
// with 95th percentile at 30-38% across all file sizes. This 0.4x multiplier avoids
// reallocation for 95%+ of files.
let capacity = (context.source_text().len() * 2) / 5;

let mut state = FormatState::new(context);
let mut buffer = VecBuffer::with_capacity(arguments.items().len(), &mut state);
let mut buffer = VecBuffer::with_capacity(capacity, &mut state);

buffer.write_fmt(arguments)?;

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