share one binary search across large string maps

[alii]

comptime_string_map expands every map into an unrolled compare tree per map, and for large maps that tree is big: the case-insensitive MIME/header lookup in bun_http_types alone was 37 KB, with the JSX entity table, which and a few bun_jsc maps adding another 27 KB. All of them are the same algorithm with different data.

Maps at or above a key-count threshold now emit their keys as data (a byte blob, offsets, indexes sorted by length then bytes, per-length buckets) and share one #[inline(never)] bucketed binary search; small maps keep the compare tree. Pre-LTO the rlib text drops by about 63 KB (bun_http_types −37 KB, bun_jsc −19 KB, css and misc −7 KB). A lookup on a large map is now a length-bucket slice plus a binary search (a handful of memcmp calls) instead of the unrolled tree.

Behavior is unchanged; the macro crate gains a test that a large map resolves every key, misses near-keys and preserves declaration order. Header, MIME, which and JSX tests pass on the debug build.

[robobun]

^{Updated 1:45 AM PT - Aug 18th, 2026}

✅ @autofix-ci[bot], your commit 148645cd7ad8a922254e26ec7534016ffa20d368 passed in Build #100479! 🎉

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[coderabbitai]

Walkthrough

The change adds SortedKeys for large compile-time string maps. Macro expansion generates sorted indexes, offsets, and length buckets at 200 keys, while smaller maps retain unrolled comparisons. Lookup paths and tests now cover exact, comparator, case-insensitive, and miss cases.

Changes

Compile-time string map lookup

Layer / File(s)	Summary
Sorted key representation and lookup src/bun_core/comptime_string_map.rs	SortedKeys stores key data and supports declaration-order access, length buckets, and binary-search lookup. A 240-entry test validates lookup variants and miss cases.
Large-map table generation src/bun_core_macros/comptime_string_map.rs	Maps with at least 200 keys generate length-grouped u16 indexes and offsets. Generation rejects oversized key counts and key blobs.
Lookup path selection src/bun_core_macros/comptime_string_map.rs	Generated storage includes optional sorted tables. Large maps use sorted lookups, small maps retain unrolled comparisons, and comparator and ASCII case-insensitive paths use the selected implementation.

Merge Risk: 🔵 Low · up to 14864

Large maps now use a shared bucketed binary search, but an empty-key map can panic instead of returning a miss when queried with usize::MAX. This is a localized low-risk issue that should be fixed or explicitly accepted before merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: sharing one binary-search implementation across large string maps.
Description check	✅ Passed	The description explains the implementation, performance impact, unchanged behavior, and verification results, although it omits the template headings.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[alii]

@robobun adopt

[robobun]

Superseded by #39770, which landed on main with the same change (threshold 64, #[inline] opt-out, sets included). I checked it on main with the tests from #39532 (all 1184 extensions, named JSX entities, MIME types by extension): all pass, details in #39532 (comment). Closed #39532 and closing this one too; reopen if you still want any part of it.

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@src/bun_core_macros/comptime_string_map.rs`:
- Around line 255-265: Update the too_large error construction in the comptime
string map validation to include the rejected key count or total key-byte value,
identify the u16::MAX constraint, and state that the map must be reduced to at
most u16::MAX entries or bytes. Pass the relevant measured value from each
validation branch while preserving the existing error span and resource-specific
labels.

In `@src/bun_core/comptime_string_map.rs`:
- Around line 102-108: Update SortedKeys::bucket to compute the next bucket
index with i.checked_add(1), returning an empty bucket if the addition overflows
before indexing buckets; add a regression test asserting bucket(usize::MAX)
returns empty.

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• src/bun_core/comptime_string_map.rs
• src/bun_core_macros/comptime_string_map.rs

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[claude]

I reviewed this PR and didn't find any bugs. Because it swaps the lookup strategy for every ≥200-key comptime_string_map! (HTTP headers, MIME types, JSX entities, etc.) from inline compares to a shared binary search — a code-size vs. hot-path-speed tradeoff — a human look at the threshold choice and perf impact would still be worthwhile.

What was reviewed:

• SortedKeys::bucket/index_of bounds: checked_sub guards len < min_len, i + 1 >= buckets.len() guards len > max_len; the CodeRabbit usize::MAX overflow requires a slice spanning the address space, not reachable.
• sorted_tables builds starts/sorted/offsets consistent with length_buckets' (length, bytes) ordering, so the per-bucket binary search sees byte-sorted keys.
• get_ascii_case_insensitive rerouted through __key_index(buf) — equivalent to the old get_with_len_and_eql(.., |a, b| a == b) for both small and large maps.
• Large-map eql_body linear-scans the length bucket, matching the old unrolled eql_arms iteration order and count.

Extended reasoning...

Overview

This PR changes the comptime_string_map! proc-macro so that maps with ≥200 keys emit static index tables (SortedKeys: concatenated key blob, u16 prefix offsets, declaration indexes sorted by (length, bytes), per-length bucket starts) and route __key_index through one shared #[inline(never)] bucketed binary search in bun_core. Maps below the threshold keep the existing unrolled per-key compare tree. get_ascii_case_insensitive is rewired to call __key_index on the lowercased buffer directly. A 240-entry unit test exercises get, get_ascii_case_insensitive, get_with_eql, declaration-order iteration, and below/at/above-range misses.

Security risks

None. Inputs are compile-time string literals in Bun's own source; runtime lookups take a &[u8] and either miss or hit a static value. No allocation, no unsafe, no external data. The one arithmetic edge CodeRabbit flagged (i + 1 overflow when min_len == 0 and len == usize::MAX) would require a slice occupying the entire address space; even in release the subsequent slice index is bounds-checked, so it panics rather than reading OOB.

Level of scrutiny

High. This macro backs hot lookups across the runtime — HTTP header name matching, MIME-type resolution, the JSX entity table, which, and several bun_jsc maps per the PR description. The change trades ~63 KB of pre-LTO text for a length-bucket slice + binary_search_by (a handful of memcmps) per lookup on those tables. That's a real design tradeoff on hot paths, and the 200-key threshold is a judgment call. The PR description says header/MIME/which/JSX tests pass on debug, but doesn't show a lookup-latency benchmark; per the repo's Performance review guidance a maintainer should confirm the runtime cost is acceptable on the affected hot paths.

Other factors

I traced the table construction against length_buckets: it sorts by (len, bytes), so each bucket's indexes are already byte-ordered when sorted_tables copies them into sorted, and starts records cumulative positions per length including empty runs — matching SortedKeys::bucket's buckets[i]..buckets[i+1] slicing. The u16 bounds checks on key count and blob_total prevent silent truncation of the generated offsets/sorted arrays. The large-map eql_body (custom comparator path) does a linear scan over the same bucket the old eql_arms unrolled, so get_with_eql semantics and comparison count are unchanged. The two CodeRabbit notes are minor (compile-time error-message wording; an unreachable overflow) and don't affect correctness. Deferring because the perf/size tradeoff and threshold are decisions a maintainer should sign off on, not because of a correctness concern.