Add UTF8.valid?, a byte-level UTF-8 validator

[carpentry-agent]

What

Adds UTF8.valid? (Fn [(Ref (Array Byte))] Bool) — a byte-level check for whether an array is well-formed UTF-8 per RFC 3629. It rejects:

• overlong encodings (e.g. C0 80, E0 9F BF)
• UTF-16 surrogate code points U+D800..U+DFFF (e.g. ED A0 80)
• code points beyond U+10FFFF (e.g. F4 90 80 80)
• lone continuation bytes, truncated sequences, and out-of-range lead bytes (F5..FF)

The scan is allocation-free and pure Carp. The lead-byte classification maps directly to the Unicode standard's Table 3-7: each lead byte fixes how many continuation bytes follow and the tightened valid range of the first one (E0→A0..BF, ED→80..9F, F0→90..BF, F4→80..8F), which is what kills overlongs, surrogates, and over-max in a single pass.

Why

Follow-up to carpentry-org/web#40 (WebSocket text-frame UTF-8 validation). The review there was: "We should use the utf8 library for that." — but this library had no validator to reuse. This PR adds one so web (and anyone else) can depend on it instead of hand-rolling the check. from-string and String.from-bytes both assume well-formed input, so a standalone guard for untrusted bytes is a natural fit here.

The API is a predicate on raw bytes rather than on a UTF8/String value on purpose: validity is a property of bytes, and callers (like web, whose WebSocket payloads are (Array Byte)) can check before constructing anything — no allocation, no throwaway string on the reject path.

Tests

Adds 18 asserts to test/utf8.carp: ASCII and empty input; valid 2/3/4-byte sequences (U+00A9, U+20AC, U+1F600); the boundaries either side of the surrogate gap (U+D7FF, U+E000) and of the 4-byte range (U+10000 min, U+10FFFF max); and malformed input — overlong 2- and 3-byte, lone/truncated continuations, both surrogate ends (U+D800, U+DFFF), >U+10FFFF, and F5/FF leads.

Verification

• carp -x test/utf8.carp → 35/35 pass (17 existing + 18 new), run locally.
• carp-fmt --check and angler clean on utf8.carp and test/utf8.carp.
• docs/UTF8.html regenerated via gendocs.carp.

Once this is released, web#40 can drop its own WebSocket.valid-utf8? and call UTF8.valid?.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

Checked out claude/add-valid-predicate locally and ran the suite:
carp -x test/utf8.carp → 35/35 pass (17 existing + 18 new). CI is green on both ubuntu-latest and macos-latest; the single test job also runs angler lint, carp-fmt --check, and gendocs.carp, so those passed too.

Findings

I read the whole function, checked it against Unicode Table 3-7 / RFC 3629, then went well beyond the suite — 20 extra adversarial cases, all of which behave correctly:

• Lead-byte classification (utf8.carp:164-174) is exhaustive and correct. 0xC0/0xC1, the lone-continuation range 0x80-0xBF, and 0xF5-0xFF all fall through to (set! ok false). The tightened first-continuation ranges for E0 (≥A0), ED (≤9F), F0 (≥90), F4 (≤8F) correctly kill overlongs, surrogates, and >U+10FFFF in a single pass. I confirmed each boundary empirically (E0 9F→reject / E0 A0→accept; F4 90→reject / F4 8F→accept).
• Continuation handling (utf8.carp:175-183) is sound. The truncation guard (> (+ i need) (- len 1)) is correct, and every unsafe-nth is provably in bounds (all indices ≤ len-1 after that guard). The first continuation is checked against the tightened [lo,hi]; the rest against [0x80,0xBF] via the for loop — I verified a bad 2nd/3rd continuation byte in 3- and 4-byte sequences is rejected (E2 82 00, F0 9F 98 00), which the suite doesn't cover.
• Multi-codepoint advancement works. aé€😀 and back-to-back 3-byte sequences validate; interior faults after a valid prefix (61 FF 62, abc 80) are caught — it's not just checking the first character.
• The unsigned-byte assumption holds. Byte.to-int returns 0-255 (confirmed 255→255), which the ≥194 comparisons depend on.
• Style fits. This let-do/for/set! imperative idiom already appears in to-runes (utf8.carp:80-89), so it's not a new pattern for the file.
• Docs/changelog: docs/UTF8.html was regenerated; the repo has no CHANGELOG and the README is a narrative tutorial with no API list, so nothing else to update.
• API shape — a predicate over (Ref (Array Byte)) rather than a UTF8/String — is the right call: validity is a byte-level property, and callers like web#40 can check before allocating.

Found no bugs, no missing edge cases, no style drift.

Verdict: merge

Correct, well-tested, and green across the full CI pipeline; I built it and ran 20 additional adversarial cases and it held up on every one. Merge is of course the maintainer's decision.