Optimize Ajv refs for external schema branches

[RelevantShannon]

Redirect buildOneOf's Ajv ref to a registered ref target when possible.

buildOneOf currently validates oneOf/anyOf branches through location.getSchemaRef(), which points back into the generated root schema even when the branch is only an external ref already registered with Ajv. This lets those branches validate against the exact external ref target instead, so Ajv can use the already-registered schema path rather than resolving through the root document.

The helper is intentionally guarded:

• only pure branches are redirected
• inline branches and refs with sibling keywords are left unchanged
• unresolved refs are left unchanged
• JSON-pointer and supported fragment refs are redirected only when the resolver can resolve the exact target
• relative root cases are only redirected when the fragment matches the registered schema id shape

This also applies the same validator-ref selection to if schemas, which use the same runtime validator path.

Verification:

• npm run lint
• git diff --check
• node --test test/oneof.test.js
• node --test test/oneof.test.js test/anyof.test.js test/if-then-else.test.js
• npm run test:unit

npm run test:unit passed locally with 488 tests and 0 failures. The local run printed Node inspector auto-attach warnings, but exited successfully.

[mcollina]

lgtm

[Fdawgs]

Any supporting benchmarks for this optimisation?

[Copilot]

Pull request overview

This pull request optimizes how fast-json-stringify selects the Ajv schema reference used for runtime branch validation in oneOf/anyOf and if schemas, allowing validation to target already-registered external schema refs directly when the branch is a “pure” $ref.

Changes:

• Add getValidatorSchemaRef() to redirect validator refs from root-relative JSON-pointer paths to external registered $ref targets when safely resolvable.
• Use the new validator-ref selection in buildOneOf and buildIfThenElse.
• Add regression/behavior tests covering bare refs, $id refs (absolute and relative), named fragments, JSON-pointer subpaths, and non-redirected cases (inline branches and $ref with sibling keywords).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
index.js	Introduces getValidatorSchemaRef() and applies it to Ajv validation calls in oneOf/anyOf and if codegen paths.
test/oneof.test.js	Adds targeted tests asserting standalone output uses direct external validator refs for eligible oneOf branches (and preserves behavior for ineligible cases).
test/anyof.test.js	Adds a test ensuring anyOf branch validator refs resolve directly to registered external targets.
test/if-then-else.test.js	Adds tests ensuring if schema validator refs resolve directly to registered external targets (and are not redirected when $ref has siblings).

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[RelevantShannon]

Added a benchmark covering the affected path: cold fast-json-stringify compile plus first stringify() for a oneOf field whose branches are external registered $ref schemas.

Local before/after using the same benchmark workload:

Branch	Result
main	462.0 ops/sec
this PR	1,529.3 ops/sec

That works out to roughly:

• 3.31x faster
• +231% throughput
• ~69.8% less time per compile + first stringify
• 2.16ms -> 0.65ms per operation

I also ran the full benchmark suite locally after adding the benchmark, and the new case reported:

oneOf external refs compile and first stringify x 1,584 ops/sec ±1.09% (1534 runs sampled)