Optimize object key lookup through reducing String initialization

[Joannis]

Achieved by hashing the string (with FNV1) and storing the hash inline in the description

[Copilot]

Pull request overview

This PR optimizes object key lookup performance by storing pre-computed FNV-1a hashes alongside object keys in the JSON description structure. This allows the decoder to skip expensive memcmp operations when hash comparisons fail, achieving faster key lookups through early rejection.

Changes:

• Added FNV-1a hash computation during JSON parsing for object keys, stored inline in the description (13 bytes vs 9 bytes)
• Implemented hash-based comparison in keyOffset lookup to skip memcmp when hashes don't match
• Added sequential access hint optimization for decoder to improve performance on ordered key access

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
Sources/_NIOJSON/Core/JSONDescription.swift	Added FNV-1a hash function, objectKeyFound handler, and hash-based key lookup optimization with hint support
Sources/_NIOJSON/Core/JSONDescription+Constants.swift	Added constants for object key index length (13 bytes) and hash offset (9 bytes)
Sources/_NIOJSON/Types/JSONObject.swift	Added handling for objectKey types in value conversion, returning nil
Sources/_NIOJSON/Types/JSONArray.swift	Added fatalError for unexpected objectKey types in arrays
Sources/_NIOJSON/Codable/JSONDecoder.swift	Added lastKeySearchHint tracking and refactored key lookup to use valueOffset helper with hash optimization
Sources/_JSONCore/Parser/JSONTokenizerDestination.swift	Added objectKeyFound protocol method with default implementation for backward compatibility
Sources/_JSONCore/Parser/JSONParser+Parsing.swift	Added scanObjectKey function that computes FNV-1a hash during parsing
Sources/_JSONCore/Errors.swift	Updated TypeConversionError to use two generic parameters and made it Sendable
Package.swift	Added IkigaJSONCore library product and StrictConcurrency feature for _JSONCore target

Comments suppressed due to low confidence (1)

Sources/_NIOJSON/Core/JSONDescription.swift:1

• The FNV-1a hash computation is duplicated in two places: the fnv1aHashString function (lines 9-15) and inline in scanObjectKey (lines 330-334). This code duplication creates a maintenance burden. Consider extracting a shared implementation or calling fnv1aHashString with the appropriate buffer slice.

import Foundation

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[Copilot]

For empty keys (when keyLength is 0), the range 1...0 is invalid and will cause a runtime error. The loop should use ..< instead of ... or guard against empty keys. Change to for i in 1..<(keyLength + 1) or for i in 1...max(1, keyLength) to handle the empty key case safely.

Suggested change

	for i in 1...keyLength {
	for i in 1..<(keyLength + 1) {

[Joannis]

@copilot make a unit test demonstrating this potential bug

[Copilot]

@Joannis I've opened a new pull request, #53, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

@Joannis I've opened a new pull request, #54, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Comments suppressed due to low confidence (1)

Sources/_NIOJSON/Codable/JSONDecoder.swift:370

• The subDecoder method doesn't reset lastKeySearchHint when creating a child decoder for nested objects. This causes the child decoder to inherit the parent's hint value, which points to an invalid offset in the child's index space. The hint should be reset to Constants.firstArrayObjectChildOffset for nested decoders. While the test at line 1634 documents this as working correctly due to wraparound logic, it's still an optimization inefficiency that causes unnecessary wraparound searches.

    func subDecoder(offsetBy offset: Int) -> _JSONDecoder {
        let subDescription = self.description.unsafeReadOnlySubDescription(offset: offset)
        return _JSONDecoder(description: subDescription, codingPath: _codingPath, pointer: pointer, settings: settings)
    }

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