Double roundtripping

[tayloraswift]

fix #83
fix #91

[gemini-code-assist]

Summary of Changes

Hello @tayloraswift, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a substantial overhaul of how JSON numbers are represented, parsed, and encoded within the library. The primary goal is to improve the accuracy and robustness of numeric handling, particularly for floating-point values and non-finite numbers, by adopting a more flexible and efficient internal representation. This refactoring also aligns the project with the latest Swift 6.2 toolchain and enhances the overall testing coverage for numeric types.

Highlights

• Swift Version Upgrade: The project has been updated to require Swift 6.2, with corresponding changes in Package.swift and README.md. New Swift settings for upcoming features and warnings have also been enabled.
• Refactored JSON Number Representation: The core JSON.Number type has been refactored from a struct to an enum, now supporting inline (for efficient fixed-point numbers), fallback (for complex numbers represented as strings), infinity, nan, and snan. A new JSON.Number.Inline struct was introduced for memory-efficient fixed-point number storage.
• Improved Floating-Point Handling and Roundtripping: The parsing and encoding logic for numbers has been significantly enhanced to correctly handle and roundtrip floating-point values, including Float16, Float, Double, and Float80, as well as non-finite numbers like NaN, sNaN, and Infinity. This ensures better precision and interoperability.
• Simplified JSON Node and Object Structures: The JSON.Node enum no longer has a separate numberExtension_ case, consolidating all number types under the number case. Additionally, the JSON.Object's special init(encoding:) for numbers has been removed, streamlining the API.
• Enhanced Numeric Testing: A new dedicated test suite, ParsingNumerics.swift, has been added to provide comprehensive testing for number parsing and roundtripping, covering various integer and floating-point edge cases.

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[gemini-code-assist]

Code Review

This pull request introduces a significant refactoring of how numeric values are handled to ensure lossless round-tripping of floating-point numbers, which resolves issues #83 and #91. The JSON.Number type is changed from a struct to an enum to support non-finite numbers and values that don't fit into a 64-bit representation, using a string fallback. The parsing and encoding logic is updated accordingly, with new tests to verify the round-tripping behavior.

My review has identified a critical issue in the number parsing logic where an integer overflow can occur, potentially crashing the application. I've also suggested a simplification for the Swift settings configuration in Package.swift to improve readability and maintainability.

[gemini-code-assist]

As noted in the comment, places += exponent.magnitude can cause an integer overflow, which would lead to a crash in debug builds or incorrect parsing in release builds. You should use addingReportingOverflow to safely perform the addition and fall back to the string representation if an overflow occurs.

Suggested change

	case .minus:
	// note: potential crash if `exponent.magnitude` is absurdly large
	places += exponent.magnitude
	case .minus:
	let (sum, overflow) = places.addingReportingOverflow(exponent.magnitude)
	if overflow { break representable }
	places = sum

[gemini-code-assist]

This block of code for adding Swift settings is overly complex and uses an obscure pattern with an immediately-invoked closure and an inout parameter. This makes the code hard to read and maintain. A more straightforward approach using a guard statement and direct property assignment would be much clearer.

package.targets = package.targets.map {
    guard $0.type != .plugin, $0.type != .binary else {
        return $0
    }

    $0.swiftSettings = ($0.swiftSettings ?? []) + [
        .enableUpcomingFeature("ExistentialAny"),
        .treatWarning("ExistentialAny", as: .error),
        .treatWarning("MutableGlobalVariable", as: .error),
    ]
    return $0
}