fix: Improve performance of the UTF-8 string comparison logic

[dconeybe]

The real PR is: googleapis#2380

The semantics of this logic were originally fixed by googleapis#2275, but this fix caused a material performance degradation, which was then improved by googleapis#2299 The performance was, however, still suboptimal, and this PR further improves the speed back to close to its original speed and, serendipitously, simplifies the algorithm too.

This commit is a port of firebase/firebase-js-sdk#9143

[gemini-code-assist]

Summary of Changes

Hello @dconeybe, 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 optimizes the UTF-8 string comparison logic to improve performance and simplify the algorithm. It achieves this by directly comparing UTF-16 code units and handling surrogate pairs appropriately. The changes result in faster string comparisons without sacrificing correctness.

Highlights

• Performance Improvement: The primary goal of this PR is to improve the performance of UTF-8 string comparison logic, bringing it back to near its original speed before a previous fix introduced a performance degradation.
• Algorithm Simplification: Serendipitously, the performance improvements also led to a simplification of the string comparison algorithm.
• UTF-16 Code Unit Comparison: The comparison logic now directly compares UTF-16 code units (chars) for efficiency, leveraging the way UTF-8 and UTF-16 represent Unicode code points.
• Surrogate Handling: The code handles surrogate pairs correctly, ensuring that strings containing surrogates are ordered appropriately relative to non-surrogate strings.
• New Helper Function: Introduced a new helper function isSurrogate to determine if a character is a surrogate.
• Code Removal: Removed the getUtf8SafeSubstring function and the TextEncoder instantiation, simplifying the code and improving performance.

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[Copilot]

Pull Request Overview

This PR rewrites compareUtf8Strings to remove costly TextEncoder usage and byte-level comparisons, instead comparing UTF-16 code units directly and handling surrogate pairs via a new isSurrogate helper.

• Replaces multi-step UTF-8 byte encoding with direct character comparison.
• Introduces isSurrogate and removes getUtf8SafeSubstring and TextEncoder logic.
• Falls back to length-based comparison when one string is a prefix of the other.

Comments suppressed due to low confidence (2)

dev/src/order.ts:297

• [nitpick] Add a JSDoc comment for isSurrogate to explain its purpose, parameters, return value, and the expected range of inputs (e.g., that it checks for UTF-16 surrogate code units).

export function isSurrogate(s: string): boolean {

dev/src/order.ts:276

• Add unit tests covering the new code paths in compareUtf8Strings, including cases with surrogate pairs, non-surrogate characters, and prefix comparisons to ensure correct ordering for astral characters and edge cases.

  const length = Math.min(left.length, right.length);

[gemini-code-assist]

Code Review

This pull request significantly improves the performance and simplifies the logic for UTF-8 string comparison by replacing a complex implementation that used TextEncoder with a more direct character-by-character comparison. The new approach is clever and well-commented.

I have a couple of suggestions to further improve the new implementation's readability and encapsulation. Overall, this is a solid improvement.