fix: Further improve performance of the UTF-8 string comparison logic

[dconeybe]

This PR is a mirror of googleapis#2182

The semantics of the UTF-8 string comparison logic were originally fixed by googleapis#1967, but this fix caused a material performance degradation, which was then improved by googleapis#2021. The performance was, however, still sub-optimal, and this PR further improves the speed back to close to its original speed and, serendipitously, simplifies the algorithm too.

This PR effectively ports the following two PRs from the firebase-android-sdk repository:

• fix(firestore): Further improved performance of UTF-8 string comparison logic firebase/firebase-android-sdk#7098
• Firestore: use Integer.compare() and Long.compare() instead Firestore bespoke implementations firebase/firebase-android-sdk#7109

[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 delivers a crucial performance enhancement and simplification to the UTF-8 string comparison logic within the Firestore client library. It addresses prior performance regressions by introducing a highly optimized and more elegant algorithm for string ordering, ensuring efficient and correct data sorting.

Highlights

• Performance Optimization: The core compareUtf8Strings method in Order.java has been completely rewritten to significantly improve performance and simplify its logic. The new algorithm leverages the relationship between UTF-8 and UTF-16 representations for more efficient string comparison, avoiding costly byte string conversions.
• Algorithm Simplification: The previous complex logic involving codePointAt and ByteString.copyFromUtf8 for non-ASCII characters has been replaced with a more straightforward character-by-character comparison that intelligently handles surrogate pairs.
• Test Robustness: The OrderTest.java has been updated to improve the robustness of the compareUtf8Strings test. Instead of asserting exact return values, it now checks the signum (sign) of the comparison result, which is a more appropriate way to validate comparator behavior.

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[Copilot]

Pull Request Overview

This PR further improves performance of Firebase’s UTF-8 string comparison by simplifying the algorithm and adjusts the corresponding tests to validate only the comparison sign.

• Simplified compareUtf8Strings in Order.java to a single-pass UTF-16 char comparison leveraging surrogate checks.
• Updated OrderTest.java to compare only the sign of results and introduced a signName helper for clearer error messages.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Order.java	Replaced the multi-step code-point and byte-encoding logic with a direct UTF-16 char loop using Character.isSurrogate and Character.compare for performance.
OrderTest.java	Changed test assertions to use Integer.signum, added signName(int) to describe result signs, and updated error output accordingly.

Comments suppressed due to low confidence (1)

google-cloud-firestore/src/test/java/com/google/cloud/firestore/OrderTest.java:235

• The error message prints only the descriptive expected sign but omits the actual numeric expected value. For consistency with the actual output, include (" + expected + ") after signName(expected) to display the numeric value as well.

                + signName(expected)

[gemini-code-assist]

Code Review

This pull request improves the performance of UTF-8 string comparison. The new implementation is simpler and the test suite has been updated to verify the comparator's behavior.