RFR: 8366224: Introduce DecimalDigits.appendPair for efficient two-digit formatting and refactor DateTimeHelper [v4]

[Johannes Graham]

On Mon, 8 Sep 2025 02:27:10 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> This PR introduces a new efficient API for appending two-digit integers to StringBuilders and refactors DateTimeHelper to leverage this new functionality.
>> 
>> Changes include:
>> 
>> 1. New `appendPair` method for efficient two-digit integer formatting (00-99):
>>    - Added `AbstractStringBuilder.appendPair(int i)` with core implementation
>>    - Added `JavaLangAccess.appendPair(StringBuilder, int)` for internal access
>>    - Added `System.JavaLangAccessImpl.appendPair(StringBuilder, int)` bridge
>>    - Added `DecimalDigits.appendPair(StringBuilder, int)` public static utility method
>>    - Enhanced Javadoc documentation for all new methods
>> 
>> 2. Refactored `DateTimeHelper` to use the new `DecimalDigits.appendPair`:
>>    - Updated `DateTimeHelper.formatTo` methods for `LocalDate` and `LocalTime`
>>    - Replaced manual formatting logic with the new efficient two-digit appending
>>    - Improved code clarity and consistency in date/time formatting
>> 
>> These changes improve code clarity and performance when formatting two-digit numbers, particularly in date/time formatting scenarios.
>
> Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
> 
>  - Merge remote-tracking branch 'upstream/master' into appendPair_202508
>    
>    # Conflicts:
>    #	src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java
>  - Add DecimalDigitsTest to verify appendPair method with LATIN1 and UTF16 encoding
>  - Use DecimalDigits.appendPair for formatting in time classes
>    
>    This change modifies the toString() methods in MonthDay, YearMonth, ZoneOffset, and ChronoLocalDateImpl to use DecimalDigits.appendPair for formatting two-digit numbers. This provides a more efficient and consistent way to format these values.
>    
>    Also added a comment in ChronoLocalDateImpl.toString() to explain why get() is used instead of getLong() for performance reasons, as the values are guaranteed to be within the int range for all chronologies.
>    
>    Co-authored-by: Qwen-Coder <qwen-coder at alibabacloud.com>
>  - Optimize year formatting in DateTimeHelper by reducing modulo operation
>    
>    Co-authored-by: Qwen-Coder <qwen-coder at alibabacloud.com>
>    
>    Refactored the year formatting logic to use subtraction instead of modulo for calculating the lower two digits, which can be slightly more efficient. Added a comment to clarify the safety of the input range for DecimalDigits.appendPair.
>  - Refactor DateTimeHelper to use DecimalDigits.appendPair for date/time formatting\n\nThis change updates DateTimeHelper.formatTo methods for LocalDate and LocalTime\nto use the new DecimalDigits.appendPair method for formatting month, day, hour,\nminute, and second components. This improves code clarity and leverages the\nnewly introduced efficient two-digit integer appending functionality.
>    
>    Co-authored-by: Qwen-Coder <qwen-coder at alibabacloud.com>
>  - Introduce appendPair method for efficient two-digit integer appending\n\nThis change adds a new internal API to efficiently append two-digit integers\n(00-99) to a StringBuilder. It includes:\n- AbstractStringBuilder.appendPair(int i): The core implementation.\n- JavaLangAccess.appendPair(StringBuilder, int): For internal access.\n- System.JavaLangAccessImpl.appendPair(StringBuilder, int): Bridge to AbstractStringBuilder.\n- DecimalDigits.appendPair(StringBuilder, int): Public static utility method.\n\nImproved Javadoc comments for clarity and consistency across all new methods.
>    
>    Co-authored-by: Qwen-Coder <qwen-coder at alibabacloud.com>

What is the performance like with a simpler implementation of DecimalDigits.appendPair? One that uses the DIGITS table and then simply appends each character. This would avoid the JavaLangAccess concern, but might still provide some of the benefit.

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PR Comment: https://git.openjdk.org/jdk/pull/26911#issuecomment-3289899032