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

[Mikhail Yankelevich]

On Sat, 23 Aug 2025 04:06:13 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.

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 928:

> 926:             DecimalDigits.uncheckedPutPairLatin1(value, count, i);
> 927:         } else {
> 928:             DecimalDigits.uncheckedPutPairUTF16(value, count, i);

I believe this is not covered by current tests, so could you please add the test if it's easy to reach?

src/java.base/share/classes/jdk/internal/util/DecimalDigits.java line 455:

> 453:      * If the value is between 0 and 9, it is formatted with a leading zero
> 454:      * (e.g., 5 becomes "05"). If the value is outside the range 0-99,
> 455:      * the behavior is unspecified.

Just out of interest, why leave the behaviour as unspecified instead of validating and throwing an exception?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26911#discussion_r2303663156
PR Review Comment: https://git.openjdk.org/jdk/pull/26911#discussion_r2303685649