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

[Johannes Döbler]

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.

Nice performance gains, but still this is quite a complex solution with a narrow use case (optimizes toString() of some java.time classes).
What about broadening the scope?
Situation: 
You want to build a Latin1 string and number of added chars is more or less known (so we can optimize byte[] allocations)

Solution sketch: Introduce a new Latin1Builder
- owning a byte[] value and int count field
- allowing only latin1 chars to be added
Example:


	public static void formatTo(Latin1Builder buf, LocalDate date) {
		buf.ensureCapacity(10); // most common size
		int year  = date.getYear();
		if (year < 0)
			buf.append('-');
		else if (year > 9999)
			buf.append('+');
		buf.appendInt(year);
		buf.append('-');
		buf.appendIntPair(buf, date.getMonthValue());
		buf.append('-');
		buf.appendIntPair(buf, date.getDayOfMonth());
	}

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PR Comment: https://git.openjdk.org/jdk/pull/26911#issuecomment-3229130870