RFR: 8370013: Refactor Double.toHexString to eliminate regex and StringBuilder [v12]

[Raffaello Giulietti]

On Tue, 21 Oct 2025 07:22:47 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 17 commits:
>> 
>>  - codestyle
>>  - Merge remote-tracking branch 'upstream/master' into float_to_hex_str_2510
>>    
>>    # Conflicts:
>>    #	test/jdk/java/lang/Double/ToHexString.java
>>  - copyright
>>  - fix test, from @jddarcy
>>  - extract doubleToLongBits, from @jddarcy
>>  - fix test comment
>>  - add tests for removed trailing zeros, from @jddarcy
>>  - from @rgiulietti
>>  - from @rgiulietti
>>  - code style, from @rgiulietti
>>  - ... and 7 more: https://git.openjdk.org/jdk/compare/eee29088...822c0259
>
> src/java.base/share/classes/java/lang/Double.java line 768:
> 
>> 766:             // Integer.digits maps values 0-15 to '0'-'f' characters
>> 767:             chars[index++] = Integer.digits[((int)(signifBits >> ((12 - i) << 2))) & 0xF];
>> 768:         }
> 
> Suggestion:
> 
>         for (int sh = 48, end = 4 * trailingZeros; sh >= end; sh -= 4) {
>             // Extract 4 bits at a time from left to right
>             // Shift right by sh positions and mask with 0xF
>             // Integer.digits maps values 0-15 to '0'-'f' characters
>             chars[index++] = Integer.digits[((int)(signifBits >> sh)) & 0xF];
>         }

Or even

        for (int sh = 4 * 12, end = 4 * trailingZeros; sh >= end; sh -= 4) {
            ...

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PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2447019997