RFR: 8377427: Reduce substring allocations in Color.web(String, double) [v10]

[Michael Strauß]

On Fri, 20 Feb 2026 00:59:24 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> Here are some test cases that may be worth including, because they relate to different code paths, or might break in case some of the rounding rules aren't implemented correctly. They come directly from examples in the algorithm's original paper. Rounding in the code is done exactly as prescribed in the algorithm everywhere I checked.
>> 
>> assertSameDouble(0.2, "0.2");
>> assertSameDouble(0.2, "0.20000000000000001110");
>> assertSameDouble(0.19999999999999998335, "0.19999999999999998335");
>> assertSameDouble(9000000000000000., "9000000000000000.5");
>> assertSameDouble(9000000000000002., "9000000000000001.5");
>> assertSameDouble(9000000000000002., "9000000000000002.5");
>> assertSameDouble(1e22, "1e22");
>> assertSameDouble(1e23, "1e23");
>> assertSameDouble(7450580596923828125e-27, "7450580596923828125e-27");
>> assertSameDouble(2440254496e57, "2440254496e57");
>> assertSameDouble(9007199254740993., "9007199254740993");
>> assertSameDouble(9.109e-31, "9.109e-31");
>> 
>> Testing indicates all but the third one work. There, the algorithm isn't even used, though.
>
> I've added these tests. The reason why `assertSameDouble(0.19999999999999998335, "0.19999999999999998335")` fails is because there are more digits in the number than fit in a `long`, and thus the actual number being parsed is truncated.

Interestingly, the test passes if we use Lemire's algorithm with the truncated significand instead of using the fallback. This might be a coincidence.

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PR Review Comment: https://git.openjdk.org/jfx/pull/2069#discussion_r2830940879