RFR: 8356709: Avoid redundant String formatting in BigDecimal.valueOf(double) [v2]

[Raffaello Giulietti]

On Tue, 13 May 2025 13:03:42 GMT, Johannes Graham <duke at openjdk.org> wrote:

>> Optimize `BigDecimal.valueOf(double)` by using `FormattedFPDecimal` instead of converting to decimal string and then parsing it. This results in an approximate 6x improvement for me.
>
> Johannes Graham has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix code tag in javadoc

src/java.base/share/classes/java/math/BigDecimal.java line 1408:

> 1406:         }
> 1407: 
> 1408:         return valueOf(s, fmt.getScale(), fmt.getPrecision());

I'd prefer to have a `getExp()` rather than `getScale()` in `FormattedFPDecimal`.

src/java.base/share/classes/jdk/internal/math/FormattedFPDecimal.java line 100:

> 98:                 // Keep digits to left of decimal, plus leave a
> 99:                 // trailing zero
> 100:                     (expR + 1) + 1;

I think that the comment is sufficiently clear to be able to replace `(expR + 1) + 1` with `expR + 2`

src/java.base/share/classes/jdk/internal/math/FormattedFPDecimal.java line 106:

> 104:                 // there is a single digit
> 105:                     2;
> 106:         };

Can we have a chain of `if`s or conditional expressions (`? :`)?

src/java.base/share/classes/jdk/internal/math/FormattedFPDecimal.java line 129:

> 127: 
> 128:         return fd;
> 129:     }

Please be consistent with the usage of `final` for local vars in the context of a single method.

src/java.base/share/classes/jdk/internal/math/FormattedFPDecimal.java line 147:

> 145:     public int getScale() {
> 146:         return -e;
> 147:     }

I'd prefer to have a `getExp()` rather than `getScale()` here, as everything here uses an exponent rather than a scale.
Then there's no need for the JavaDoc comment.

test/jdk/java/math/BigDecimal/ValueOfDouble.java line 41:

> 39: 
> 40: public class ValueOfDouble {
> 41:     private static final String DIGITS = "1234567899123456789"; // Enough digits to fill a long

Suggestion:

    private static final String DIGITS = "1234567000003456789"; // Enough digits to fill a long

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PR Review Comment: https://git.openjdk.org/jdk/pull/25173#discussion_r2088553632
PR Review Comment: https://git.openjdk.org/jdk/pull/25173#discussion_r2088558336
PR Review Comment: https://git.openjdk.org/jdk/pull/25173#discussion_r2088561193
PR Review Comment: https://git.openjdk.org/jdk/pull/25173#discussion_r2088593281
PR Review Comment: https://git.openjdk.org/jdk/pull/25173#discussion_r2088563587
PR Review Comment: https://git.openjdk.org/jdk/pull/25173#discussion_r2088576102