Ryu Implementation for Float-to-string Conversion

[zihaowang678]

Overview

This PR adds an implementation of the Ryu algorithm for efficient and accurate float-to-string conversion in ion-java. The Ryu algorithm provides performance improvements over traditional approaches while maintaining correctness.

Changes

• Added RyuDouble.java which implements the core Ryu algorithm for double values
• Added RoundingMode.java to support different rounding strategies
• Integrated with the existing Ion Java codebase for floating-point serialization

Benefits

• Performance comparable to or faster than the native Double.toString() method in JDK 21
• Consistent serialized size with runtime JDK 21; reduced serialized size with runtime JDK 17 and lower versions
• Improved memory allocation efficiency for JDK 17 and lower versions
• Guaranteed shortest correctly rounded string representation

References

• Original Ryu paper: "Ryū: fast float-to-string conversion" by Ulf Adams
• Implementation based on the reference Java version with adaptations for Ion's requirements

License

The implementation maintains the original Apache 2.0 license from Ulf Adams' reference implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 55.96026% with 133 lines in your changes missing coverage. Please review.

Project coverage is 67.71%. Comparing base (3c1b6b1) to head (88cdaac).
Report is 112 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/main/java/com/amazon/ion/impl/ryu/RyuDouble.java	56.12%	99 Missing and 30 partials ⚠️
...ain/java/com/amazon/ion/impl/ryu/RoundingMode.java	42.85%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1078      +/-   ##
============================================
+ Coverage     67.23%   67.71%   +0.48%     
- Complexity     5484     5603     +119     
============================================
  Files           159      162       +3     
  Lines         23025    23387     +362     
  Branches       4126     4218      +92     
============================================
+ Hits          15481    15837     +356     
+ Misses         6262     6247      -15     
- Partials       1282     1303      +21     

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[tgregg]

Here's something to try.

Since we'd be copying in the code with attribution instead of taking a dependency, we can modify the algorithm slightly to fit our needs.

You could try modifying the algorithm to output text Ion floats natively without the post-processing steps here: https://github.com/amazon-ion/ion-java/pull/1078/files#diff-b3377504685a7b6e1dddbc955be3d9c24e4f2e57e2e26a705c2733d2e5d88dabR844-R856

Maybe that would enable additional performance benefits.

[zihaowang678]

Implement Schubfach instead