use StringBuilder instead of Buffer in builtin and json package

[hackwaly]

This PR introduce StringBuilder in builtin package. Thus avoid expensive Buffer operations when use Show trait in js/wasm-gc target.

[coderabbitai]

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[coveralls]

Pull Request Test Coverage Report for Build 3332

Details

• 25 of 31 (80.65%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.08%) to 83.201%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtin/assert.mbt	0	1	0.0%
builtin/autoloc.mbt	0	1	0.0%
builtin/string.mbt	2	6	33.33%

Totals
Change from base Build 3330:	-0.08%
Covered Lines:	4096
Relevant Lines:	4923

---

💛 - Coveralls

[hackwaly]

Mark todo: add wasm-gc here once we have support for jsstring proposal on wasm-gc

[bobzhang]

do we have some concrete data points to justify the change?

[Yoorkin]

I was supposed the StringBuilder is a faster (in js backend) and safer abstraction than Buffer. The Byte/Buffer is more suitable to writing binary stuff, programmers use it at their own risk: the Buffer::to_string do not check the output string. While StringBuilder will ensure the result is valid, and the String and Char being writing into are also already checked.

But in this PR StringBuilder::sub_string will write unpaired surrogate into result string, which is not valid in UTF16. Furthermore the StringBuilder is specialized for js backend and in others backends it just a wrapper of Buffer. It seems possible to do the same thing for Buffer's implementation, which leads the StringBuilder becoming a subset of Buffer.

I wonder why we still want to introduce a new abstraction. What functionalities should StringBuilder and Buffer provide separately?

[hackwaly]

write_sub_string will be removed in StringBuilder. For now, to keep it just to make the diff smaller.

I wonder why we still want to introduce a new abstraction. What functionalities should StringBuilder and Buffer provide separately?

As you said, Buffer is more suitable to writing binary stuff, and StringBuilder only cares string concatenation. Buffer is not always the best approach to concat strings on each targets. It's too slow compared to concat based approach on js platform even wasm-gc platform if we support JsStringBuiltins proposal.

I'll present some data result later to show that.

[hackwaly]

The test result use this code: https://gist.github.com/hackwaly/24c2c16e25ef660772f3b06625ddb7a4

On my machine. Under js target: buffer based cost 5.7s. concat based cost 1.25s. @bobzhang

This PR also reduces simple println code javascript output by 120 SLOC.

[Yoorkin]

and the String and Char being writing into are also already checked.

I was wrong because the invalid UTF16 string (contains unpaired surrogate) can still produced by String::sub_string, or just take a char by String::op_get and write it into StringBuilder. Will StringBuilder validate the input and result string in future?

[hackwaly]

Probably not. It will align with the javascript's behavior. I think validation should be opt-in for users. It's best placed in unicode package?

[peter-jerry-ye-code-review]

From the provided git diff output, several changes have been made to update code to use a StringBuilder instead of a Buffer in various parts of the codebase. Here are the key observations and potential issues:

1. Renaming and Consolidation:

   • The introduction of StringBuilder as a replacement for Buffer in many contexts suggests a desire to consolidate string-building operations under a more semantically appropriate name. This is generally a good practice as it improves readability and maintainability.
   • The split into stringbuilder_buffer.mbt and stringbuilder_concat.mbt based on the target environment (js or non-js) indicates a need for different implementations of string concatenation in different runtime environments. This could be due to performance considerations or specific native support for string operations in JavaScript engines.

2. Potential Performance Implications:

   • The new StringBuilder implementations in stringbuilder_buffer.mbt and stringbuilder_concat.mbt should be carefully validated for performance, especially in high-throughput scenarios where string concatenation is frequent. The choice between Buffer and direct string concatenation (+ operator) can have significant performance differences.
   • The non-JavaScript implementation (stringbuilder_buffer.mbt) uses a Buffer internally, which is likely to be more efficient for large-scale concatenation due to its mutable nature. However, this needs to be benchmarked against the previous implementation to ensure there are no regressions.
   • The JavaScript-specific implementation (stringbuilder_concat.mbt) directly concatenates strings, which might be optimized by the JavaScript engine but could still lead to performance bottlenecks if not handled properly.

3. Error Handling and Validation:

   • The substring function and its related error-prone conditions have been moved and possibly consolidated. This is a good practice for maintaining code consistency and reducing duplication.
   • Ensure that the error messages and the conditions under which they are thrown are consistent and provide clear guidance for debugging. The new implementations should pass existing tests and any new tests should be added to cover edge cases, especially those related to negative indices and out-of-bound errors.

Recommendations:

1. Performance Testing:

   • Conduct thorough performance testing to ensure that the new StringBuilder implementations do not introduce performance regressions, especially in scenarios that involve heavy string manipulation.

2. Code Review and Validation:

   • Review the new implementations for any potential edge cases or inconsistencies in behavior compared to the old Buffer-based implementations.
   • Ensure all new and modified functions are covered by unit tests to catch any regressions or bugs introduced by the refactoring.

3. Documentation Updates:

   • Update any relevant documentation to reflect the new StringBuilder type and its usage, including any performance considerations for different environments.

By addressing these points, the transition to using StringBuilder should result in clearer, more maintainable code with minimal impact on performance.

[hackwaly]

By replace the internal StringBuilder previously used in json package. The performance @json.parse on wasm also be improved after this MR.