Refactor fastJsonFormat for high-performance JSON formatting and Unicode decoding

[Sumith-Kumar-Saini]

Description

This PR refactors fastJsonFormat in src/index.js to improve speed, memory efficiency, and Unicode handling.
Key updates:

• Added decodeUnicodeString() for proper \uXXXX decoding (including surrogate pairs).
• Used Uint8Array lookup tables for structural and whitespace characters.
• Switched to chunked string building for better performance on large inputs.
• Removed legacy helper functions (scanString, scanAtom, etc.) and simplified logic.

Technical Details

• Fully backward compatible with previous output.
• Cleaner and more maintainable code with reduced overhead.
• Improved large-file handling and Unicode correctness.

Benchmark

• CPU: Intel(R) Core(TM) i5-8279U CPU @ 2.40GHz
• RAM: 16.0 GB
• GPU: Intel(R) Iris(R) Plus Graphics 655

Testing

• Manually verified formatting accuracy and Unicode output.
• Compared outputs with JSON.stringify and legacy implementation.
• Passed all existing fastJsonFormat tests.

[Sumith-Kumar-Saini]

@helloanoop, I would be grateful if you could review my pull request.

[helloanoop]

@Sumith-Kumar-Saini can you share screenshots of benchmark run before and after your changes, so that I can see the exact improvement across different data sizes

[Sumith-Kumar-Saini]

Hey @helloanoop 👋
Please find the benchmark screenshots before and after the changes below.
The left side shows results from the old code (upstream/main), and the right side shows results after my changes.

Benchmarks:

System (for reference):

• Intel Core i5 (4 cores / 8 threads, ~2.4 GHz)
• 16 GB RAM
• Integrated Intel GPU
• Windows 10 Pro

Let me know if you’d like additional sizes or runs.

[helloanoop]

@Sumith-Kumar-Saini This is fantastic. Wanted to have further discussion, Can you accept my connection on Linkedin. Lets talk there

[Sumith-Kumar-Saini]

Thanks! Glad you found it useful. I've accepted your LinkedIn connection — happy to continue the discussion there.

[helloanoop]

Hey @Sumith-Kumar-Saini Really impressed with the perf improvement 👏 👏

I ran benchmark on my system, while the performance doubles for files around 100kb, it gets worse than the current performance for larger data sizes. Could you check what might be causing that ?

I am running this on Macbook M4 Air 16GB Ram

[Sumith-Kumar-Saini]

Thanks for catching that, @helloanoop, I’ll dig into the large file performance issue and see what’s causing the slowdown — will share an update soon.