Ascii85 Improvements

[tehtelev]

Summary

• Replace StringBuilder with string.Create: Accurate length calculation and direct writing via Span completely eliminate heap allocations and unnecessary memory copies during encoding.
• Replace MemoryStream with a byte[] array: Direct writing by index (byteIdx) eliminates stream overhead and dynamic expansion of internal buffers during decoding.
• Switch from char[] arrays to stackalloc: Temporary buffers are allocated on the stack, which is instantaneous and does not burden the garbage collector (GC).
• Move stackalloc outside the loop: Reusing a single stack buffer prevents unnecessary stack pointer shifts on each iteration.
• Add static to lambda: The modifier disables closure capture of external variables, preventing hidden heap allocations of closure classes during method invocation.
• Method inlining (EncodeValue, DecodeValue): Inlining logic directly into loops reduces method call overhead and improves the JIT compiler's optimization capabilities.
• Tighter overflow checking (checked): Fixing implicit arithmetic and adding a try/catch to the padding block ensures that invalid data will cause a FormatException rather than silently corrupting the result due to uint overflow.

Also sent to Stratum

Performance numbers

Tested on the generation of 10200 chunk columns:

• Total GC memory allocations: reduced from 733 MB to 157 MB.

Before

After