Summary

Add a fast path to Int32ArrayFormatter.Deserialize that bulk-reads fixint-encoded int arrays in a single pass instead of calling ReadInt32() per element.

How it works

MessagePack encodes small integers (0–127) as a single byte called "positive fixint" — the byte value is the integer. The existing code deserializes int[] by calling ReadInt32() in a loop, which for each element: dispatches through format detection, decodes the value, and advances the reader.

The new TryReadFixIntArray method iterates the unread span once, validating each byte is ≤ 0x7f and copying it to the output array in the same loop. If any byte fails the check, it bails early and the caller falls back to the original element-by-element ReadInt32 loop — the fallback overwrites any partial data.

Before: for each element → ReadInt32() → TryReadInt32() → format detection → decode → Advance
After:  single loop: validate + copy each byte → one Advance

Fixint values (0–127) are common in int arrays: enum values, indices, counts, flags, IDs, and game state data.

Per-array cost (100-element fixint array)

Changes

• MessagePackReader.cs — add internal TryReadFixIntArray(Int32[] array)
• PrimitiveFormatter.cs — call TryReadFixIntArray before element-by-element fallback in Int32ArrayFormatter.Deserialize

Profiling

Profiled with metreja using a 1M serialize+deserialize harness, method_stats events, inlining enabled. 5 runs for statistical confidence.

metreja v1.0.20, macOS ARM64 (Apple M1 Pro), .NET 10.0.2

Test plan

• All 1,000 MessagePack.Tests pass
• 5-run profiling for statistical confidence
• No regression on non-fixint arrays (falls back to original loop)

🤖 Generated with Claude Code