add cos.Builder (micro-optimizations)

* separately, cos.BytePack to return unsafe string

Signed-off-by: Alex Aizman <[email protected]>

cmn/cos/bytepack.go

@@ -1,6 +1,6 @@
 // Package cos provides common low-level types and utilities for all aistore projects
 /*
- * Copyright (c) 2018-2022, NVIDIA CORPORATION. All rights reserved.
+ * Copyright (c) 2018-2025, NVIDIA CORPORATION. All rights reserved.
  */
 package cos
 
@@ -195,7 +195,7 @@ func (br *ByteUnpack) ReadString() (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return string(bytes), nil
+	return UnsafeS(bytes), nil
 }
 
 func (br *ByteUnpack) ReadAny(st Unpacker) error {

cmn/cos/sbuilder.go

@@ -0,0 +1,39 @@
+// Package cos provides common low-level types and utilities for all aistore projects
+/*
+ * Copyright (c) 2025, NVIDIA CORPORATION. All rights reserved.
+ */
+package cos
+
+import (
+	"unsafe"
+)
+
+// implementation:
+// - reusable, single-threaded, best-effort, and once-allocated
+// motivation:
+// - to optimally replace `strings.Builder` when applicable
+
+type Builder struct {
+	buf []byte
+}
+
+func (b *Builder) String() string {
+	return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
+}
+
+func (b *Builder) Reset(size int) {
+	switch {
+	case b.buf == nil:
+		b.buf = make([]byte, 0, size)
+	case len(b.buf) >= size-size>>1: // vs. previous usage
+		b.buf = make([]byte, 0, size<<1)
+	default:
+		b.buf = b.buf[:0]
+	}
+}
+
+func (b *Builder) Len() int { return len(b.buf) }
+func (b *Builder) Cap() int { return cap(b.buf) }
+
+func (b *Builder) WriteByte(c byte)     { b.buf = append(b.buf, c) }
+func (b *Builder) WriteString(s string) { b.buf = append(b.buf, s...) }