perf: Speed up UTF-16 decoding with ASCII fast-path

pkg/util/utf16/utf16.go

@@ -36,27 +36,45 @@ const (
 func isHighSurrogate(r rune) bool { return r >= surr1 && r <= 0xdbff }
 func isLowSurrogate(r rune) bool  { return r >= surr2 && r <= 0xdfff }
 
-// Decode decodes the UTF16-encoded string to UTF-8 string. This function
-// exhibits much better performance than the standard library counterpart.
-// All credits go to: https://gist.github.com/skeeto/09f1410183d246f9b18cba95c4e602f0
+// Decode decodes the UTF16-encoded string to UTF-8 string using fast ASCII path.
+// This function exhibits much better performance than the standard library counterpart.
 func Decode(p []uint16) string {
-	s := make([]byte, 0, 2*len(p))
+	n := len(p)
+	if n == 0 {
+		return ""
+	}
+
+	s := make([]byte, 0, n*2)
+
 	for i := 0; i < len(p); i++ {
-		r := rune(0xfffd)
+		// ascii fast-path (0x0000–0x007F)
+		if p[i] <= 0x7F {
+			s = append(s, byte(p[i]))
+			continue
+		}
+
 		r1 := rune(p[i])
-		if isHighSurrogate(r1) {
-			if i+1 < len(p) {
-				r2 := rune(p[i+1])
-				if isLowSurrogate(r2) {
-					i++
-					r = 0x10000 + (r1-surr1)<<10 + (r2 - surr2)
-				}
+
+		// surrogate pair handling
+		if isHighSurrogate(r1) && i+1 < n {
+			r2 := rune(p[i+1])
+			if isLowSurrogate(r2) {
+				i++
+				r := 0x10000 + (r1-surr1)<<10 + (r2 - surr2)
+				s = utf8.AppendRune(s, r)
+				continue
 			}
-		} else if !isLowSurrogate(r) {
-			r = r1
 		}
-		s = utf8.AppendRune(s, r)
+
+		// non-surrogate BMP code point or malformed surrogate
+		if !isLowSurrogate(r1) {
+			s = utf8.AppendRune(s, r1)
+		} else {
+			// lone low surrogate to replacement char
+			s = utf8.AppendRune(s, utf8.RuneError)
+		}
 	}
+
 	return string(s)
 }