Use strings.Builder in unpackString (#746)

* Add test case and benchmark for unpackString helper

* Use strings.Builder in unpackString

msg_helpers.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"net"
 	"strconv"
+	"strings"
 )
 
 // helper functions called from the generated zmsg.go
@@ -267,29 +268,27 @@ func unpackString(msg []byte, off int) (string, int, error) {
 	if off+l+1 > len(msg) {
 		return "", off, &Error{err: "overflow unpacking txt"}
 	}
-	s := make([]byte, 0, l)
+	var s strings.Builder
+	s.Grow(l)
 	for _, b := range msg[off+1 : off+1+l] {
-		switch b {
-		case '"', '\\':
-			s = append(s, '\\', b)
-		default:
-			if b < 32 || b > 127 { // unprintable
-				var buf [3]byte
-				bufs := strconv.AppendInt(buf[:0], int64(b), 10)
-				s = append(s, '\\')
-				for i := 0; i < 3-len(bufs); i++ {
-					s = append(s, '0')
-				}
-				for _, r := range bufs {
-					s = append(s, r)
-				}
-			} else {
-				s = append(s, b)
+		switch {
+		case b == '"' || b == '\\':
+			s.WriteByte('\\')
+			s.WriteByte(b)
+		case b < 32 || b > 127: // unprintable
+			var buf [3]byte
+			bufs := strconv.AppendInt(buf[:0], int64(b), 10)
+			s.WriteByte('\\')
+			for i := len(bufs); i < 3; i++ {
+				s.WriteByte('0')
 			}
+			s.Write(bufs)
+		default:
+			s.WriteByte(b)
 		}
 	}
 	off += 1 + l
-	return string(s), off, nil
+	return s.String(), off, nil
 }
 
 func packString(s string, msg []byte, off int) (int, error) {

msg_helpers_test.go

@@ -106,3 +106,33 @@ func TestPackDataNsec(t *testing.T) {
 		})
 	}
 }
+
+func TestUnpackString(t *testing.T) {
+	msg := []byte("\x00abcdef\x0f\\\"ghi\x04mmm")
+	msg[0] = byte(len(msg) - 1)
+
+	got, _, err := unpackString(msg, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if want := `abcdef\015\\\"ghi\004mmm`; want != got {
+		t.Errorf("expected %q, got %q", want, got)
+	}
+}
+
+func BenchmarkUnpackString(b *testing.B) {
+	msg := []byte("\x00abcdef\x0f\\\"ghi\x04mmm")
+	msg[0] = byte(len(msg) - 1)
+
+	for n := 0; n < b.N; n++ {
+		got, _, err := unpackString(msg, 0)
+		if err != nil {
+			b.Fatal(err)
+		}
+
+		if want := `abcdef\015\\\"ghi\004mmm`; want != got {
+			b.Errorf("expected %q, got %q", want, got)
+		}
+	}
+}