net: don't import bytes or fmt in mac.go

Also add some more MAC tests.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5728065

src/pkg/net/mac.go

@@ -6,24 +6,26 @@
 
 package net
 
-import (
-	"bytes"
-	"errors"
-	"fmt"
-)
+import "errors"
+
+const hexDigit = "0123456789abcdef"
 
 // A HardwareAddr represents a physical hardware address.
 type HardwareAddr []byte
 
 func (a HardwareAddr) String() string {
-	var buf bytes.Buffer
+	if len(a) == 0 {
+		return ""
+	}
+	buf := make([]byte, 0, len(a)*3-1)
 	for i, b := range a {
 		if i > 0 {
-			buf.WriteByte(':')
+			buf = append(buf, ':')
 		}
-		fmt.Fprintf(&buf, "%02x", b)
+		buf = append(buf, hexDigit[b>>4])
+		buf = append(buf, hexDigit[b&0xF])
 	}
-	return buf.String()
+	return string(buf)
 }
 
 // ParseMAC parses s as an IEEE 802 MAC-48, EUI-48, or EUI-64 using one of the

src/pkg/net/mac_test.go

@@ -43,12 +43,24 @@ func match(err error, s string) bool {
 	return err != nil && strings.Contains(err.Error(), s)
 }
 
-func TestParseMAC(t *testing.T) {
-	for _, tt := range mactests {
+func TestMACParseString(t *testing.T) {
+	for i, tt := range mactests {
 		out, err := ParseMAC(tt.in)
 		if !reflect.DeepEqual(out, tt.out) || !match(err, tt.err) {
 			t.Errorf("ParseMAC(%q) = %v, %v, want %v, %v", tt.in, out, err, tt.out,
 				tt.err)
 		}
+		if tt.err == "" {
+			// Verify that serialization works too, and that it round-trips.
+			s := out.String()
+			out2, err := ParseMAC(s)
+			if err != nil {
+				t.Errorf("%d. ParseMAC(%q) = %v", i, s, err)
+				continue
+			}
+			if !reflect.DeepEqual(out2, out) {
+				t.Errorf("%d. ParseMAC(%q) = %v, want %v", i, s, out2, out)
+			}
+		}
 	}
 }