opitimize json interface methods

A common helper function is used for UUID.String(), UUID.URN(), and
UUID.MarshalJSON().  Any perforance hit in UUID.String() and
UUID.Marshal() appears to be negligable.  The benefit to
UUID.MarshalJSON() is several hundred nanoseconds (23% faster) and 2
allocations (21% fewer bytes).

Some redundant checks are removed from UUID.UnmarshalJSON() method.  The
"encoding/json".Unmarshaler interface specifies that implementations can
assume input is valid JSON content.  This allows one to assume that (1)
input is not empty and (2) if index 0 is a quote, then the content is a
json string and the last index will contain a terminating quote.  The
second point is not completely explicit in the documentation but it is
true in practice (and it is safe to assume -- errors will be caught).

json.go

@@ -7,17 +7,21 @@ package uuid
 import "errors"
 
 func (u UUID) MarshalJSON() ([]byte, error) {
-	if len(u) == 0 {
+	if len(u) != 16 {
 		return []byte(`""`), nil
 	}
-	return []byte(`"` + u.String() + `"`), nil
+	var js [38]byte
+	js[0] = '"'
+	encodeHex(js[1:], u)
+	js[37] = '"'
+	return js[:], nil
 }
 
 func (u *UUID) UnmarshalJSON(data []byte) error {
-	if len(data) == 0 || string(data) == `""` {
+	if string(data) == `""` {
 		return nil
 	}
-	if len(data) < 2 || data[0] != '"' || data[len(data)-1] != '"' {
+	if data[0] != '"' {
 		return errors.New("invalid UUID format")
 	}
 	data = data[1 : len(data)-1]

uuid.go

@@ -83,15 +83,7 @@ func (uuid UUID) String() string {
 		return ""
 	}
 	var buf [36]byte
-	hex.Encode(buf[:], uuid[:4])
-	buf[8] = '-'
-	hex.Encode(buf[9:13], uuid[4:6])
-	buf[13] = '-'
-	hex.Encode(buf[14:18], uuid[6:8])
-	buf[18] = '-'
-	hex.Encode(buf[19:23], uuid[8:10])
-	buf[23] = '-'
-	hex.Encode(buf[24:], uuid[10:])
+	encodeHex(buf[:], uuid)
 	return string(buf[:])
 }
 
@@ -103,19 +95,22 @@ func (uuid UUID) URN() string {
 	}
 	var buf [36 + 9]byte
 	copy(buf[:], "urn:uuid:")
-	_buf := buf[9:]
-	hex.Encode(_buf[:], uuid[:4])
-	_buf[8] = '-'
-	hex.Encode(_buf[9:13], uuid[4:6])
-	_buf[13] = '-'
-	hex.Encode(_buf[14:18], uuid[6:8])
-	_buf[18] = '-'
-	hex.Encode(_buf[19:23], uuid[8:10])
-	_buf[23] = '-'
-	hex.Encode(_buf[24:], uuid[10:])
+	encodeHex(buf[9:], uuid)
 	return string(buf[:])
 }
 
+func encodeHex(dst []byte, uuid UUID) {
+	hex.Encode(dst[:], uuid[:4])
+	dst[8] = '-'
+	hex.Encode(dst[9:13], uuid[4:6])
+	dst[13] = '-'
+	hex.Encode(dst[14:18], uuid[6:8])
+	dst[18] = '-'
+	hex.Encode(dst[19:23], uuid[8:10])
+	dst[23] = '-'
+	hex.Encode(dst[24:], uuid[10:])
+}
+
 // Variant returns the variant encoded in uuid.  It returns Invalid if
 // uuid is invalid.
 func (uuid UUID) Variant() Variant {