Add payload conversion method

Add a payload conversion method to ease UTF8 conversion.

message.go

@@ -121,12 +121,6 @@ const (
 	MsgReplyUtf16 = byte(193)
 )
 
-// Payload represents an encoded message payload
-type Payload struct {
-	Encoding PayloadEncoding
-	Data     []byte
-}
-
 // Message represents a WebWire protocol message
 type Message struct {
 	fulfill func(reply Payload)

payload.go

@@ -0,0 +1,37 @@
+package webwire
+
+import (
+	"bytes"
+	"fmt"
+	"unicode/utf16"
+	"unicode/utf8"
+)
+
+// Payload represents an encoded message payload
+type Payload struct {
+	Encoding PayloadEncoding
+	Data     []byte
+}
+
+// Utf8 returns a UTF8 representation of the payload data
+func (pld *Payload) Utf8() (string, error) {
+	if pld.Encoding == EncodingUtf16 {
+		if len(pld.Data)%2 != 0 {
+			return "", fmt.Errorf("Cannot convert invalid UTF16 payload data to UTF8")
+		}
+		u16str := make([]uint16, 1)
+		// TODO: check whether the builtin append would be more efficient than bytes.Buffer
+		utf8str := &bytes.Buffer{}
+		utf8buf := make([]byte, 4)
+		for i := 0; i < len(pld.Data); i += 2 {
+			u16str[0] = uint16(pld.Data[i]) + (uint16(pld.Data[i+1]) << 8)
+			rn := utf16.Decode(u16str)
+			rnSize := utf8.EncodeRune(utf8buf, rn[0])
+			utf8str.Write(utf8buf[:rnSize])
+		}
+		return utf8str.String(), nil
+	}
+
+	// Binary and UTF8 encoded payloads should pass through untouched
+	return string(pld.Data), nil
+}