stream/mts/meta: using order slice so that encoded order reamins consistent with order that we added metadata. And also now using binary.BigEndian.PutUint16() to encode len into metdata rather than doing ugly byte manipulation

stream/mts/meta/meta.go

@@ -45,8 +45,7 @@ const (
 
 // Indices of bytes for uint16 metadata length.
 const (
-	dataLenIdx1 = 2
-	dataLenIdx2 = 3
+	dataLenIdx = 2
 )
 
 var (
@@ -58,9 +57,10 @@ var (
 // Metadata provides functionality for the storage and encoding of metadata
 // using a map.
 type Metadata struct {
-	mu   sync.RWMutex
-	data map[string]string
-	enc  []byte
+	mu    sync.RWMutex
+	data  map[string]string
+	order []string
+	enc   []byte
 }
 
 // New returns a pointer to a new Metadata.
@@ -80,6 +80,7 @@ func New() *Metadata {
 func (m *Metadata) Add(key, val string) {
 	m.mu.Lock()
 	m.data[key] = val
+	m.order = append(m.order, key)
 	m.mu.Unlock()
 }
 
@@ -111,6 +112,12 @@ func (m *Metadata) Delete(key string) error {
 	defer m.mu.Unlock()
 	if _, ok := m.data[key]; ok {
 		delete(m.data, key)
+		for i, k := range m.order {
+			if k == key {
+				m.order = append(m.order[:i], m.order[i+1:]...)
+				break
+			}
+		}
 		return nil
 	}
 	return errKeyAbsent
@@ -123,12 +130,11 @@ func (m *Metadata) Encode() []byte {
 
 	// Iterate over map and append entries, only adding tab if we're not on the
 	// last entry.
-	var i int
 	var entry string
-	for k, v := range m.data {
-		i++
+	for i, k := range m.order {
+		v := m.data[k]
 		entry += k + "=" + v
-		if i < len(m.data) {
+		if i+1 < len(m.data) {
 			entry += "\t"
 		}
 	}
@@ -136,9 +142,7 @@ func (m *Metadata) Encode() []byte {
 
 	// Calculate and set data length in encoded meta header.
 	dataLen := len(m.enc[headSize:])
-	m.enc[dataLenIdx1] = byte(dataLen >> 8)
-	m.enc[dataLenIdx2] = byte(dataLen)
-
+	binary.BigEndian.PutUint16(m.enc[dataLenIdx:dataLenIdx+2], uint16(dataLen))
 	return m.enc
 }