refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/header.go

@@ -0,0 +1,46 @@
+package peer
+
+import "unsafe"
+
+// ----------------------------------------------------------------------------
+
+const (
+	headerSize        = 12
+	controlStreamID   = 2
+	controlHeaderSize = 24
+	dataStreamID      = 1
+	dataHeaderSize    = 12
+)
+
+type header struct {
+	Version  byte
+	StreamID byte
+	SourceIP byte
+	DestIP   byte
+	Counter  uint64 // Init with time.Now().Unix << 30 to ensure monotonic.
+}
+
+func parseHeader(b []byte) (h header) {
+	h.Version = b[0]
+	h.StreamID = b[1]
+	h.SourceIP = b[2]
+	h.DestIP = b[3]
+	h.Counter = *(*uint64)(unsafe.Pointer(&b[4]))
+	return h
+}
+
+func (h *header) Parse(b []byte) {
+	h.Version = b[0]
+	h.StreamID = b[1]
+	h.SourceIP = b[2]
+	h.DestIP = b[3]
+	h.Counter = *(*uint64)(unsafe.Pointer(&b[4]))
+}
+
+func (h *header) Marshal(buf []byte) {
+	buf[0] = h.Version
+	buf[1] = h.StreamID
+	buf[2] = h.SourceIP
+	buf[3] = h.DestIP
+	*(*uint64)(unsafe.Pointer(&buf[4])) = h.Counter
+}