refactor/serializing (#1)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: https://git.tornberg.me/mats/go-cart-actor/pulls/1

message.go

@@ -0,0 +1,111 @@
+package main
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"time"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+	"google.golang.org/protobuf/proto"
+)
+
+type StorableMessage interface {
+	Write(w io.Writer) error
+}
+
+type Message struct {
+	Type      uint16
+	TimeStamp *int64
+	Content   interface{}
+}
+
+type MessageWriter struct {
+	io.Writer
+}
+
+type StorableMessageHeader struct {
+	Version    uint16
+	Type       uint16
+	TimeStamp  int64
+	DataLength uint64
+}
+
+func GetData(fn func(w io.Writer) error) ([]byte, error) {
+	var buf bytes.Buffer
+	err := fn(&buf)
+	if err != nil {
+		return nil, err
+	}
+	b := buf.Bytes()
+	return b, nil
+}
+
+func (m Message) Write(w io.Writer) error {
+	data, err := GetData(func(wr io.Writer) error {
+		if m.Type == AddRequestType {
+			messageBytes, err := proto.Marshal(m.Content.(*messages.AddRequest))
+			if err != nil {
+				return err
+			}
+			wr.Write(messageBytes)
+		} else if m.Type == AddItemType {
+			messageBytes, err := proto.Marshal(m.Content.(*messages.AddItem))
+			if err != nil {
+				return err
+			}
+			wr.Write(messageBytes)
+		}
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+	ts := time.Now().Unix()
+	if m.TimeStamp != nil {
+		ts = *m.TimeStamp
+	}
+
+	err = binary.Write(w, binary.LittleEndian, StorableMessageHeader{
+		Version:    1,
+		Type:       m.Type,
+		TimeStamp:  ts,
+		DataLength: uint64(len(data)),
+	})
+	w.Write(data)
+
+	return err
+}
+
+func MessageFromReader(reader io.Reader, m *Message) error {
+	header := StorableMessageHeader{}
+	err := binary.Read(reader, binary.LittleEndian, &header)
+	if err != nil {
+		return err
+	}
+	messageBytes := make([]byte, header.DataLength)
+	_, err = reader.Read(messageBytes)
+	if err != nil {
+		return err
+	}
+	switch header.Type {
+	case AddRequestType:
+		msg := &messages.AddRequest{}
+		err = proto.Unmarshal(messageBytes, msg)
+		m.Content = msg
+	case AddItemType:
+		msg := &messages.AddItem{}
+		err = proto.Unmarshal(messageBytes, msg)
+		m.Content = msg
+	default:
+		return fmt.Errorf("unknown message type")
+	}
+	if err != nil {
+		return err
+	}
+	m.Type = header.Type
+	m.TimeStamp = &header.TimeStamp
+
+	return nil
+}