feat: add packet

internal/utils/packets/packet.go

@@ -0,0 +1,80 @@
+package packets
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+const magicBytes = "EKO"
+
+const (
+	FlagHandshake byte = 0b1000_0000
+	FlagV1        byte = 0b0000_0000
+)
+
+type Packet struct {
+	flag byte
+	data []byte
+}
+
+func NewPacket(flag byte, data []byte) Packet {
+	return Packet{flag, data}
+}
+
+func (p Packet) Write(w io.Writer) error {
+	if err := binary.Write(w, binary.LittleEndian, magicBytes); err != nil {
+		return err
+	}
+	if err := binary.Write(w, binary.LittleEndian, p.flag); err != nil {
+		return err
+	}
+	if err := binary.Write(w, binary.LittleEndian, uint32(len(p.data))); err != nil {
+		return err
+	}
+	if err := binary.Write(w, binary.LittleEndian, p.data); err != nil {
+		return err
+	}
+	return nil
+}
+
+func ReadPacket(r io.Reader) (Packet, error) {
+	buffer := [len(magicBytes) + 1 + 4]byte{}
+	bytesRead, err := r.Read(buffer[:])
+	if bytesRead != len(buffer) {
+		if err != nil && err != io.EOF {
+			return Packet{}, err
+		}
+		return Packet{}, fmt.Errorf("invalid packet size: got %v, want %v", bytesRead, len(buffer))
+	}
+
+	magic := buffer[:len(magicBytes)]
+	if !bytes.Equal(magic, []byte(magicBytes)) {
+		return Packet{}, fmt.Errorf("invalid magic number: got %v, want %v", magic, magicBytes)
+	}
+
+	flag := buffer[len(magicBytes)]
+
+	// TODO: Consider adding some data limit (maybe 65k?)
+	var length uint32
+	err = binary.Read(
+		bytes.NewReader(buffer[len(magicBytes):len(magicBytes)+4]),
+		binary.LittleEndian,
+		&length,
+	)
+	if err != nil {
+		panic(fmt.Errorf("Assertion Failed in packet.ReadPacket(io.Reader): %v", err))
+	}
+
+	data := make([]byte, length)
+	bytesRead, err = r.Read(data)
+	if uint32(bytesRead) != length {
+		if err != nil {
+			return Packet{}, err
+		}
+		return Packet{}, fmt.Errorf("invalid data size: got %v, want %v", bytesRead, length)
+	}
+
+	return Packet{flag, data}, nil
+}