add public header parser

public_header.go

@@ -0,0 +1,84 @@
+package quic
+
+import "io"
+
+// The PublicHeader of a QUIC packet
+type PublicHeader struct {
+	VersionFlag bool
+	ResetFlag   bool
+
+	ConnectionIDLength uint8
+	ConnectionID       uint64
+
+	QuicVersion uint32
+
+	PacketNumberLength uint8
+	PacketNumber       uint64
+}
+
+// ParsePublicHeader parses a QUIC packet's public header
+func ParsePublicHeader(b io.ByteReader) (*PublicHeader, error) {
+	header := &PublicHeader{}
+
+	// First byte
+	publicFlagByte, err := b.ReadByte()
+	if err != nil {
+		return nil, err
+	}
+	header.VersionFlag = publicFlagByte&0x01 > 0
+	header.ResetFlag = publicFlagByte&0x02 > 0
+	switch publicFlagByte & 0x0C {
+	case 0x0C:
+		header.ConnectionIDLength = 8
+	case 0x08:
+		header.ConnectionIDLength = 4
+	case 0x04:
+		header.ConnectionIDLength = 1
+	}
+	switch publicFlagByte & 0x30 {
+	case 0x30:
+		header.PacketNumberLength = 6
+	case 0x20:
+		header.PacketNumberLength = 4
+	case 0x10:
+		header.PacketNumberLength = 2
+	case 0x00:
+		header.PacketNumberLength = 1
+	}
+
+	// Connection ID
+	header.ConnectionID, err = readUint64(b, header.ConnectionIDLength)
+	if err != nil {
+		return nil, err
+	}
+
+	// Version (optional)
+	if header.VersionFlag {
+		var v uint64
+		v, err = readUint64(b, 4)
+		if err != nil {
+			return nil, err
+		}
+		header.QuicVersion = uint32(v)
+	}
+
+	// Packet number
+	header.PacketNumber, err = readUint64(b, header.PacketNumberLength)
+	if err != nil {
+		return nil, err
+	}
+
+	return header, nil
+}
+
+func readUint64(b io.ByteReader, length uint8) (uint64, error) {
+	var res uint64
+	for i := uint8(0); i < length; i++ {
+		bt, err := b.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		res = res<<8 + uint64(bt)
+	}
+	return res, nil
+}