implement parsing and writing of varint packet numbers

internal/utils/varint_packetnumber.go

@@ -0,0 +1,50 @@
+package utils
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+)
+
+// ReadVarIntPacketNumber reads a number in the QUIC varint packet number format
+func ReadVarIntPacketNumber(b *bytes.Reader) (protocol.PacketNumber, protocol.PacketNumberLen, error) {
+	b1, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	if b1&0x80 == 0 {
+		return protocol.PacketNumber(b1), protocol.PacketNumberLen1, nil
+	}
+	b2, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	if b1&0x40 == 0 {
+		return protocol.PacketNumber(uint64(b1&0x3f)<<8 + uint64(b2)), protocol.PacketNumberLen2, nil
+	}
+	b3, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	b4, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	return protocol.PacketNumber(uint64(b1&0x3f)<<24 + uint64(b2)<<16 + uint64(b3)<<8 + uint64(b4)), protocol.PacketNumberLen4, nil
+}
+
+// WriteVarIntPacketNumber writes a packet number in the QUIC varint packet number format
+func WriteVarIntPacketNumber(b *bytes.Buffer, i protocol.PacketNumber, len protocol.PacketNumberLen) error {
+	switch len {
+	case protocol.PacketNumberLen1:
+		b.WriteByte(uint8(i & 0x7f))
+	case protocol.PacketNumberLen2:
+		b.Write([]byte{(uint8(i>>8) & 0x3f) | 0x80, uint8(i)})
+	case protocol.PacketNumberLen4:
+		b.Write([]byte{(uint8(i>>24) & 0x3f) | 0xc0, uint8(i >> 16), uint8(i >> 8), uint8(i)})
+	default:
+		return fmt.Errorf("invalid packet number length: %d", len)
+	}
+	return nil
+}