package chunk7

const (
	chunkFlagVital  = 1
	chunkFlagResend = 2
)

type ChunkFlags struct {
	Vital  bool
	Resend bool
}

func (flags *ChunkFlags) ToInt() int {
	v := 0
	if flags.Resend {
		v |= chunkFlagResend
	}
	if flags.Vital {
		v |= chunkFlagVital
	}
	return v
}

type ChunkHeader struct {
	Flags ChunkFlags
	Size  int
	// sequence number
	// will be acknowledged in the packet header ack
	Seq int
}

type Chunk struct {
	Header ChunkHeader
	Data   []byte
}

func (header *ChunkHeader) Pack() []byte {
	len := 2
	if header.Flags.Vital {
		len = 3
	}
	data := make([]byte, len)
	data[0] = (byte(header.Flags.ToInt()&0x03) << 6) | ((byte(header.Size) >> 6) & 0x3f)
	data[1] = (byte(header.Size) & 0x3f)
	if header.Flags.Vital {
		data[1] |= (byte(header.Seq) >> 2) & 0xc0
		data[2] = byte(header.Seq) & 0xff
	}
	return data
}

func (header *ChunkHeader) Unpack(data []byte) {
	flagBits := (data[0] >> 6) & 0x03
	header.Flags.Vital = (flagBits & chunkFlagVital) != 0
	header.Flags.Resend = (flagBits & chunkFlagResend) != 0
	header.Size = (int(data[0]&0x3F) << 6) | (int(data[1]) & 0x3F)

	if header.Flags.Vital {
		header.Seq = int((data[1]&0xC0)<<2) | int(data[2])
	}
}