package tls

import (
	"context"

	"github.com/v2fly/v2ray-core/v4/common"
	"github.com/v2fly/v2ray-core/v4/common/dice"
)

// DTLS writes header as DTLS. See https://tools.ietf.org/html/rfc6347
type DTLS struct {
	epoch    uint16
	length   uint16
	sequence uint32
}

// Size implements PacketHeader.
func (*DTLS) Size() int32 {
	return 1 + 2 + 2 + 6 + 2
}

// Serialize implements PacketHeader.
func (d *DTLS) Serialize(b []byte) {
	b[0] = 23 // application data
	b[1] = 254
	b[2] = 253
	b[3] = byte(d.epoch >> 8)
	b[4] = byte(d.epoch)
	b[5] = 0
	b[6] = 0
	b[7] = byte(d.sequence >> 24)
	b[8] = byte(d.sequence >> 16)
	b[9] = byte(d.sequence >> 8)
	b[10] = byte(d.sequence)
	d.sequence++
	b[11] = byte(d.length >> 8)
	b[12] = byte(d.length)
	d.length += 17
	if d.length > 100 {
		d.length -= 50
	}
}

// New creates a new UTP header for the given config.
func New(ctx context.Context, config interface{}) (interface{}, error) {
	return &DTLS{
		epoch:    dice.RollUint16(),
		sequence: 0,
		length:   17,
	}, nil
}

func init() {
	common.Must(common.RegisterConfig((*PacketConfig)(nil), New))
}