package header

import (
	"encoding/binary"
	"errors"

	"github.com/xtls/xray-core/common/dice"
)

type dns struct {
	header []byte
}

func (h *dns) Size() int {
	return len(h.header)
}

func (h *dns) Serialize(b []byte) {
	copy(b, h.header)
	binary.BigEndian.PutUint16(b[0:], dice.RollUint16())
}

func NewHeaderDNS(domain string) (*dns, error) {
	var header []byte
	header = binary.BigEndian.AppendUint16(header, 0x0000) // Transaction ID
	header = binary.BigEndian.AppendUint16(header, 0x0100) // Flags: Standard query
	header = binary.BigEndian.AppendUint16(header, 0x0001) // Questions
	header = binary.BigEndian.AppendUint16(header, 0x0000) // Answer RRs
	header = binary.BigEndian.AppendUint16(header, 0x0000) // Authority RRs
	header = binary.BigEndian.AppendUint16(header, 0x0000) // Additional RRs
	buf := make([]byte, 0x100)
	off1, err := packDomainName(domain+".", buf)
	if err != nil {
		return nil, err
	}
	header = append(header, buf[:off1]...)
	header = binary.BigEndian.AppendUint16(header, 0x0001) // Type: A
	header = binary.BigEndian.AppendUint16(header, 0x0001) // Class: IN
	return &dns{header: header}, nil
}

func packDomainName(s string, msg []byte) (off1 int, err error) {
	off := 0
	ls := len(s)
	// Each dot ends a segment of the name.
	// We trade each dot byte for a length byte.
	// Except for escaped dots (\.), which are normal dots.
	// There is also a trailing zero.

	// Emit sequence of counted strings, chopping at dots.
	var (
		begin int
		bs    []byte
	)
	for i := 0; i < ls; i++ {
		var c byte
		if bs == nil {
			c = s[i]
		} else {
			c = bs[i]
		}

		switch c {
		case '\\':
			if off+1 > len(msg) {
				return len(msg), errors.New("buffer size too small")
			}

			if bs == nil {
				bs = []byte(s)
			}

			copy(bs[i:ls-1], bs[i+1:])
			ls--
		case '.':
			labelLen := i - begin
			if labelLen >= 1<<6 { // top two bits of length must be clear
				return len(msg), errors.New("bad rdata")
			}

			// off can already (we're in a loop) be bigger than len(msg)
			// this happens when a name isn't fully qualified
			if off+1+labelLen > len(msg) {
				return len(msg), errors.New("buffer size too small")
			}

			// The following is covered by the length check above.
			msg[off] = byte(labelLen)

			if bs == nil {
				copy(msg[off+1:], s[begin:i])
			} else {
				copy(msg[off+1:], bs[begin:i])
			}
			off += 1 + labelLen
			begin = i + 1
		default:
		}
	}

	if off < len(msg) {
		msg[off] = 0
	}

	return off + 1, nil
}
