package codecs

import (
	"errors"
)

var errTruncated = errors.New("truncated packet")

// Flags holds parsed RTP header flags.
type Flags struct {
	Seqno  uint16
	Pid    uint16
	Marker bool
}

// PacketFlags extracts basic RTP header flags from a raw packet buffer.
func PacketFlags(codec string, buf []byte) (Flags, error) {
	if len(buf) < 4 {
		return Flags{}, errTruncated
	}

	var flags Flags
	flags.Seqno = (uint16(buf[2]) << 8) | uint16(buf[3])
	flags.Marker = (buf[1] & 0x80) != 0
	return flags, nil
}

// RewritePacket rewrites the sequence number and marker bit in a raw RTP packet.
func RewritePacket(codec string, data []byte, setMarker bool, seqno uint16, delta uint16) error {
	if len(data) < 12 {
		return errTruncated
	}

	if setMarker {
		data[1] |= 0x80
	}

	data[2] = uint8(seqno >> 8)
	data[3] = uint8(seqno)
	if delta == 0 {
		return nil
	}

	offset := 12
	offset += int(data[0]&0x0F) * 4
	if len(data) <= offset {
		return errTruncated
	}

	if (data[0] & 0x10) != 0 {
		length := uint16(data[offset+2])<<8 | uint16(data[offset+3])
		offset += 4 + int(length)*4
		if len(data) < offset+4 {
			return errTruncated
		}
	}

	return nil
}