// Package control implements the VPN-internal peer control protocol.
// Peers exchange Ping packets over UDP on the VPN control port to maintain
// liveness and discover external endpoints for direct connection attempts.
package control

import (
	"encoding/binary"
	"fmt"
	"net/netip"
)

const (
	version = 1
	Size    = 59 // 1 version + 8 ID + 8 PingTS + 6 SrcV4 + 18 SrcV6 + 18 Dst
)

// Ping is the single control packet type exchanged between VPN peers.
//
// In each peer pair, the peer with the lower VPN IP is the client: it sets ID
// and PingTS and sends pings on a timer. The server echoes ID and PingTS back
// in its response, allowing the client to compute RTT = now - PingTS.
//
// Both client and server populate SrcV4, SrcV6, and Dst on every packet so
// endpoint information flows in both directions.
//
// Dst is the recipient's external endpoint as observed by the sender from the
// WireGuard handshake source. Zero if the sender has not observed a handshake
// from the recipient.
type Ping struct {
	ID     int64          // Client ping ID.
	PingTS int64          // Client ping send time in nanoseconds.
	SrcV4  netip.AddrPort // Sender's discovered IPv4 address and port.
	SrcV6  netip.AddrPort // Sender's discovered IPv6 address and port.
	Dst    netip.AddrPort
}

// Marshal encodes p into a fixed-size 59-byte array.
func (p Ping) Marshal() [Size]byte {
	var buf [Size]byte
	buf[0] = version
	binary.BigEndian.PutUint64(buf[1:9], uint64(p.ID))
	binary.BigEndian.PutUint64(buf[9:17], uint64(p.PingTS))
	if p.SrcV4.IsValid() {
		a4 := p.SrcV4.Addr().As4()
		copy(buf[17:21], a4[:])
		binary.BigEndian.PutUint16(buf[21:23], p.SrcV4.Port())
	}
	a16 := p.SrcV6.Addr().As16()
	copy(buf[23:39], a16[:])
	binary.BigEndian.PutUint16(buf[39:41], p.SrcV6.Port())
	a16 = p.Dst.Addr().As16()
	copy(buf[41:57], a16[:])
	binary.BigEndian.PutUint16(buf[57:59], p.Dst.Port())
	return buf
}

// Unmarshal decodes a Ping from a fixed-size 59-byte array.
func Unmarshal(buf [Size]byte) (Ping, error) {
	if buf[0] != version {
		return Ping{}, fmt.Errorf("unknown ping version %d", buf[0])
	}
	p := Ping{
		ID:     int64(binary.BigEndian.Uint64(buf[1:9])),
		PingTS: int64(binary.BigEndian.Uint64(buf[9:17])),
	}
	if addr := netip.AddrFrom4([4]byte(buf[17:21])); !addr.IsUnspecified() {
		p.SrcV4 = netip.AddrPortFrom(addr, binary.BigEndian.Uint16(buf[21:23]))
	}
	if addr := netip.AddrFrom16([16]byte(buf[23:39])); !addr.IsUnspecified() {
		p.SrcV6 = netip.AddrPortFrom(addr, binary.BigEndian.Uint16(buf[39:41]))
	}
	if addr := netip.AddrFrom16([16]byte(buf[41:57])).Unmap(); !addr.IsUnspecified() {
		p.Dst = netip.AddrPortFrom(addr, binary.BigEndian.Uint16(buf[57:59]))
	}
	return p, nil
}