package udp

import (
	"net/netip"

	"github.com/slackhq/nebula/config"
)

const MTU = 9001

// MaxWriteBatch is the largest batch any Conn.WriteBatch implementation is
// required to accept. Callers SHOULD NOT pass more than this per call; Linux
// backends preallocate sendmmsg scratch sized to this value, so exceeding it
// only costs a chunked retry.
const MaxWriteBatch = 128

type EncReader func(
	addr netip.AddrPort,
	payload []byte,
)

type Conn interface {
	Rebind() error
	LocalAddr() (netip.AddrPort, error)
	ListenOut(r EncReader) error
	WriteTo(b []byte, addr netip.AddrPort) error
	// WriteBatch sends a contiguous batch of packets, each with its own
	// destination. bufs and addrs must have the same length. Linux uses
	// sendmmsg(2) for a single syscall; other backends fall back to a
	// WriteTo loop. Returns on the first error; callers may observe a
	// partial send if some packets went out before the error.
	WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error
	// WriteSegmented sends bufs as a single UDP GSO sendmsg when the kernel
	// supports it: all bufs go to the same addr, each must be exactly segSize
	// bytes except the last which may be shorter. The kernel emits one
	// datagram per buf on the wire. Backends / kernels without GSO support
	// fall back to a per-packet WriteTo loop. Returns on the first error.
	WriteSegmented(bufs [][]byte, addr netip.AddrPort, segSize int) error
	// SupportsGSO reports whether WriteSegmented takes the single-syscall
	// GSO path. Callers use this to decide at batch-assembly time whether
	// the uniform-size / same-dst check is worth running.
	SupportsGSO() bool
	ReloadConfig(c *config.C)
	SupportsMultipleReaders() bool
	Close() error
}

type NoopConn struct{}

func (NoopConn) Rebind() error {
	return nil
}
func (NoopConn) LocalAddr() (netip.AddrPort, error) {
	return netip.AddrPort{}, nil
}
func (NoopConn) ListenOut(_ EncReader) error {
	return nil
}
func (NoopConn) SupportsMultipleReaders() bool {
	return false
}
func (NoopConn) WriteTo(_ []byte, _ netip.AddrPort) error {
	return nil
}
func (NoopConn) WriteBatch(_ [][]byte, _ []netip.AddrPort) error {
	return nil
}
func (NoopConn) WriteSegmented(_ [][]byte, _ netip.AddrPort, _ int) error {
	return nil
}
func (NoopConn) SupportsGSO() bool {
	return false
}
func (NoopConn) ReloadConfig(_ *config.C) {
	return
}
func (NoopConn) Close() error {
	return nil
}