// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package quic

import (
	"net"
	"net/netip"
)

// Per-plaform consts describing support for various features.
//
// const udpECNSupport indicates whether the platform supports setting
// the ECN (Explicit Congestion Notification) IP header bits.
//
// const udpInvalidLocalAddrIsError indicates whether sending a packet
// from an local address not associated with the system is an error.
// For example, assuming 127.0.0.2 is not a local address, does sending
// from it (using IP_PKTINFO or some other such feature) result in an error?

// unmapAddrPort returns a with any IPv4-mapped IPv6 address prefix removed.
func unmapAddrPort(a netip.AddrPort) netip.AddrPort {
	if a.Addr().Is4In6() {
		return netip.AddrPortFrom(
			a.Addr().Unmap(),
			a.Port(),
		)
	}
	return a
}

func localAddrFor(local, remote netip.AddrPort) (netip.AddrPort, error) {
	if local.Addr().IsValid() && !local.Addr().IsUnspecified() {
		return local, nil
	}
	// Simplest portable approach: Bind a socket and see what address it gets.
	remoteAddr := net.UDPAddrFromAddrPort(remote)
	uc, err := net.DialUDP("udp", nil, remoteAddr)
	if err != nil {
		return netip.AddrPort{}, err
	}
	boundAddr := uc.LocalAddr().(*net.UDPAddr).AddrPort()
	uc.Close()
	return netip.AddrPortFrom(boundAddr.Addr(), local.Port()), nil
}