func pickLocalAddr(laddrs []ma.Multiaddr, raddr ma.Multiaddr) (laddr ma.Multiaddr) {
	if len(laddrs) < 1 {
		return nil
	}

	// make sure that we ONLY use local addrs that match the remote addr.
	laddrs = manet.AddrMatch(raddr, laddrs)
	if len(laddrs) < 1 {
		return nil
	}

	// make sure that we ONLY use local addrs that CAN dial the remote addr.
	// filter out all the local addrs that aren't capable
	raddrIPLayer := ma.Split(raddr)[0]
	raddrIsLoopback := manet.IsIPLoopback(raddrIPLayer)
	raddrIsLinkLocal := manet.IsIP6LinkLocal(raddrIPLayer)
	laddrs = addrutil.FilterAddrs(laddrs, func(a ma.Multiaddr) bool {
		laddrIPLayer := ma.Split(a)[0]
		laddrIsLoopback := manet.IsIPLoopback(laddrIPLayer)
		laddrIsLinkLocal := manet.IsIP6LinkLocal(laddrIPLayer)
		if laddrIsLoopback { // our loopback addrs can only dial loopbacks.
			return raddrIsLoopback
		}
		if laddrIsLinkLocal {
			return raddrIsLinkLocal // out linklocal addrs can only dial link locals.
		}
		return true
	})

	// TODO pick with a good heuristic
	// we use a random one for now to prevent bad addresses from making nodes unreachable
	// with a random selection, multiple tries may work.
	return laddrs[rand.Intn(len(laddrs))]
}

// ResolveUnspecifiedAddress expands an unspecified ip addresses (/ip4/0.0.0.0, /ip6/::) to
// use the known local interfaces. If ifaceAddr is nil, we request interface addresses
// from the network stack. (this is so you can provide a cached value if resolving many addrs)
func ResolveUnspecifiedAddress(resolve ma.Multiaddr, ifaceAddrs []ma.Multiaddr) ([]ma.Multiaddr, error) {
	// split address into its components
	split := ma.Split(resolve)

	// if first component (ip) is not unspecified, use it as is.
	if !manet.IsIPUnspecified(split[0]) {
		return []ma.Multiaddr{resolve}, nil
	}

	out := make([]ma.Multiaddr, 0, len(ifaceAddrs))
	for _, ia := range ifaceAddrs {
		// must match the first protocol to be resolve.
		if ia.Protocols()[0].Code != resolve.Protocols()[0].Code {
			continue
		}

		split[0] = ia
		joined := ma.Join(split...)
		out = append(out, joined)
		log.Debug("adding resolved addr:", resolve, joined, out)
	}
	if len(out) < 1 {
		return nil, fmt.Errorf("failed to resolve: %s", resolve)
	}
	return out, nil
}

// ParseMultiaddr parses a multiaddr into an IPFSAddr
func ParseMultiaddr(m ma.Multiaddr) (a IPFSAddr, err error) {
	// never panic.
	defer func() {
		if r := recover(); r != nil {
			log.Debug("recovered from panic: ", r)
			a = nil
			err = ErrInvalidAddr
		}
	}()

	if m == nil {
		return nil, ErrInvalidAddr
	}

	// make sure it's an ipfs addr
	parts := ma.Split(m)
	if len(parts) < 1 {
		return nil, ErrInvalidAddr
	}
	ipfspart := parts[len(parts)-1] // last part
	if ipfspart.Protocols()[0].Code != ma.P_IPFS {
		return nil, ErrInvalidAddr
	}

	// make sure ipfs id parses as a peer.ID
	peerIdParts := strings.Split(ipfspart.String(), "/")
	peerIdStr := peerIdParts[len(peerIdParts)-1]
	id, err := peer.IDB58Decode(peerIdStr)
	if err != nil {
		return nil, err
	}

	return ipfsAddr{ma: m, id: id}, nil
}

// AddrOverNonLocalIP returns whether the addr uses a non-local ip link
func AddrOverNonLocalIP(a ma.Multiaddr) bool {
	split := ma.Split(a)
	if len(split) < 1 {
		return false
	}
	if manet.IsIP6LinkLocal(split[0]) {
		return false
	}
	return true
}

// AddrIsShareableOnWAN returns whether the given address should be shareable on the
// wide area network (wide internet).
func AddrIsShareableOnWAN(addr ma.Multiaddr) bool {
	s := ma.Split(addr)
	if len(s) < 1 {
		return false
	}
	a := s[0]
	if manet.IsIPLoopback(a) || manet.IsIP6LinkLocal(a) || manet.IsIPUnspecified(a) {
		return false
	}
	return manet.IsThinWaist(a)
}

func toPeerInfo(bp config.BootstrapPeer) peer.PeerInfo {
	// for now, we drop the "ipfs addr" part of the multiaddr. the rest
	// of the codebase currently uses addresses without the peerid part.
	m := bp.Multiaddr()
	s := ma.Split(m)
	m = ma.Join(s[:len(s)-1]...)

	return peer.PeerInfo{
		ID:    bp.ID(),
		Addrs: []ma.Multiaddr{m},
	}
}

func TestTransport(t *testing.T) {
	for _, g := range good {
		a, err := ParseString(g)
		if err != nil {
			t.Error("failed to parse", g, err)
			continue
		}

		m := newMultiaddr(t, g)
		split := ma.Split(m)
		m = ma.Join(split[:len(split)-1]...)
		if a.Multiaddr().Equal(m) {
			t.Error("should not be equal", a.Multiaddr(), m)
		}
		if !Transport(a).Equal(m) {
			t.Error("should be equal", Transport(a), m)
		}
	}
}

// observerGroup is a function that determines what part of
// a multiaddr counts as a different observer. for example,
// two ipfs nodes at the same IP/TCP transport would get
// the exact same NAT mapping; they would count as the
// same observer. This may protect against NATs who assign
// different ports to addresses at different IP hosts, but
// not TCP ports.
//
// Here, we use the root multiaddr address. This is mostly
// IP addresses. In practice, this is what we want.
func observerGroup(m ma.Multiaddr) string {
	return ma.Split(m)[0].String()
}

func Transport(iaddr IPFSAddr) (maddr ma.Multiaddr) {
	maddr = iaddr.Multiaddr()
	split := ma.Split(maddr)
	maddr = ma.Join(split[:len(split)-1]...)
	return
}