add method to lookup values in a multiaddr

interface.go

@@ -39,4 +39,7 @@ type Multiaddr interface {
 	//      /ip4/1.2.3.4/tcp/80 decapsulate /ip4/1.2.3.4 = /tcp/80
 	//
 	Decapsulate(Multiaddr) Multiaddr
+
+	// ValueForProtocol returns the value (if any) following the specified protocol
+	ValueForProtocol(code int) (string, error)
 }

multiaddr.go

@@ -113,3 +113,32 @@ func (m *multiaddr) Decapsulate(o Multiaddr) Multiaddr {
 	}
 	return ma
 }
+
+var ErrProtocolNotFound = fmt.Errorf("protocol not found in multiaddr")
+
+func (m *multiaddr) ValueForProtocol(code int) (string, error) {
+	protos := m.Protocols()
+	found := -1
+	index := 2
+
+	for i, p := range protos {
+		if code == p.Code {
+			if p.Size == 0 {
+				return "", nil
+			}
+			found = i
+			break
+		} else {
+			index += 2
+			if p.Size == 0 {
+				index--
+			}
+		}
+	}
+
+	if found == -1 {
+		return "", ErrProtocolNotFound
+	}
+
+	return strings.Split(m.String(), "/")[index], nil
+}

multiaddr_test.go

@@ -300,3 +300,34 @@ func TestEncapsulate(t *testing.T) {
 		t.Error("decapsulate /ip4 failed.", "/", s)
 	}
 }
+
+func assertValueForProto(t *testing.T, a Multiaddr, p int, exp string) {
+	t.Logf("checking for %s in %s", ProtocolWithCode(p).Name, a)
+	fv, err := a.ValueForProtocol(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if fv != exp {
+		t.Fatalf("expected %q for %d in %d, but got %q instead", exp, p, a, fv)
+	}
+}
+
+func TestGetValue(t *testing.T) {
+	a := newMultiaddr(t, "/ip4/127.0.0.1/utp/tcp/5555/udp/1234/utp/ipfs/QmbHVEEepCi7rn7VL7Exxpd2Ci9NNB6ifvqwhsrbRMgQFP")
+	assertValueForProto(t, a, P_IP4, "127.0.0.1")
+	assertValueForProto(t, a, P_UTP, "")
+	assertValueForProto(t, a, P_TCP, "5555")
+	assertValueForProto(t, a, P_UDP, "1234")
+	assertValueForProto(t, a, P_IPFS, "QmbHVEEepCi7rn7VL7Exxpd2Ci9NNB6ifvqwhsrbRMgQFP")
+
+	_, err := a.ValueForProtocol(P_IP6)
+	switch err {
+	case ErrProtocolNotFound:
+		break
+	case nil:
+		t.Fatal("expected value lookup to fail")
+	default:
+		t.Fatalf("expected ErrProtocolNotFound but got: %s", err)
+	}
+}