// DialGRPC returns a GRPC connection for use communicating with a Google cloud
// service, configured with the given ClientOptions.
func DialGRPC(ctx context.Context, opt ...cloud.ClientOption) (*grpc.ClientConn, error) {
	var o opts.DialOpt
	for _, opt := range opt {
		opt.Resolve(&o)
	}
	if o.HTTPClient != nil {
		return nil, errors.New("unsupported HTTP base transport specified")
	}
	if o.GRPCClient != nil {
		return o.GRPCClient, nil
	}
	if o.TokenSource == nil {
		var err error
		o.TokenSource, err = google.DefaultTokenSource(ctx, o.Scopes...)
		if err != nil {
			return nil, fmt.Errorf("google.DefaultTokenSource: %v", err)
		}
	}
	grpcOpts := []grpc.DialOption{
		grpc.WithPerRPCCredentials(oauth.TokenSource{o.TokenSource}),
		grpc.WithTransportCredentials(credentials.NewClientTLSFromCert(nil, "")),
	}
	grpcOpts = append(grpcOpts, o.GRPCDialOpts...)
	if o.UserAgent != "" {
		grpcOpts = append(grpcOpts, grpc.WithUserAgent(o.UserAgent))
	}
	return grpc.Dial(o.Endpoint, grpcOpts...)
}

// DialGRPC returns a GRPC connection for use communicating with a Google cloud
// service, configured with the given ClientOptions.
func DialGRPC(ctx context.Context, opt ...cloud.ClientOption) (*grpc.ClientConn, error) {
	o := make([]option.ClientOption, 0, len(opt))
	for _, opt := range opt {
		o = append(o, opt.Resolve())
	}
	return transport.DialGRPC(ctx, o...)
}

// NewProtoClient returns a ProtoClient for communicating with a Google cloud service,
// configured with the given ClientOptions.
func NewProtoClient(ctx context.Context, opt ...cloud.ClientOption) (*ProtoClient, error) {
	var o opts.DialOpt
	for _, opt := range opt {
		opt.Resolve(&o)
	}
	if o.GRPCClient != nil {
		return nil, errors.New("unsupported GRPC base transport specified")
	}
	var client *http.Client
	switch {
	case o.HTTPClient != nil:
		if o.TokenSource != nil {
			return nil, errors.New("at most one of WithTokenSource or WithBaseHTTP may be provided")
		}
		client = o.HTTPClient
	case o.TokenSource != nil:
		client = oauth2.NewClient(ctx, o.TokenSource)
	default:
		var err error
		client, err = google.DefaultClient(ctx, o.Scopes...)
		if err != nil {
			return nil, err
		}
	}

	return &ProtoClient{
		client:    client,
		endpoint:  o.Endpoint,
		userAgent: o.UserAgent,
	}, nil
}

// NewHTTPClient returns an HTTP client for use communicating with a Google cloud
// service, configured with the given ClientOptions. It also returns the endpoint
// for the service as specified in the options.
func NewHTTPClient(ctx context.Context, opt ...cloud.ClientOption) (*http.Client, string, error) {
	o := make([]option.ClientOption, 0, len(opt))
	for _, opt := range opt {
		o = append(o, opt.Resolve())
	}
	return transport.NewHTTPClient(ctx, o...)
}

// NewHTTPClient returns an HTTP client for use communicating with a Google cloud
// service, configured with the given ClientOptions. It also returns the endpoint
// for the service as specified in the options.
func NewHTTPClient(ctx context.Context, opt ...cloud.ClientOption) (*http.Client, string, error) {
	var o opts.DialOpt
	for _, opt := range opt {
		opt.Resolve(&o)
	}
	if o.GRPCClient != nil {
		return nil, "", errors.New("unsupported GRPC base transport specified")
	}
	// TODO(djd): Wrap all http.Clients with appropriate internal version to add
	// UserAgent header and prepend correct endpoint.
	if o.HTTPClient != nil {
		return o.HTTPClient, o.Endpoint, nil
	}
	if o.TokenSource == nil {
		var err error
		o.TokenSource, err = google.DefaultTokenSource(ctx, o.Scopes...)
		if err != nil {
			return nil, "", fmt.Errorf("google.DefaultTokenSource: %v", err)
		}
	}
	return oauth2.NewClient(ctx, o.TokenSource), o.Endpoint, nil
}