package grpcreflect

import (
	"fmt"

	"google.golang.org/grpc"

	"github.com/jhump/protoreflect/desc"
)

// LoadServiceDescriptors loads the service descriptors for all services exposed by the
// given GRPC server.
func LoadServiceDescriptors(s *grpc.Server) (map[string]*desc.ServiceDescriptor, error) {
	descs := map[string]*desc.ServiceDescriptor{}
	for name, info := range s.GetServiceInfo() {
		file, ok := info.Metadata.(string)
		if !ok {
			return nil, fmt.Errorf("service %q has unexpected metadata: expecting a string; got %v", name, info.Metadata)
		}
		fd, err := desc.LoadFileDescriptor(file)
		if err != nil {
			return nil, err
		}
		d := fd.FindSymbol(name)
		if d == nil {
			return nil, fmt.Errorf("file descriptor for %q has no element named %q", file, name)
		}
		sd, ok := d.(*desc.ServiceDescriptor)
		if !ok {
			return nil, fmt.Errorf("file descriptor for %q has incorrect element named %q: expecting a service descriptor; got %v", file, name, d)
		}
		descs[name] = sd
	}
	return descs, nil
}

// LoadServiceDescriptor loads a rich descriptor for a given service description
// generated by protoc-gen-go. Generated code contains an unexported symbol with
// a name like "_<Service>_serviceDesc" which is the service's description. It
// is used internally to register a service implementation with a GRPC server.
// But it can also be used by this package to retrieve the rich descriptor for
// the service.
func LoadServiceDescriptor(svc *grpc.ServiceDesc) (*desc.ServiceDescriptor, error) {
	file, ok := svc.Metadata.(string)
	if !ok {
		return nil, fmt.Errorf("service %q has unexpected metadata: expecting a string; got %v", svc.ServiceName, svc.Metadata)
	}
	fd, err := desc.LoadFileDescriptor(file)
	if err != nil {
		return nil, err
	}
	d := fd.FindSymbol(svc.ServiceName)
	if d == nil {
		return nil, fmt.Errorf("file descriptor for %q has no element named %q", file, svc.ServiceName)
	}
	sd, ok := d.(*desc.ServiceDescriptor)
	if !ok {
		return nil, fmt.Errorf("file descriptor for %q has incorrect element named %q: expecting a service descriptor; got %v", file, svc.ServiceName, d)
	}
	return sd, nil
}