/*
 * Minio Cloud Storage, (C) 2018 Minio, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package rpc

import (
	"bytes"
	"encoding/gob"
	"errors"
	"fmt"
	"net/http"
	"reflect"
	"strings"
	"unicode"
	"unicode/utf8"
)

// Authenticator - validator of first argument of any RPC call.
type Authenticator interface {
	// Method to validate first argument of any RPC call.
	Authenticate() error
}

// reflect.Type of error interface.
var errorType = reflect.TypeOf((*error)(nil)).Elem()

// reflect.Type of Authenticator interface.
var authenticatorType = reflect.TypeOf((*Authenticator)(nil)).Elem()

func gobEncodeBuf(e interface{}, buf *bytes.Buffer) error {
	return gob.NewEncoder(buf).Encode(e)
}

func gobDecode(data []byte, e interface{}) error {
	return gob.NewDecoder(bytes.NewReader(data)).Decode(e)
}

// Returns whether given type is exported or builin type or not.
func isExportedOrBuiltinType(t reflect.Type) bool {
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}

	rune, _ := utf8.DecodeRuneInString(t.Name())
	return unicode.IsUpper(rune) || t.PkgPath() == ""
}

// Makes method name map from given type.
func getMethodMap(receiverType reflect.Type) map[string]reflect.Method {
	methodMap := make(map[string]reflect.Method)
	for i := 0; i < receiverType.NumMethod(); i++ {
		// Method.PkgPath is empty for this package.
		method := receiverType.Method(i)

		// Methods must have three arguments (receiver, args, reply)
		if method.Type.NumIn() != 3 {
			continue
		}

		// First argument must be exported.
		if !isExportedOrBuiltinType(method.Type.In(1)) {
			continue
		}

		// First argument must be Authenticator.
		if !method.Type.In(1).Implements(authenticatorType) {
			continue
		}

		// Second argument must be exported or builtin type.
		if !isExportedOrBuiltinType(method.Type.In(2)) {
			continue
		}

		// Second argument must be a pointer.
		if method.Type.In(2).Kind() != reflect.Ptr {
			continue
		}

		// Method must return one value.
		if method.Type.NumOut() != 1 {
			continue
		}

		// The return type of the method must be error.
		if method.Type.Out(0) != errorType {
			continue
		}

		methodMap[method.Name] = method
	}

	return methodMap
}

// Server - HTTP based RPC server.
type Server struct {
	serviceName   string
	receiverValue reflect.Value
	methodMap     map[string]reflect.Method
}

// RegisterName - registers receiver with given name to handle RPC requests.
func (server *Server) RegisterName(name string, receiver interface{}) error {
	server.serviceName = name

	server.receiverValue = reflect.ValueOf(receiver)
	if !reflect.Indirect(server.receiverValue).IsValid() {
		return fmt.Errorf("nil receiver")
	}

	receiverName := reflect.Indirect(server.receiverValue).Type().Name()
	receiverType := reflect.TypeOf(receiver)
	server.methodMap = getMethodMap(receiverType)
	if len(server.methodMap) == 0 {
		str := "rpc.Register: type " + receiverName + " has no exported methods of suitable type"

		// To help the user, see if a pointer receiver would work.
		if len(getMethodMap(reflect.PtrTo(receiverType))) != 0 {
			str += " (hint: pass a pointer to value of that type)"
		}

		return errors.New(str)
	}

	return nil
}

// call - call service method in receiver.
func (server *Server) call(serviceMethod string, argBytes []byte, replyBytes *bytes.Buffer) (err error) {
	tokens := strings.SplitN(serviceMethod, ".", 2)
	if len(tokens) != 2 {
		return fmt.Errorf("invalid service/method request ill-formed %v", serviceMethod)
	}

	serviceName := tokens[0]
	if serviceName != server.serviceName {
		return fmt.Errorf("can't find service %v", serviceName)
	}

	methodName := tokens[1]
	method, found := server.methodMap[methodName]
	if !found {
		return fmt.Errorf("can't find method %v", methodName)
	}

	var argv reflect.Value

	// Decode the argument value.
	argIsValue := false // if true, need to indirect before calling.
	if method.Type.In(1).Kind() == reflect.Ptr {
		argv = reflect.New(method.Type.In(1).Elem())
	} else {
		argv = reflect.New(method.Type.In(1))
		argIsValue = true
	}

	if err = gobDecode(argBytes, argv.Interface()); err != nil {
		return err
	}

	if argIsValue {
		argv = argv.Elem()
	}

	// call Authenticate() method.
	authMethod, ok := method.Type.In(1).MethodByName("Authenticate")
	if !ok {
		panic("Authenticate() method not found. This should not happen.")
	}
	returnValues := authMethod.Func.Call([]reflect.Value{argv})
	errInter := returnValues[0].Interface()
	if errInter != nil {
		err = errInter.(error)
	}
	if err != nil {
		return err
	}

	replyv := reflect.New(method.Type.In(2).Elem())

	switch method.Type.In(2).Elem().Kind() {
	case reflect.Map:
		replyv.Elem().Set(reflect.MakeMap(method.Type.In(2).Elem()))
	case reflect.Slice:
		replyv.Elem().Set(reflect.MakeSlice(method.Type.In(2).Elem(), 0, 0))
	}

	returnValues = method.Func.Call([]reflect.Value{server.receiverValue, argv, replyv})
	errInter = returnValues[0].Interface()
	if errInter != nil {
		err = errInter.(error)
	}
	if err != nil {
		return err
	}

	return gobEncodeBuf(replyv.Interface(), replyBytes)
}

// CallRequest - RPC call request parameters.
type CallRequest struct {
	Method   string
	ArgBytes []byte
}

// CallResponse - RPC call response parameters.
type CallResponse struct {
	Error      string
	ReplyBytes []byte
}

// ServeHTTP - handles RPC on HTTP request.
func (server *Server) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if req.Method != http.MethodPost {
		w.WriteHeader(http.StatusMethodNotAllowed)
		return
	}

	var callRequest CallRequest
	if err := gob.NewDecoder(req.Body).Decode(&callRequest); err != nil {
		w.WriteHeader(http.StatusBadRequest)
		return
	}

	callResponse := CallResponse{}

	buf := bytes.NewBuffer(make([]byte, 0, 1024))

	if err := server.call(callRequest.Method, callRequest.ArgBytes, buf); err != nil {
		callResponse.Error = err.Error()
	}
	callResponse.ReplyBytes = buf.Bytes()

	gob.NewEncoder(w).Encode(callResponse)

	w.(http.Flusher).Flush()
}

// NewServer - returns new RPC server.
func NewServer() *Server {
	return &Server{}
}