// Package jsonrpc2 implements a minimal JSON-RPC 2.0 server over HTTP.
//
// This server supports only single JSON-RPC 2.0 requests.
// Batch requests (arrays of requests) are explicitly unsupported and
// will be rejected with an Invalid Request (-32600) error.
//
// Author: NiuJiuRu
// Email: niujiuru@qq.com

// See: https://www.jsonrpc.org/specification

package jsonrpc2

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"maps"
	"net/http"
	"runtime/debug"
	"sort"
	"strings"
	"sync"
	"sync/atomic"

	"github.com/sirupsen/logrus"
)

const (
	defaultMaxBodySize = 1 << 20 // 1MB
)

// MethodHandler defines the execution logic for a JSON-RPC method.
type MethodHandler func(ctx context.Context, input *Request) *Response

// MethodMap maps JSON-RPC method names to handlers.
type MethodMap map[string]MethodHandler

// RPCServer implements a JSON-RPC 2.0 HTTP server.
//
// Request processing rules:
//   - Only individual (non-batch) requests are supported.
//   - Batch requests are explicitly disallowed and rejected.
//   - Notifications (requests without an id) are supported and do not produce a response.
//
// Handler execution model:
//   - Each handler runs with a context that is canceled when the server stops.
//   - No request-level timeout is enforced; handlers may run indefinitely.
//   - Handlers are expected to respect context cancellation.
//
// Built-in behavior:
//   - A built-in "ping" method is provided for health checks.
//   - The server is thread-safe and supports concurrent request handling.
//   - Maximum request body size can be configured to prevent abuse.
//   - Panics in handlers are recovered and converted into internal error responses.
//   - Incoming requests and outgoing responses are logged for debugging.
type RPCServer struct {
	name        string // 服务器名称, 用于日志的打印前缀
	ctx         context.Context
	cancel      context.CancelFunc
	mu          sync.RWMutex // 保护 handlers的并发访问
	handlers    MethodMap
	maxBodySize atomic.Int64 // 允许的最大请求负载字节数
	logger      *logrus.Logger
}

// NewRPCServer creates and initializes a JSON-RPC 2.0 server.
func NewRPCServer(name string, logger *logrus.Logger) (*RPCServer, error) {
	if logger == nil {
		return nil, errors.New("logger cannot be nil")
	}

	ctx, cancel := context.WithCancel(context.Background())

	s := &RPCServer{
		name:     name,
		ctx:      ctx,
		cancel:   cancel,
		handlers: make(MethodMap),
		logger:   logger,
	}

	s.maxBodySize.Store(defaultMaxBodySize)

	s.handlers["ping"] = s.handlePing // 注册内置方法, 例如: ping -> pong

	return s, nil
}

// Stop gracefully shuts down the server.
// It is safe to call multiple times.
func (s *RPCServer) Stop() {
	s.cancel()
}

// SetMaxBodySize sets the maximum allowed request body size.
func (s *RPCServer) SetMaxBodySize(size int64) {
	if size > 0 {
		s.maxBodySize.Store(size)
	}
}

// RegisterMethods registers multiple JSON-RPC methods atomically.
// Either all methods are registered successfully, or none are registered if validation fails.
func (s *RPCServer) RegisterMethods(methods MethodMap) error {
	if len(methods) == 0 {
		return errors.New("methods map cannot be empty")
	}

	s.mu.Lock()
	defer s.mu.Unlock()

	tmp := make(MethodMap, len(s.handlers)+len(methods))
	maps.Copy(tmp, s.handlers)

	for name, handler := range methods {
		if name == "" {
			return errors.New("method name cannot be empty, all methods must have a valid name")
		}
		if handler == nil {
			return fmt.Errorf("method %q has nil handler", name)
		}
		if _, exists := tmp[name]; exists {
			return fmt.Errorf("method %q already registered", name)
		}
		tmp[name] = handler
	}

	s.handlers = tmp
	return nil
}

// ListMethods returns all registered JSON-RPC method names.
// The returned list is sorted alphabetically for deterministic output.
func (s *RPCServer) ListMethods() []string {
	s.mu.RLock()
	defer s.mu.RUnlock()

	methods := make([]string, 0, len(s.handlers))
	for m := range s.handlers {
		methods = append(methods, m)
	}

	sort.Strings(methods)
	return methods
}

// ServeHTTP handles incoming HTTP requests.
//
// Example:
//
//	server := jsonrpc2.NewRPCServer("MyRPCServer", baseapp.Logger)
//	http.Handle("/rpc", server)
//	http.ListenAndServe(":8080", nil)
//
// Example usage from a client:
//
//	// Standard request (expects a response)
//	curl -X POST http://localhost:8080/rpc \
//	  -H "Content-Type: application/json" \
//	  -d '{"jsonrpc":"2.0","method":"ping","id":1}'
//
//	// Notification, no response is returned
//	curl -X POST http://localhost:8080/rpc \
//	  -H "Content-Type: application/json" \
//	  -d '{"jsonrpc":"2.0","method":"ping"}'
func (s *RPCServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	defer r.Body.Close()

	ctx, cancel := context.WithCancel(r.Context()) // per-request context
	defer cancel()

	stop := context.AfterFunc(s.ctx, cancel) // tie request lifetime to server shutdown(Go >= 1.21)
	defer stop()

	remoteAddr := r.Header.Get("X-Forwarded-For")
	if remoteAddr != "" {
		parts := strings.Split(remoteAddr, ",")
		remoteAddr = strings.TrimSpace(parts[0])
	} else {
		remoteAddr = r.RemoteAddr
	}

	if r.Method != http.MethodPost {
		s.logger.Warnf(
			"[%s] invalid http method | remote=%s | method=%s",
			s.name, remoteAddr, r.Method,
		)
		http.Error(w, "Only POST allowed", http.StatusMethodNotAllowed)
		return
	}

	r.Body = http.MaxBytesReader(w, r.Body, s.maxBodySize.Load())
	body, err := io.ReadAll(r.Body)
	if err != nil {
		s.logger.Warnf(
			"[%s] failed to read request body | remote=%s | err=%v",
			s.name, remoteAddr, err,
		)
		http.Error(w, "Failed to read body", http.StatusBadRequest)
		return
	}

	if isBatchRequest(body) {
		s.logger.Warnf(
			"[%s] batch request rejected | remote=%s",
			s.name, remoteAddr,
		)
		resp := BuildError(nil, ErrInvalidRequest, "Batch requests are not supported")
		s.writeResponse(w, resp, remoteAddr)
		return
	}

	req, err := ParseRequest(body)
	if err != nil {
		s.logger.Warnf(
			"[%s] failed to parse RPC request | remote=%s | err=%v",
			s.name, remoteAddr, err,
		)
		resp := BuildError(nil, ErrParse, err.Error())
		s.writeResponse(w, resp, remoteAddr)
		return
	}

	s.logger.Debugf(
		"[%s] RPC request received | remote=%s | payload=%s",
		s.name, remoteAddr, string(body),
	)

	handler, ok := s.getHandler(req.Method)
	if !ok {
		s.logger.Warnf(
			"[%s] method not found | remote=%s | method=%s",
			s.name, remoteAddr, req.Method,
		)
		resp := BuildError(req, ErrMethodNotFound, "Method not found")
		s.writeResponse(w, resp, remoteAddr)
		return
	}

	var resp *Response
	func() {
		defer func() {
			if r := recover(); r != nil {
				stack := string(debug.Stack())
				s.logger.Errorf(
					"[%s] handler panic | remote=%s | method=%s | panic=%v | stack=%s",
					s.name, remoteAddr, req.Method, r, stack,
				)
				resp = BuildError(req, ErrInternal, "Internal server error")
			}
		}()

		resp = handler(ctx, req)
	}() // 闭包处理, 捕获handler中的panic, 转换为JSON-RPC错误

	if req.IsNotification() { // 如果是通知消息, 则不需要响应
		s.logger.Debugf(
			"[%s] RPC notification received | remote=%s | method=%s",
			s.name, remoteAddr, req.Method,
		)
		return
	}

	if resp == nil { // handler没有返回结果, 则构建一个空响应
		s.logger.Warnf(
			"[%s] handler returned nil response | remote=%s | method=%s",
			s.name, remoteAddr, req.Method,
		)
		resp = BuildResponse(req, nil, nil)
	}

	s.writeResponse(w, resp, remoteAddr)
}

func (s *RPCServer) handlePing(ctx context.Context, req *Request) *Response {
	return BuildResponse(req, "pong", nil)
}

func isBatchRequest(body []byte) bool {
	var raw any
	if err := json.Unmarshal(body, &raw); err != nil {
		return false
	}
	_, ok := raw.([]any)
	return ok
}

func (s *RPCServer) getHandler(method string) (MethodHandler, bool) {
	s.mu.RLock()
	defer s.mu.RUnlock()
	h, ok := s.handlers[method]
	return h, ok
}

func (s *RPCServer) writeResponse(w http.ResponseWriter, resp *Response, remoteAddr string) {
	if resp == nil {
		w.WriteHeader(http.StatusNoContent)
		return
	}

	data, err := json.Marshal(resp)
	if err != nil {
		s.logger.Errorf(
			"[%s] failed to marshal json-rpc response | remote=%s | err=%v",
			s.name, remoteAddr, err,
		)
		http.Error(w, "Failed to encode response", http.StatusInternalServerError)
		return
	}

	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusOK)

	if _, err := w.Write(data); err != nil {
		s.logger.Errorf(
			"[%s] failed to write response | remote=%s | err=%v",
			s.name, remoteAddr, err,
		)
	}

	s.logger.Debugf(
		"[%s] RPC response sent | remote=%s | payload=%s",
		s.name, remoteAddr, string(data),
	)
}