/*
 * Copyright (C) 2020-2022, IrineSistiana
 *
 * This file is part of mosdns.
 *
 * mosdns is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * mosdns is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

package server_handler

import (
	"context"
	"sync"
	"time"

	"github.com/IrineSistiana/mosdns/v5/mlog"
	"github.com/IrineSistiana/mosdns/v5/pkg/query_context"
	"github.com/IrineSistiana/mosdns/v5/pkg/server"
	"github.com/IrineSistiana/mosdns/v5/pkg/utils"
	"github.com/IrineSistiana/mosdns/v5/plugin/executable/sequence"
	"github.com/miekg/dns"
	"go.uber.org/zap"
)

const (
	defaultQueryTimeout = time.Second * 5
)

var (
	nopLogger = mlog.Nop()

	// options that can forward to upstream
	queryForwardEDNS0Option = map[uint16]struct{}{
		dns.EDNS0SUBNET: {},
	}

	// options that useless for downstream
	respRemoveEDNS0Option = map[uint16]struct{}{
		dns.EDNS0PADDING: {},
	}
)

type EntryHandlerOpts struct {
	// Logger is used for logging. Default is a noop logger.
	Logger *zap.Logger

	// Required.
	Entry sequence.Executable

	// QueryTimeout limits the timeout value of each query.
	// Default is defaultQueryTimeout.
	QueryTimeout time.Duration
}

func (opts *EntryHandlerOpts) init() {
	if opts.Logger == nil {
		opts.Logger = nopLogger
	}
	utils.SetDefaultNum(&opts.QueryTimeout, defaultQueryTimeout)
}

type EntryHandler struct {
	opts EntryHandlerOpts
}

var _ server.Handler = (*EntryHandler)(nil)

func NewEntryHandler(opts EntryHandlerOpts) *EntryHandler {
	opts.init()
	return &EntryHandler{opts: opts}
}

// ServeDNS implements server.Handler.
// If entry returns an error, a SERVFAIL response will be returned.
// If entry returns without a response, a REFUSED response will be returned.
func (h *EntryHandler) Handle(ctx context.Context, q *dns.Msg, serverMeta server.QueryMeta, packMsgPayload func(m *dns.Msg) (*[]byte, error)) *[]byte {
	// 记录查询开始时间（用于统计）
	startTime := time.Now()

	// basic query check.
	if q.Response || len(q.Question) != 1 || len(q.Answer)+len(q.Ns) > 0 || len(q.Extra) > 1 {
		return nil
	}

	ddl := time.Now().Add(h.opts.QueryTimeout)
	ctx, cancel := context.WithDeadline(ctx, ddl)
	defer cancel()

	qCtx := query_context.NewContext(q)
	qCtx.ServerMeta = serverMeta

	// exec entry
	err := h.opts.Entry.Exec(ctx, qCtx)
	var resp *dns.Msg
	success := true
	if err != nil {
		h.opts.Logger.Warn("entry err", qCtx.InfoField(), zap.Error(err))
		resp = new(dns.Msg)
		resp.SetReply(q)
		resp.Rcode = dns.RcodeServerFailure
		success = false
	} else {
		resp = qCtx.R()
	}

	if resp == nil {
		resp = new(dns.Msg)
		resp.SetReply(q)
		resp.Rcode = dns.RcodeRefused
		success = false
	}
	// We assume that our server is a forwarder.
	resp.RecursionAvailable = true

	// add respOpt back to resp
	if respOpt := qCtx.RespOpt(); respOpt != nil {
		resp.Extra = append(resp.Extra, respOpt)
	}

	if serverMeta.FromUDP {
		udpSize := getValidUDPSize(qCtx.ClientOpt())
		resp.Truncate(udpSize)
	}

	payload, err := packMsgPayload(resp)
	if err != nil {
		h.opts.Logger.Error("internal err: failed to pack resp msg", qCtx.InfoField(), zap.Error(err))
		return nil
	}

	// 记录查询统计
	responseTime := time.Since(startTime).Milliseconds()
	cached := checkIfCachedResponse(qCtx) // 检查是否命中缓存
	recordQueryStats(success, cached, responseTime)

	return payload
}

// opt can be nil.
func getValidUDPSize(opt *dns.OPT) int {
	var s uint16
	if opt != nil {
		s = opt.UDPSize()
	}
	if s < dns.MinMsgSize {
		s = dns.MinMsgSize
	}
	return int(s)
}

func newOpt() *dns.OPT {
	opt := new(dns.OPT)
	opt.Hdr.Name = "."
	opt.Hdr.Rrtype = dns.TypeOPT
	return opt
}

// checkIfCachedResponse 检查响应是否来自缓存
// 简化实现：当前版本暂时假设所有查询都未命中缓存
// TODO: 未来可以通过缓存插件在 qCtx 中设置标记来识别缓存命中
func checkIfCachedResponse(qCtx *query_context.Context) bool {
	// 简化实现，始终返回 false
	// 缓存统计需要在缓存插件中单独实现
	return false
}

// 全局查询统计变量（内部实现，避免循环依赖）
var (
	totalQueries      int64
	successfulQueries int64
	failedQueries     int64
	statsMutex        sync.RWMutex
)

// recordQueryStats 记录查询统计（直接实现，避免循环依赖）
func recordQueryStats(success bool, cached bool, responseTimeMs int64) {
	statsMutex.Lock()
	defer statsMutex.Unlock()

	totalQueries++
	if success {
		successfulQueries++
	}
	// 简化版本：暂时不记录缓存命中、响应时间等
	// 这些可以通过 Prometheus metrics 或者单独的统计插件实现
}

// GetQueryStats 返回查询统计数据（供 coremain 调用）
func GetQueryStats() (total, successful, failed int64) {
	statsMutex.RLock()
	defer statsMutex.RUnlock()
	return totalQueries, successfulQueries, failedQueries
}