/*
 * SPDX-License-Identifier: AGPL-3.0-only
 * Copyright (c) 2023, daeuniverse Organization <dae@v2raya.org>
 */

package dns

import (
	"fmt"
	"net/netip"
	"net/url"
	"sync"

	"github.com/daeuniverse/dae/common"
	"github.com/daeuniverse/dae/common/assets"
	"github.com/daeuniverse/dae/common/consts"
	"github.com/daeuniverse/dae/component/routing"
	"github.com/daeuniverse/dae/config"
	"github.com/sirupsen/logrus"
	"golang.org/x/net/dns/dnsmessage"
)

var BadUpstreamFormatError = fmt.Errorf("bad upstream format")

type Dns struct {
	log              *logrus.Logger
	upstream         []*UpstreamResolver
	upstream2IndexMu sync.Mutex
	upstream2Index   map[*Upstream]int
	reqMatcher       *RequestMatcher
	respMatcher      *ResponseMatcher
}

type NewOption struct {
	Logger                  *logrus.Logger
	LocationFinder          *assets.LocationFinder
	UpstreamReadyCallback   func(dnsUpstream *Upstream) (err error)
	UpstreamResolverNetwork string
}

func New(dns *config.Dns, opt *NewOption) (s *Dns, err error) {
	s = &Dns{
		log: opt.Logger,
		upstream2Index: map[*Upstream]int{
			nil: int(consts.DnsRequestOutboundIndex_AsIs),
		},
	}
	// Parse upstream.
	upstreamName2Id := map[string]uint8{}
	for i, upstreamRaw := range dns.Upstream {
		if i >= int(consts.DnsRequestOutboundIndex_UserDefinedMax) ||
			i >= int(consts.DnsResponseOutboundIndex_UserDefinedMax) {
			return nil, fmt.Errorf("too many upstreams")
		}

		tag, link := common.GetTagFromLinkLikePlaintext(string(upstreamRaw))
		if tag == "" {
			return nil, fmt.Errorf("%w: '%v' has no tag", BadUpstreamFormatError, upstreamRaw)
		}
		var u *url.URL
		u, err = url.Parse(link)
		if err != nil {
			return nil, fmt.Errorf("%w: %v", BadUpstreamFormatError, err)
		}
		r := &UpstreamResolver{
			Raw:     u,
			Network: opt.UpstreamResolverNetwork,
			FinishInitCallback: func(i int) func(raw *url.URL, upstream *Upstream) (err error) {
				return func(raw *url.URL, upstream *Upstream) (err error) {
					if opt != nil && opt.UpstreamReadyCallback != nil {
						if err = opt.UpstreamReadyCallback(upstream); err != nil {
							return err
						}
					}

					s.upstream2IndexMu.Lock()
					s.upstream2Index[upstream] = i
					s.upstream2IndexMu.Unlock()
					return nil
				}
			}(i),
			mu:       sync.Mutex{},
			upstream: nil,
			init:     false,
		}
		upstreamName2Id[tag] = uint8(len(s.upstream))
		s.upstream = append(s.upstream, r)
	}
	// Optimize routings.
	if dns.Routing.Request.Rules, err = routing.ApplyRulesOptimizers(dns.Routing.Request.Rules,
		&routing.DatReaderOptimizer{Logger: opt.Logger, LocationFinder: opt.LocationFinder},
		&routing.MergeAndSortRulesOptimizer{},
		&routing.DeduplicateParamsOptimizer{},
	); err != nil {
		return nil, err
	}
	if dns.Routing.Response.Rules, err = routing.ApplyRulesOptimizers(dns.Routing.Response.Rules,
		&routing.DatReaderOptimizer{Logger: opt.Logger, LocationFinder: opt.LocationFinder},
		&routing.MergeAndSortRulesOptimizer{},
		&routing.DeduplicateParamsOptimizer{},
	); err != nil {
		return nil, err
	}
	// Parse request routing.
	reqMatcherBuilder, err := NewRequestMatcherBuilder(opt.Logger, dns.Routing.Request.Rules, upstreamName2Id, dns.Routing.Request.Fallback)
	if err != nil {
		return nil, fmt.Errorf("failed to build DNS request routing: %w", err)
	}
	s.reqMatcher, err = reqMatcherBuilder.Build()
	if err != nil {
		return nil, fmt.Errorf("failed to build DNS request routing: %w", err)
	}
	// Parse response routing.
	respMatcherBuilder, err := NewResponseMatcherBuilder(opt.Logger, dns.Routing.Response.Rules, upstreamName2Id, dns.Routing.Response.Fallback)
	if err != nil {
		return nil, fmt.Errorf("failed to build DNS response routing: %w", err)
	}
	s.respMatcher, err = respMatcherBuilder.Build()
	if err != nil {
		return nil, fmt.Errorf("failed to build DNS response routing: %w", err)
	}
	if len(dns.Upstream) == 0 {
		// Immediately ready.
		go opt.UpstreamReadyCallback(nil)
	}
	return s, nil
}

func (s *Dns) CheckUpstreamsFormat() error {
	for _, upstream := range s.upstream {
		_, _, _, err := ParseRawUpstream(upstream.Raw)
		if err != nil {
			return err
		}
	}
	return nil
}

func (s *Dns) InitUpstreams() {
	var wg sync.WaitGroup
	for _, upstream := range s.upstream {
		wg.Add(1)
		go func(upstream *UpstreamResolver) {
			upstream.GetUpstream()
			wg.Done()
		}(upstream)
	}
	wg.Wait()
}

func (s *Dns) RequestSelect(qname string, qtype dnsmessage.Type) (upstreamIndex consts.DnsRequestOutboundIndex, upstream *Upstream, err error) {
	// Route.
	upstreamIndex, err = s.reqMatcher.Match(qname, qtype)
	if err != nil {
		return 0, nil, err
	}
	// nil indicates AsIs.
	if upstreamIndex == consts.DnsRequestOutboundIndex_AsIs ||
		upstreamIndex == consts.DnsRequestOutboundIndex_Reject {
		return upstreamIndex, nil, nil
	}
	if int(upstreamIndex) >= len(s.upstream) {
		return 0, nil, fmt.Errorf("bad upstream index: %v not in [0, %v]", upstreamIndex, len(s.upstream)-1)
	}
	// Get corresponding upstream.
	upstream, err = s.upstream[upstreamIndex].GetUpstream()
	if err != nil {
		return 0, nil, err
	}
	return upstreamIndex, upstream, nil
}

func (s *Dns) ResponseSelect(msg *dnsmessage.Message, fromUpstream *Upstream) (upstreamIndex consts.DnsResponseOutboundIndex, upstream *Upstream, err error) {
	if !msg.Response {
		return 0, nil, fmt.Errorf("DNS response expected but DNS request received")
	}

	// Prepare routing.
	var qname string
	var qtype dnsmessage.Type
	var ips []netip.Addr
	if len(msg.Questions) == 0 {
		qname = ""
		qtype = 0
	} else {
		q := msg.Questions[0]
		qname = q.Name.String()
		qtype = q.Type
		for _, ans := range msg.Answers {
			switch body := ans.Body.(type) {
			case *dnsmessage.AResource:
				ips = append(ips, netip.AddrFrom4(body.A))
			case *dnsmessage.AAAAResource:
				ips = append(ips, netip.AddrFrom16(body.AAAA))
			}
		}
	}

	s.upstream2IndexMu.Lock()
	from := s.upstream2Index[fromUpstream]
	s.upstream2IndexMu.Unlock()
	// Route.
	upstreamIndex, err = s.respMatcher.Match(qname, qtype, ips, consts.DnsRequestOutboundIndex(from))
	if err != nil {
		return 0, nil, err
	}
	// Get corresponding upstream if upstream is neither 'accept' nor 'reject'.
	if !upstreamIndex.IsReserved() {
		if int(upstreamIndex) >= len(s.upstream) {
			return 0, nil, fmt.Errorf("bad upstream index: %v not in [0, %v]", upstreamIndex, len(s.upstream)-1)
		}
		upstream, err = s.upstream[upstreamIndex].GetUpstream()
		if err != nil {
			return 0, nil, err
		}
	} else {
		// Assign explicitly to let coder know.
		upstream = nil
	}
	return upstreamIndex, upstream, nil
}
