package resolver

import (
	"context"
	"errors"
	"fmt"
	"net"
	"time"

	"github.com/miekg/dns"
)

var (
	ErrMaxDelegations = errors.New("max delegations exceeded")
	ErrNoServers      = errors.New("no nameservers available")
)

type Resolver struct {
	roots          []string
	maxDelegations int
	timeout        time.Duration
	retries        int
	client *dns.Client
}

type Option func(*Resolver)

func New(opts ...Option) *Resolver {
	r := &Resolver{
		roots:          loadRootServers(),
		maxDelegations: 30,
		timeout:        2 * time.Second,
		retries:        2,
		client: &dns.Client{
			Net: "udp",
			UDPSize: 4096,
			Timeout: 2 * time.Second,
		},
	}
	for _, opt := range opts {
		opt(r)
	}
	return r
}

func WithRootAddresses(addrs []string) Option {
	return func(r *Resolver) {
		r.roots = addrs
	}
}

func WithTimeout(d time.Duration) Option {
	return func(r *Resolver) {
		r.timeout = d
	}
}

func (r *Resolver) Lookup(ctx context.Context, qname string, qtype uint16) (*dns.Msg, error) {
	if ctx == nil {
		ctx = context.Background()
	}
	return r.resolve(ctx, qname, qtype)
}

func (r *Resolver) resolve(ctx context.Context, qname string, qtype uint16) (*dns.Msg, error) {
	servers := make([]string, len(r.roots))
	copy(servers, r.roots)
	for depth := 0; depth < r.maxDelegations; depth++ {
		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		default:
		}
		if len(servers) == 0 {
			return nil, ErrNoServers
		}
		reply, err := r.exchangeWithRetries(ctx, servers, qname, qtype)
		if err != nil {
			return nil, fmt.Errorf("resolve %s %s: %w",
				qname, dns.TypeToString[qtype], err)
		}
		switch {
		case reply.Rcode == dns.RcodeSuccess && len(reply.Answer) > 0:
			return reply, nil
		case reply.Rcode == dns.RcodeNameError:
			return reply, nil
		case reply.Rcode == dns.RcodeSuccess && isReferral(reply):
			next, err := r.nextServers(ctx, reply)
			if err != nil {
				return nil, err
			}
			servers = next
		case reply.Rcode == dns.RcodeSuccess:
			return reply, nil
		default:
			if len(servers) > 1 {
				servers = servers[1:]
			} else {
				return reply, nil
			}
		}

	}
	return nil, ErrMaxDelegations
}

func isReferral(msg *dns.Msg) bool {
	return !msg.Authoritative && len(msg.Ns) > 0
}

func (r *Resolver) nextServers(ctx context.Context, msg *dns.Msg) ([]string, error) {
	var targets []string
	glue := make(map[string]string)

	for _, rr := range msg.Ns {
		ns, ok := rr.(*dns.NS)
		if !ok {
			continue
		}
		targets = append(targets, ns.Ns)
	}
	for _, rr := range msg.Extra {
		switch v := rr.(type) {
		case *dns.A:
			if _, exists := glue[v.Hdr.Name]; !exists {
				glue[v.Hdr.Name] = v.A.String()
			}
		case *dns.AAAA:
			if _, exists := glue[v.Hdr.Name]; !exists {
				glue[v.Hdr.Name] = v.AAAA.String()
			}
		}
	}

	var addrs []string
	var unresolved []string
	for _, t := range targets {
		if ip, ok := glue[t]; ok {
			addrs = append(addrs, ip)
		} else {
			unresolved = append(unresolved, t)
		}
	}

	if len(addrs) > 0 {
		return addrs, nil
	}

	for _, name := range unresolved {
		reply, err := r.resolve(ctx, name, dns.TypeA)
		if err != nil {
			continue
		}
		if reply.Rcode != dns.RcodeSuccess {
			continue
		}
		for _, rr := range reply.Answer {
			if a, ok := rr.(*dns.A); ok {
				addrs = append(addrs, a.A.String())
				break
			}
		}
	}

	if len(addrs) == 0 {
		return nil, ErrNoServers
	}
	return addrs, nil
}

func (r *Resolver) exchangeWithRetries(ctx context.Context, servers []string,
	qname string, qtype uint16) (*dns.Msg, error) {

	msg := new(dns.Msg)
	msg.SetQuestion(qname, qtype)
	msg.SetEdns0(4096, false)
	msg.RecursionDesired = false

	type result struct {
		reply *dns.Msg
		err error
	}

	ch := make(chan result, len(servers))
	for _, srv := range servers {
		addr := srv
		if _, _, err := net.SplitHostPort(srv); err != nil {
			addr = net.JoinHostPort(srv, "53")
		}
		go func(addr string) {
			for attempt := 0; attempt < r.retries; attempt++ {
				reply, _, err := r.client.ExchangeContext(ctx, msg, addr)
				if err == nil {
					if reply.Truncated {
						tcpClient := &dns.Client{Net: "tcp", Timeout: r.timeout}
						reply, _, err = tcpClient.ExchangeContext(ctx, msg, addr)
						if err == nil {
							ch <- result{reply: reply, err: nil}
							return
						}
					}
					ch <- result{reply: reply, err: nil}
					return
				}
				delay := time.Duration(1<<attempt) * 100 * time.Millisecond
				select {
				case <-time.After(delay):
				case <-ctx.Done():
					ch <- result{nil, ctx.Err()}
					return
				}
			}
			ch <- result{nil, fmt.Errorf("all retries failed for %s", addr)}
		}(addr)
	}
	var lastErr error
	for i := 0; i < len(servers); i++ {
		res := <-ch
		if res.err == nil {
			return res.reply, nil
		}
		lastErr = res.err
	}
	return nil, lastErr
}