package checker

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"net"
	"strings"
	"sync"
	"time"

	"github.com/miekg/dns"
)

const dnsTimeout = 5 * time.Second

// dnsExchange sends a single query against a host:port server.
// rd controls the RD bit (set false when querying an authoritative server),
// dnssec controls the DO bit so the server returns RRSIG / NSEC[3] records.
func dnsExchange(ctx context.Context, proto, server string, q dns.Question, rd, dnssec bool) (*dns.Msg, error) {
	client := dns.Client{Net: proto, Timeout: dnsTimeout}

	m := new(dns.Msg)
	m.Id = dns.Id()
	m.Question = []dns.Question{q}
	m.RecursionDesired = rd
	m.SetEdns0(4096, dnssec)

	if deadline, ok := ctx.Deadline(); ok {
		if d := time.Until(deadline); d > 0 && d < client.Timeout {
			client.Timeout = d
		}
	}

	r, _, err := client.Exchange(m, server)
	if err != nil {
		return nil, err
	}
	if r == nil {
		return nil, fmt.Errorf("nil response from %s", server)
	}
	return r, nil
}

func recursiveExchange(ctx context.Context, server string, q dns.Question, dnssec bool) (*dns.Msg, error) {
	return dnsExchange(ctx, "", server, q, true, dnssec)
}

func systemResolver() string {
	cfg, err := dns.ClientConfigFromFile("/etc/resolv.conf")
	if err != nil || len(cfg.Servers) == 0 {
		return net.JoinHostPort("1.1.1.1", "53")
	}
	return net.JoinHostPort(cfg.Servers[0], cfg.Port)
}

func hostPort(host, port string) string {
	return net.JoinHostPort(strings.TrimSuffix(host, "."), port)
}

func lowerFQDN(name string) string {
	return strings.ToLower(dns.Fqdn(name))
}

// resolveAuthNS returns "host:port" addresses for every authoritative NS of
// zone, asking the bootstrap resolver. The list is deduplicated and sorted
// only by NS host order so the per-server section of the report is stable.
// Per-host lookup failures are returned as nsErrors so the caller can surface
// them without aborting the whole collection.
func resolveAuthNS(ctx context.Context, zone, resolver string) (hosts []string, addrs []string, nsErrors []string, err error) {
	q := dns.Question{Name: dns.Fqdn(zone), Qtype: dns.TypeNS, Qclass: dns.ClassINET}
	r, err := recursiveExchange(ctx, resolver, q, false)
	if err != nil {
		return nil, nil, nil, fmt.Errorf("NS lookup for %s: %w", zone, err)
	}
	if r.Rcode != dns.RcodeSuccess {
		return nil, nil, nil, fmt.Errorf("NS lookup for %s: rcode %s", zone, dns.RcodeToString[r.Rcode])
	}

	for _, rr := range r.Answer {
		if ns, ok := rr.(*dns.NS); ok {
			hosts = append(hosts, strings.ToLower(strings.TrimSuffix(ns.Ns, ".")))
		}
	}
	if len(hosts) == 0 {
		return nil, nil, nil, fmt.Errorf("no NS records for %s", zone)
	}

	results := make([][]string, len(hosts))
	errs := make([]string, len(hosts))
	var wg sync.WaitGroup
	wg.Add(len(hosts))
	for i, host := range hosts {
		go func() {
			defer wg.Done()
			a, err := net.DefaultResolver.LookupHost(ctx, host)
			if err != nil {
				errs[i] = fmt.Sprintf("address lookup for %s: %v", host, err)
				return
			}
			out := make([]string, 0, len(a))
			for _, ip := range a {
				out = append(out, hostPort(ip, "53"))
			}
			results[i] = out
		}()
	}
	wg.Wait()

	seen := map[string]struct{}{}
	for _, batch := range results {
		for _, a := range batch {
			if _, ok := seen[a]; ok {
				continue
			}
			seen[a] = struct{}{}
			addrs = append(addrs, a)
		}
	}
	for _, e := range errs {
		if e != "" {
			nsErrors = append(nsErrors, e)
		}
	}
	return hosts, addrs, nsErrors, nil
}

// hasParentDS asks the bootstrap resolver whether the parent zone publishes
// a DS for zone. Failures are reported as "false, nil" because absence-of-
// evidence is the practical fallback when the network is glitchy.
func hasParentDS(ctx context.Context, zone, resolver string) bool {
	q := dns.Question{Name: dns.Fqdn(zone), Qtype: dns.TypeDS, Qclass: dns.ClassINET}
	r, err := recursiveExchange(ctx, resolver, q, true)
	if err != nil || r == nil || r.Rcode != dns.RcodeSuccess {
		return false
	}
	for _, rr := range r.Answer {
		if _, ok := rr.(*dns.DS); ok {
			return true
		}
	}
	return false
}

// randomLabel returns a 32-hex-char label used as the leftmost component of
// the NXDOMAIN probe name. 32 hex chars = 128 bits of entropy: collision
// with an existing wildcard or zone name is statistically impossible.
func randomLabel() string {
	var b [16]byte
	_, _ = rand.Read(b[:])
	return hex.EncodeToString(b[:])
}