package certs

import (
	"context"
	"errors"
	"fmt"
	"slices"
	"strings"
	"time"

	"github.com/miekg/dns"
)

// resolver asks recursive resolvers (acme.resolvers: public ones by
// default), to see a domain as the CA does rather than as this server's
// own views answer it.
type resolver struct {
	servers []string
	timeout time.Duration
}

func (r resolver) query(ctx context.Context, name string, qtype uint16) (*dns.Msg, error) {
	if len(r.servers) == 0 {
		return nil, errors.New("no resolvers configured (acme.resolvers)")
	}
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(name), qtype)
	m.SetEdns0(1232, false)
	m.RecursionDesired = true
	timeout := r.timeout
	if timeout == 0 {
		timeout = 5 * time.Second
	}
	var errs []error
	for _, s := range r.servers {
		c := &dns.Client{Timeout: timeout}
		in, _, err := c.ExchangeContext(ctx, m, s)
		if err == nil && in.Truncated {
			c.Net = "tcp"
			in, _, err = c.ExchangeContext(ctx, m, s)
		}
		if err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", s, err))
			continue
		}
		if in.Rcode == dns.RcodeServerFailure || in.Rcode == dns.RcodeRefused {
			errs = append(errs, fmt.Errorf("%s answered %s", s, dns.RcodeToString[in.Rcode]))
			continue
		}
		return in, nil
	}
	return nil, fmt.Errorf("looking up %s %s: %w", strings.TrimSuffix(name, "."), dns.TypeToString[qtype], errors.Join(errs...))
}

// zoneOf returns the zone that name is in on the internet (the owner of
// the SOA record that answers for it).
func (r resolver) zoneOf(ctx context.Context, name string) (string, error) {
	name = dns.Fqdn(strings.ToLower(name))
	for cand := name; ; {
		in, err := r.query(ctx, cand, dns.TypeSOA)
		if err != nil {
			return "", err
		}
		for _, sec := range [][]dns.RR{in.Answer, in.Ns} {
			for _, rr := range sec {
				if soa, ok := rr.(*dns.SOA); ok && dns.IsSubDomain(soa.Hdr.Name, name) {
					return strings.ToLower(soa.Hdr.Name), nil
				}
			}
		}
		i, end := dns.NextLabel(cand, 0)
		if end {
			return "", fmt.Errorf("no zone found for %s", strings.TrimSuffix(name, "."))
		}
		cand = cand[i:]
	}
}

// nameServers returns the zone's name servers on the internet (lowercase,
// without trailing dots, sorted).
func (r resolver) nameServers(ctx context.Context, zone string) ([]string, error) {
	in, err := r.query(ctx, zone, dns.TypeNS)
	if err != nil {
		return nil, err
	}
	var out []string
	for _, rr := range in.Answer {
		if ns, ok := rr.(*dns.NS); ok && strings.EqualFold(ns.Hdr.Name, dns.Fqdn(zone)) {
			out = append(out, normName(ns.Ns))
		}
	}
	if len(out) == 0 {
		return nil, fmt.Errorf("%s has no name servers on the internet (%s)", strings.TrimSuffix(zone, "."), dns.RcodeToString[in.Rcode])
	}
	slices.Sort(out)
	return slices.Compact(out), nil
}

// follow returns where name leads after its CNAME records (name itself
// when it has none).
func (r resolver) follow(ctx context.Context, name string) (string, error) {
	name = dns.Fqdn(strings.ToLower(name))
	for range 10 {
		in, err := r.query(ctx, name, dns.TypeCNAME)
		if err != nil {
			return "", err
		}
		next := ""
		for _, rr := range in.Answer {
			if c, ok := rr.(*dns.CNAME); ok && strings.EqualFold(c.Hdr.Name, name) {
				next = strings.ToLower(c.Target)
			}
		}
		if next == "" {
			return name, nil
		}
		name = next
	}
	return "", fmt.Errorf("%s: more than 10 CNAME records in a row", name)
}

func normName(n string) string { return strings.ToLower(strings.TrimSuffix(n, ".")) }

// challengeName is the DNS-01 record name for a certificate name (a
// wildcard shares its base name's).
func challengeName(domain string) string {
	return "_acme-challenge." + dns.Fqdn(strings.TrimPrefix(strings.ToLower(domain), "*."))
}