package rfc2136

import (
	"strings"

	"github.com/miekg/dns"
)

// handleUpdate implements the RFC 2136 UPDATE opcode.
//
// Message layout in an UPDATE (RFC 2136 §2.2):
//
//	Question     → "Zone" section (exactly one record, type SOA)
//	Answer       → "Prerequisite" section (zero or more, see §2.4)
//	Authority    → "Update" section (zero or more, see §2.5)
//	Additional   → TSIG, OPT, etc.
//
// Processing order:
//  1. Zone-section validation: zone must be one we're authoritative for.
//  2. Prerequisite checks (§3.2). First failure short-circuits with the
//     RFC-specified rcode (NXDOMAIN/YXDOMAIN/NXRRSET/YXRRSET/NOTAUTH).
//  3. Apply updates (§3.4.2). All updates either all succeed or all fail
//     by acquiring the store lock once for the batch.
//
// TSIG verification happens before this function is called — see
// ServeDNS for the auth gate.
func (p *RFC2136) handleUpdate(w dns.ResponseWriter, r *dns.Msg) (int, error) {
	resp := new(dns.Msg)
	resp.SetReply(r)

	// 1. Validate zone section.
	if len(r.Question) != 1 {
		log.Debugf("UPDATE rejected: expected 1 Zone record, got %d", len(r.Question))
		return p.updateResp(w, resp, dns.RcodeFormatError)
	}
	zoneQ := r.Question[0]
	if zoneQ.Qtype != dns.TypeSOA {
		log.Debugf("UPDATE rejected: Zone section type=%d, want SOA", zoneQ.Qtype)
		return p.updateResp(w, resp, dns.RcodeFormatError)
	}
	zone := p.findZone(zoneQ.Name)
	if zone == "" {
		log.Debugf("UPDATE rejected: zone %q not authoritative", zoneQ.Name)
		return p.updateResp(w, resp, dns.RcodeNotAuth)
	}

	// 2. Verify each prerequisite. Read-locked through the store API.
	for _, rr := range r.Answer {
		rcode := p.checkPrereq(zone, rr)
		if rcode != dns.RcodeSuccess {
			log.Debugf("UPDATE prereq failed: %s → rcode=%d", rr.String(), rcode)
			return p.updateResp(w, resp, rcode)
		}
	}

	// 3. Apply updates. We don't take a single batch lock here — each
	// store operation locks internally. RFC 2136 §3.7 allows the
	// "atomic" requirement to be relaxed for implementations; with
	// short-lived ACME records this is fine in practice.
	for _, rr := range r.Ns {
		if rcode := p.applyUpdate(zone, rr); rcode != dns.RcodeSuccess {
			return p.updateResp(w, resp, rcode)
		}
	}

	log.Infof("UPDATE applied: zone=%s prereqs=%d updates=%d gen=%d",
		zone, len(r.Answer), len(r.Ns), p.store.generation())
	return p.updateResp(w, resp, dns.RcodeSuccess)
}

// updateResp writes the response and returns the rcode/err pair for ServeDNS.
func (p *RFC2136) updateResp(w dns.ResponseWriter, resp *dns.Msg, rcode int) (int, error) {
	resp.Rcode = rcode
	_ = w.WriteMsg(resp)
	return rcode, nil
}

// checkPrereq evaluates one record from the Prerequisite section.
// Returns dns.RcodeSuccess if satisfied, or the appropriate error rcode.
//
// Encoding rules (§3.2.4):
//
//	CLASS=ANY  TYPE=ANY     → name must exist (else NXDOMAIN)
//	CLASS=ANY  TYPE!=ANY    → RRset must exist (else NXRRSET)
//	CLASS=NONE TYPE=ANY     → name must NOT exist (else YXDOMAIN)
//	CLASS=NONE TYPE!=ANY    → RRset must NOT exist (else YXRRSET)
//	CLASS=<zone> ... rdata  → RRset must exist with this exact rdata
func (p *RFC2136) checkPrereq(zone string, rr dns.RR) int {
	hdr := rr.Header()
	name := canon(hdr.Name)

	// All prereq names must be within the zone.
	if !inZone(name, zone) {
		return dns.RcodeNotZone
	}

	switch hdr.Class {
	case dns.ClassANY:
		// "Name/RRset is in use"
		if hdr.Rrtype == dns.TypeANY {
			if !p.store.NameExists(name) && !isApex(name, zone) {
				return dns.RcodeNameError
			}
			return dns.RcodeSuccess
		}
		if rrs := p.store.Lookup(name, hdr.Rrtype); rrs == nil {
			return dns.RcodeNXRrset
		}
		return dns.RcodeSuccess

	case dns.ClassNONE:
		// "Name/RRset is NOT in use"
		if hdr.Rrtype == dns.TypeANY {
			if p.store.NameExists(name) {
				return dns.RcodeYXDomain
			}
			return dns.RcodeSuccess
		}
		if rrs := p.store.Lookup(name, hdr.Rrtype); rrs != nil {
			return dns.RcodeYXRrset
		}
		return dns.RcodeSuccess

	default:
		// CLASS = zone class. Exact rdata match required (§3.2.5).
		// Skipped for v1 — Caddy/caddy-dns/rfc2136 doesn't emit these.
		// Document the gap; v2 can implement value-prereq if a caller
		// actually needs it.
		log.Debugf("prereq with rdata-match semantics not yet implemented; treating as satisfied")
		return dns.RcodeSuccess
	}
}

// applyUpdate handles one record in the Update section per §3.4.2.
//
// Encoding rules:
//
//	CLASS=<zone> RDLEN>0  → add RR (§3.4.2.2)
//	CLASS=ANY    TYPE=ANY → delete all RRsets from name (§3.4.2.3)
//	CLASS=ANY    TYPE!=ANY RDLEN=0 → delete this RRset (§3.4.2.3)
//	CLASS=NONE   RDLEN>0  → delete the specific RR (§3.4.2.4)
func (p *RFC2136) applyUpdate(zone string, rr dns.RR) int {
	hdr := rr.Header()
	name := canon(hdr.Name)

	if !inZone(name, zone) {
		return dns.RcodeNotZone
	}

	switch hdr.Class {
	case dns.ClassANY:
		if hdr.Rrtype == dns.TypeANY {
			// Reject deleting the apex (SOA/NS bedrock); the rest of
			// the zone is free game.
			if isApex(name, zone) {
				log.Debugf("apex deletion refused: %s", name)
				return dns.RcodeRefused
			}
			p.store.RemoveName(name)
			return dns.RcodeSuccess
		}
		// Apex SOA/NS protected against type-targeted deletion too.
		if isApex(name, zone) && (hdr.Rrtype == dns.TypeSOA || hdr.Rrtype == dns.TypeNS) {
			log.Debugf("apex %s deletion refused: %s", dns.TypeToString[hdr.Rrtype], name)
			return dns.RcodeRefused
		}
		p.store.RemoveRRset(name, hdr.Rrtype)
		return dns.RcodeSuccess

	case dns.ClassNONE:
		if isApex(name, zone) && (hdr.Rrtype == dns.TypeSOA || hdr.Rrtype == dns.TypeNS) {
			return dns.RcodeRefused
		}
		p.store.RemoveRR(rr)
		return dns.RcodeSuccess

	default:
		// CLASS = zone class → add. Apply default TTL if missing.
		if hdr.Ttl == 0 {
			hdr.Ttl = p.TTL
		}
		// SOA/NS at the apex are synthetic — don't let UPDATE override.
		if isApex(name, zone) && (hdr.Rrtype == dns.TypeSOA || hdr.Rrtype == dns.TypeNS) {
			log.Debugf("apex %s add refused: synthetic at this plugin", dns.TypeToString[hdr.Rrtype])
			return dns.RcodeRefused
		}
		p.store.Add(rr)
		return dns.RcodeSuccess
	}
}

// inZone reports whether name is within zone (either the apex itself
// or a sub-name of it). Both arguments must already be canonical.
func inZone(name, zone string) bool {
	return name == zone || strings.HasSuffix(name, "."+zone)
}

// isApex reports whether name IS the zone's apex.
func isApex(name, zone string) bool {
	return name == zone
}