// Package zonefile converts between BIND-format zone files and the record
// rows stored in SQLite.
//
// Parsing is delegated to the DNS library's zone parser, so $ORIGIN, $TTL,
// $INCLUDE-free multi-line records, comments and every record type it knows
// are handled exactly as a real name server would handle them.
package zonefile

import (
	"bufio"
	"fmt"
	"io"
	"sort"
	"strings"
	"time"

	"github.com/miekg/dns"

	"github.com/owen/vibedns/internal/models"
	"github.com/owen/vibedns/internal/validate"
)

// ParseSummary reports what an import contained.
type ParseSummary struct {
	RecordsParsed int      `json:"records_parsed"`
	Skipped       int      `json:"skipped"`
	SOAFound      bool     `json:"soa_found"`
	OutOfZone     int      `json:"out_of_zone"`
	Warnings      []string `json:"warnings,omitempty"`
}

// ParseResult is the outcome of reading a zone file.
type ParseResult struct {
	Records []models.Record
	SOA     *dns.SOA
	Summary ParseSummary
}

// maxWarnings caps how much detail a badly formed file can generate.
const maxWarnings = 25

// Parse reads a zone file and converts it into storable records.
//
// origin must be a normalised FQDN. Records outside the origin are reported
// and skipped rather than silently dropped, because importing them would
// create data the server can never serve.
func Parse(r io.Reader, origin string, defaultTTL uint32) (*ParseResult, error) {
	origin = strings.ToLower(dns.Fqdn(origin))
	if defaultTTL == 0 {
		defaultTTL = 3600
	}

	res := &ParseResult{}
	zp := dns.NewZoneParser(bufio.NewReader(r), origin, "zonefile")
	zp.SetDefaultTTL(defaultTTL)
	zp.SetIncludeAllowed(false) // $INCLUDE would read arbitrary local files

	for {
		rr, ok := zp.Next()
		if !ok {
			break
		}
		if rr == nil {
			continue
		}
		owner := strings.ToLower(rr.Header().Name)

		if !dns.IsSubDomain(origin, owner) {
			res.Summary.OutOfZone++
			res.Summary.Skipped++
			addWarning(&res.Summary, fmt.Sprintf(
				"%s %s is outside zone %s and was skipped",
				owner, dns.TypeToString[rr.Header().Rrtype], origin))
			continue
		}

		if soa, isSOA := rr.(*dns.SOA); isSOA {
			if owner == origin && res.SOA == nil {
				res.SOA = soa
				res.Summary.SOAFound = true
			}
			// The SOA is stored as zone metadata, not as a record row: the
			// authoritative engine regenerates it so serials stay managed.
			continue
		}

		name, err := relativeName(owner, origin)
		if err != nil {
			res.Summary.Skipped++
			addWarning(&res.Summary, err.Error())
			continue
		}

		rdata := RData(rr)
		if strings.TrimSpace(rdata) == "" {
			res.Summary.Skipped++
			addWarning(&res.Summary, fmt.Sprintf("%s %s had empty record data and was skipped",
				owner, dns.TypeToString[rr.Header().Rrtype]))
			continue
		}

		rec := models.Record{
			Name:    name,
			Type:    typeName(rr.Header().Rrtype),
			Data:    rdata,
			Enabled: true,
		}
		if ttl := rr.Header().Ttl; ttl != defaultTTL {
			t := ttl
			rec.TTL = &t
		}
		res.Records = append(res.Records, rec)
		res.Summary.RecordsParsed++
	}

	if err := zp.Err(); err != nil {
		return nil, fmt.Errorf("zone file could not be parsed: %s", cleanError(err))
	}
	if res.Summary.RecordsParsed == 0 && res.SOA == nil {
		return nil, fmt.Errorf("no records were found in the file; check that it is a BIND zone file for %s",
			strings.TrimSuffix(origin, "."))
	}
	return res, nil
}

func addWarning(s *ParseSummary, msg string) {
	if len(s.Warnings) < maxWarnings {
		s.Warnings = append(s.Warnings, msg)
	}
}

// typeName renders a type, falling back to the RFC 3597 TYPEnnnnn form.
func typeName(t uint16) string {
	if s, ok := dns.TypeToString[t]; ok {
		return s
	}
	return fmt.Sprintf("TYPE%d", t)
}

// relativeName converts an absolute owner name into its in-zone relative form.
func relativeName(owner, origin string) (string, error) {
	if owner == origin {
		return "@", nil
	}
	if !strings.HasSuffix(owner, "."+origin) {
		return "", fmt.Errorf("%s is not inside zone %s", owner, origin)
	}
	return strings.TrimSuffix(owner, "."+origin), nil
}

// RData returns just the record data portion of an RR.
//
// The DNS library formats a record as "name<TAB>ttl<TAB>class<TAB>type<TAB>rdata",
// so splitting on the first four tabs isolates the rdata without re-implementing
// per-type formatting.
func RData(rr dns.RR) string {
	s := rr.String()
	parts := strings.SplitN(s, "\t", 5)
	if len(parts) < 5 {
		// Fall back to trimming the header's own rendering.
		header := rr.Header().String()
		return strings.TrimSpace(strings.TrimPrefix(s, header))
	}
	return strings.TrimSpace(parts[4])
}

func cleanError(err error) string {
	msg := err.Error()
	msg = strings.TrimPrefix(msg, "dns: ")
	return msg
}

// --- Export -------------------------------------------------------------

// Export writes a zone as a BIND-compatible zone file.
func Export(w io.Writer, zone models.Zone, records []models.Record) error {
	bw := bufio.NewWriter(w)
	defer bw.Flush()

	fmt.Fprintf(bw, ";; Zone file for %s\n", zone.Name)
	fmt.Fprintf(bw, ";; Exported by VibeDNS on %s\n", time.Now().UTC().Format(time.RFC3339))
	if zone.Description != "" {
		fmt.Fprintf(bw, ";; %s\n", singleLine(zone.Description))
	}
	fmt.Fprintf(bw, ";;\n")
	fmt.Fprintf(bw, "$ORIGIN %s\n", zone.Name)
	fmt.Fprintf(bw, "$TTL %d\n\n", zone.DefaultTTL)

	// The SOA is always written from zone metadata so the exported serial
	// matches what the server is actually answering with.
	soa := soaFor(zone)
	fmt.Fprintf(bw, "%s\n\n", soa.String())

	byType := map[string][]models.Record{}
	for _, r := range records {
		if strings.EqualFold(r.Type, "SOA") {
			continue // regenerated above
		}
		byType[strings.ToUpper(r.Type)] = append(byType[strings.ToUpper(r.Type)], r)
	}

	// NS records first, then everything else alphabetically: this is the
	// conventional layout and makes diffs between exports readable.
	order := []string{"NS"}
	var rest []string
	for t := range byType {
		if t != "NS" {
			rest = append(rest, t)
		}
	}
	sort.Strings(rest)
	order = append(order, rest...)

	for _, t := range order {
		recs := byType[t]
		if len(recs) == 0 {
			continue
		}
		sort.SliceStable(recs, func(i, j int) bool {
			if recs[i].Name == recs[j].Name {
				return recs[i].Data < recs[j].Data
			}
			if recs[i].Name == "@" {
				return true
			}
			if recs[j].Name == "@" {
				return false
			}
			return recs[i].Name < recs[j].Name
		})
		fmt.Fprintf(bw, ";; %s records\n", t)
		for _, r := range recs {
			writeRecord(bw, zone, r)
		}
		fmt.Fprintln(bw)
	}
	return bw.Flush()
}

func writeRecord(w io.Writer, zone models.Zone, r models.Record) {
	name := r.Name
	if name == "" {
		name = "@"
	}
	ttl := r.EffectiveTTL(zone.DefaultTTL)

	prefix := ""
	if !r.Enabled {
		// Disabled records are preserved as comments so an export/import round
		// trip does not silently discard them.
		prefix = ";; DISABLED "
	}
	line := fmt.Sprintf("%s%-24s %-7d IN  %-8s %s", prefix, name, ttl, strings.ToUpper(r.Type), r.Data)
	if r.Comment != "" {
		line += " ; " + singleLine(r.Comment)
	}
	fmt.Fprintln(w, line)
}

func soaFor(zone models.Zone) *dns.SOA {
	ns := zone.PrimaryNS
	if ns == "" {
		ns = "ns1." + zone.Name
	}
	if !strings.HasSuffix(ns, ".") {
		ns += "."
	}
	return &dns.SOA{
		Hdr: dns.RR_Header{
			Name: zone.Name, Rrtype: dns.TypeSOA, Class: dns.ClassINET, Ttl: zone.DefaultTTL,
		},
		Ns:      strings.ToLower(ns),
		Mbox:    mailbox(zone.AdminEmail, zone.Name),
		Serial:  zone.Serial,
		Refresh: orDefault(zone.Refresh, 7200),
		Retry:   orDefault(zone.Retry, 3600),
		Expire:  orDefault(zone.Expire, 1209600),
		Minttl:  orDefault(zone.Minimum, 3600),
	}
}

func mailbox(email, zone string) string {
	e := strings.TrimSpace(strings.ToLower(email))
	if e == "" {
		return "hostmaster." + zone
	}
	at := strings.LastIndex(e, "@")
	if at < 0 {
		return dns.Fqdn(e)
	}
	local := strings.ReplaceAll(e[:at], ".", `\.`)
	return local + "." + dns.Fqdn(e[at+1:])
}

func orDefault(v, def uint32) uint32 {
	if v == 0 {
		return def
	}
	return v
}

func singleLine(s string) string {
	s = strings.ReplaceAll(s, "\r", " ")
	s = strings.ReplaceAll(s, "\n", " ")
	return strings.TrimSpace(s)
}

// ZoneMetadataFromSOA copies parsed SOA values onto a zone, which is how an
// imported zone picks up its timers and serial.
func ZoneMetadataFromSOA(z *models.Zone, soa *dns.SOA) {
	if soa == nil {
		return
	}
	z.PrimaryNS = soa.Ns
	z.AdminEmail = emailFromMailbox(soa.Mbox)
	z.Serial = soa.Serial
	z.Refresh = soa.Refresh
	z.Retry = soa.Retry
	z.Expire = soa.Expire
	z.Minimum = soa.Minttl
}

// emailFromMailbox converts an SOA RNAME back into an email address.
func emailFromMailbox(mbox string) string {
	m := strings.TrimSuffix(mbox, ".")
	if m == "" {
		return ""
	}
	// The first unescaped dot separates the local part from the domain.
	var local strings.Builder
	i := 0
	for ; i < len(m); i++ {
		if m[i] == '\\' && i+1 < len(m) {
			local.WriteByte(m[i+1])
			i++
			continue
		}
		if m[i] == '.' {
			break
		}
		local.WriteByte(m[i])
	}
	if i >= len(m) {
		return m
	}
	return local.String() + "@" + m[i+1:]
}

// SuggestFilename returns a sensible download name for a zone export.
func SuggestFilename(zone string) string {
	name := strings.TrimSuffix(zone, ".")
	if name == "" {
		name = "zone"
	}
	return name + ".zone"
}

// ValidateRecords re-parses exported records to confirm they will compile.
// It is used by the importer to reject a file before anything is written.
func ValidateRecords(origin string, recs []models.Record, defaultTTL uint32) []string {
	var problems []string
	for _, r := range recs {
		if _, err := validate.BuildRR(origin, r.Name, r.Type, r.Data, r.EffectiveTTL(defaultTTL)); err != nil {
			problems = append(problems, fmt.Sprintf("%s %s: %v", r.Name, r.Type, err))
			if len(problems) >= maxWarnings {
				break
			}
		}
	}
	return problems
}