package rfc2136

import (
	"context"
	"fmt"
	"math"
	"os"
	"os/exec"
	"path/filepath"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/miekg/dns"
)

// gitCommandTimeout caps any single git invocation we shell out to. If
// the process hangs (NFS stall, gpg-sign prompt, broken pre-commit hook
// waiting on stdin, etc.) we must not block the caller — which holds
// the per-zone mutex — forever. 10s is generous for a local-disk repo.
const gitCommandTimeout = 10 * time.Second

// fileSnapshot captures the file-identity fingerprint at the moment we
// last read the zone file. We compare it to the live stat just before
// writing back to detect concurrent modification (rsync push, manual
// edit, `git checkout`). If anything changed the file out from under
// us, we refuse the UPDATE — Caddy will retry, and the next loadRRs
// will see the updated state.
type fileSnapshot struct {
	mtime time.Time
	size  int64
}

// matches returns true if `info` reports the same mtime and size we
// captured. Inode comparison would be stricter, but mtime+size is
// sufficient for the failure modes we care about (rename-over-original
// edits change both).
func (s fileSnapshot) matches(info os.FileInfo) bool {
	return s.mtime.Equal(info.ModTime()) && s.size == info.Size()
}

// zoneFile is a file-backed authority for a single DNS zone. Replaces
// the Phase-1.3 in-memory recordStore.
//
// On every UPDATE, the file is read fully into memory as parsed RRs,
// the requested adds/deletes are applied to that slice, the SOA serial
// is bumped (CalVer YYYYMMDDNN style), and the file is rewritten via
// an atomic temp-file rename. CoreDNS's `auto` plugin notices the
// mtime change within its reload interval (~30s) and re-serves the
// zone. HE eventually pulls on its SOA refresh.
//
// Concurrency: per-zone mutex serializes RFC 2136 UPDATEs against
// each other and against the plugin's own reads. It does NOT protect
// against external editors (e.g. a human running an editor while the
// plugin is mid-write); that's the operator's responsibility, and
// the typical mitigation is to do manual edits when no UPDATEs are
// in flight (or just accept the rare race — the worst case is one
// lost manual edit, easily restored from git).
type zoneFile struct {
	mu sync.Mutex

	// Path is the absolute path to the zone file on disk.
	Path string

	// Origin is the canonical (lowercase, trailing dot) zone apex.
	Origin string

	// AutoCommit, when true, runs `git add <path> && git commit ...`
	// after every successful write. Defaults to true (per the chosen
	// architecture: every dynamic update should leave a git trail).
	AutoCommit bool

	// GitAuthorName and GitAuthorEmail are passed to `git commit`
	// via -c user.name and -c user.email so the commits are
	// attributable without depending on the system git config.
	GitAuthorName  string
	GitAuthorEmail string
}

// canon normalises a DNS name to the store's internal form: lowercase
// with trailing dot. miekg/dns sometimes hands us names without the
// trailing dot; passing through this once at the boundary keeps every
// lookup, every comparison consistent.
func canon(name string) string {
	return strings.ToLower(dns.Fqdn(name))
}

// openZoneFile prepares a zoneFile handle. Does NOT read or parse the
// file; that happens lazily in each operation (so the file's content
// is always fresh and we never serve a stale snapshot).
func openZoneFile(path, origin string) *zoneFile {
	return &zoneFile{
		Path:           path,
		Origin:         canon(origin),
		AutoCommit:     true,
		GitAuthorName:  "coredns-rfc2136",
		GitAuthorEmail: "rfc2136@coredns",
	}
}

// loadRRs reads the zone file and parses it into an RR slice via
// miekg/dns's zone parser. The parser handles $ORIGIN, $TTL, multi-line
// SOA, comments, includes, etc.
//
// Returns (rrs, snapshot, error). The snapshot fingerprints the file
// identity at read time so a subsequent writeIfUnchanged can detect
// concurrent modification.
//
// Hamilton H4 — strict-parse validation: a single malformed line could
// otherwise produce a partial parse where parser.Err() returns nil but
// some records silently went missing. To catch this, we enforce a
// post-parse invariant: exactly one SOA RR, and that SOA's name equals
// the configured zone origin. A zone file that's been partially eaten
// by the parser usually loses its SOA along the way — checking SOA
// presence catches both H4 (silent truncation) and H3 (multi-SOA or
// wrong-apex SOA) with a single guard.
func (z *zoneFile) loadRRs() ([]dns.RR, fileSnapshot, error) {
	f, err := os.Open(z.Path)
	if err != nil {
		return nil, fileSnapshot{}, fmt.Errorf("open %s: %w", z.Path, err)
	}
	defer f.Close()

	info, err := f.Stat()
	if err != nil {
		return nil, fileSnapshot{}, fmt.Errorf("stat %s: %w", z.Path, err)
	}
	snap := fileSnapshot{mtime: info.ModTime(), size: info.Size()}

	parser := dns.NewZoneParser(f, z.Origin, z.Path)
	parser.SetDefaultTTL(3600)

	var rrs []dns.RR
	for rr, ok := parser.Next(); ok; rr, ok = parser.Next() {
		rrs = append(rrs, rr)
	}
	if err := parser.Err(); err != nil {
		return nil, snap, fmt.Errorf("parse %s: %w", z.Path, err)
	}
	if len(rrs) == 0 {
		return nil, snap, fmt.Errorf("%s: zero RRs parsed", z.Path)
	}

	// H3/H4 invariant: exactly one SOA, anchored at the zone origin.
	// Refuse to operate on a zone file whose SOA structure is wrong —
	// any subsequent bumpSerial or write would compound the damage.
	if err := assertSingleApexSOA(rrs, z.Origin); err != nil {
		return nil, snap, fmt.Errorf("zone %s integrity check failed: %w", z.Path, err)
	}

	return rrs, snap, nil
}

// assertSingleApexSOA enforces that rrs contains exactly one SOA and
// that its owner matches the zone origin. Returns an error otherwise.
// This is the H3+H4 zone-integrity invariant.
func assertSingleApexSOA(rrs []dns.RR, origin string) error {
	origin = canon(origin)
	var soas []*dns.SOA
	for _, rr := range rrs {
		if s, ok := rr.(*dns.SOA); ok {
			soas = append(soas, s)
		}
	}
	switch len(soas) {
	case 0:
		return fmt.Errorf("no SOA record found (expected one at %q)", origin)
	case 1:
		if canon(soas[0].Hdr.Name) != origin {
			return fmt.Errorf("SOA owner is %q, expected zone apex %q", soas[0].Hdr.Name, origin)
		}
		return nil
	default:
		names := make([]string, len(soas))
		for i, s := range soas {
			names[i] = s.Hdr.Name
		}
		return fmt.Errorf("multiple SOA records found (%d): %s", len(soas), strings.Join(names, ", "))
	}
}

// checkUnchanged returns nil if the on-disk file still matches the
// captured snapshot. If the file has been modified (mtime or size
// differs), returns an error — the caller should refuse the UPDATE
// rather than clobber the external change.
func (z *zoneFile) checkUnchanged(snap fileSnapshot) error {
	info, err := os.Stat(z.Path)
	if err != nil {
		return fmt.Errorf("stat %s: %w", z.Path, err)
	}
	if !snap.matches(info) {
		return fmt.Errorf("concurrent modification detected on %s: mtime %s/%s size %d/%d",
			z.Path, snap.mtime.Format(time.RFC3339Nano), info.ModTime().Format(time.RFC3339Nano),
			snap.size, info.Size())
	}
	return nil
}

// Lookup returns RRs in `rrs` matching (name, rtype). Both name and
// the RR header names are canonicalised for the comparison. Pass-by-
// slice rather than holding state means we can let the caller batch
// multiple operations against one snapshot of the file.
func lookupIn(rrs []dns.RR, name string, rtype uint16) []dns.RR {
	name = canon(name)
	var out []dns.RR
	for _, rr := range rrs {
		hdr := rr.Header()
		if canon(hdr.Name) == name && hdr.Rrtype == rtype {
			out = append(out, rr)
		}
	}
	return out
}

// nameExistsIn reports whether any RR's owner equals name (canonical).
func nameExistsIn(rrs []dns.RR, name string) bool {
	name = canon(name)
	for _, rr := range rrs {
		if canon(rr.Header().Name) == name {
			return true
		}
	}
	return false
}

// removeRRsetFrom returns rrs minus every RR matching (name, rtype).
func removeRRsetFrom(rrs []dns.RR, name string, rtype uint16) []dns.RR {
	name = canon(name)
	out := rrs[:0:0]
	for _, rr := range rrs {
		hdr := rr.Header()
		if canon(hdr.Name) == name && hdr.Rrtype == rtype {
			continue
		}
		out = append(out, rr)
	}
	return out
}

// removeNameFrom returns rrs minus every RR with the given owner name.
func removeNameFrom(rrs []dns.RR, name string) []dns.RR {
	name = canon(name)
	out := rrs[:0:0]
	for _, rr := range rrs {
		if canon(rr.Header().Name) == name {
			continue
		}
		out = append(out, rr)
	}
	return out
}

// removeRRFrom returns rrs minus the single RR matching the given one
// by owner + type + rdata.
//
// Hamilton M3: per RFC 2136 §2.5.4, a delete-by-exact-match UPDATE
// carries CLASS=NONE and TTL=0 as protocol flags, not as match
// criteria. The target must match a stored RR by owner+type+rdata
// alone. We normalize both sides to the same class + TTL before
// invoking dns.IsDuplicate so the comparison is correct.
func removeRRFrom(rrs []dns.RR, target dns.RR) []dns.RR {
	targetN := normalizeForCompare(target)
	out := rrs[:0:0]
	matched := false
	for _, rr := range rrs {
		if !matched && dns.IsDuplicate(normalizeForCompare(rr), targetN) {
			matched = true
			continue
		}
		out = append(out, rr)
	}
	return out
}

// addRRTo appends rr to rrs unless an identical RR already exists
// (de-dupe semantics per RFC 2136 §3.4.2.2). Same normalization as
// removeRRFrom — dedupe is TTL- and class-insensitive in the comparison
// (though the stored RR retains its original TTL/class).
func addRRTo(rrs []dns.RR, rr dns.RR) []dns.RR {
	rrN := normalizeForCompare(rr)
	for _, existing := range rrs {
		if dns.IsDuplicate(normalizeForCompare(existing), rrN) {
			return rrs
		}
	}
	return append(rrs, rr)
}

// normalizeForCompare returns a copy of rr with TTL=0 and class=IN so
// dns.IsDuplicate can be used to compare by (owner, type, rdata) alone.
// Required by RFC 2136 §2.5.4's "TTL and CLASS are flags, not match
// criteria" semantics.
func normalizeForCompare(rr dns.RR) dns.RR {
	n := dns.Copy(rr)
	n.Header().Ttl = 0
	n.Header().Class = dns.ClassINET
	return n
}

// serialCounterMul is the multiplier between the date prefix and the
// counter in our SOA-serial encoding. The format is YYMMDD*10000 + NNNN,
// giving 10000 bumps/day (NNNN ∈ [0001, 9999]). The 2-digit year keeps
// the maximum within uint32 (the RFC 1035 ceiling for SOA serials): for
// 2026-05-22, max serial 2,605,229,999 is well below 2^32-1=4,294,967,295.
// A 4-digit year (e.g., 20260522*10000) would overflow uint32.
const serialCounterMul = 10000

// bumpSerial advances the SOA's serial in CalVer YYMMDD*10000+NNNN form.
//
// Behaviour:
//   - If cur encodes today (or a future-encoded date from a prior NNNN
//     rollover), increment NNNN. On NNNN=9999, roll forward to the next
//     encoded day with NNNN=0001. The encoded date drifts ahead of wall
//     time during heavy churn and catches back up on quiet days; serial
//     numbers stay strictly monotonic, which is the only DNS hard
//     requirement.
//   - Otherwise (older serial; including legacy YYYYMMDDNN-format serials
//     left over from before this format change), jump to today*10000+1.
//     Legacy serials migrate automatically here: a value like 2026052299
//     (~2.026B) is numerically smaller than today's new-format minimum
//     2605220001 (~2.605B), so it falls to this branch and gets rewritten
//     in-place. The new value is strictly greater, so AXFR receivers (HE
//     et al.) treat it as a normal forward bump and pull cleanly.
//
// The SOA is found by type (there should be exactly one); mutated in
// place. The returned slice is the same slice with the SOA's serial
// updated.
func bumpSerial(rrs []dns.RR, now time.Time) error {
	var soa *dns.SOA
	for _, rr := range rrs {
		if s, ok := rr.(*dns.SOA); ok {
			soa = s
			break
		}
	}
	if soa == nil {
		return fmt.Errorf("zone has no SOA record")
	}

	today := now.UTC().Format("060102") // YYMMDD
	cur := fmt.Sprintf("%010d", soa.Serial)

	// Try the new-format read: cur[:6] is YYMMDD, cur[6:10] is NNNN.
	// We only honour this when the encoded date is today or later — an
	// older encoded date means a normal new-day reset (or a legacy
	// serial that happens to look like a valid YYMMDD prefix but is in
	// the past, which is the same handling: jump to today).
	if curDate := cur[:6]; isValidYYMMDD(curDate) && curDate >= today {
		nnnn := int(mustParseUint(cur[6:10]))
		if nnnn < 9999 {
			soa.Serial = uint32(mustParseUint(curDate)*serialCounterMul + uint64(nnnn+1))
			return nil
		}
		// NNNN=9999: roll to next encoded day, NNNN=0001.
		d, err := time.Parse("060102", curDate)
		if err != nil {
			return fmt.Errorf("serial date %q unparseable: %w", curDate, err)
		}
		next := d.AddDate(0, 0, 1).Format("060102")
		soa.Serial = uint32(mustParseUint(next)*serialCounterMul + 1)
		return nil
	}

	// Older or unparseable: jump to today*10000+1. Migration path for
	// legacy YYYYMMDDNN serials lives here.
	candidate := uint32(mustParseUint(today)*serialCounterMul + 1)

	// H5 — explicit MaxUint32 guard. Plain `>` comparison is correct in
	// practice (we'd never wrap during the zone's lifetime: 10000
	// bumps/day × 365 days × ~117 years = ~427M, well under 2^32). The
	// real failure mode we must prevent is wrap-to-0: if soa.Serial
	// somehow reached MaxUint32 (hand-edit, fuzz, or a future code path
	// we haven't written), `soa.Serial++` would wrap to 0, and
	// downstream secondaries per RFC 1982 treat 0-after-MaxUint32 as
	// "older" — they refuse to AXFR, and the zone goes dark. Loud
	// refusal forces the operator to manually reset the serial,
	// instead of silently bricking the zone.
	if candidate <= soa.Serial {
		if soa.Serial == math.MaxUint32 {
			return fmt.Errorf("SOA serial at uint32 max (%d) — refusing to wrap to 0; operator must reset zone serial manually (see RFC 1982 §3.2)", soa.Serial)
		}
		// Defensive monotonic advance for the unusual "current serial
		// is already > today's new-format minimum" case (e.g., a
		// hand-edit set it to a far-future value).
		soa.Serial++
		return nil
	}
	soa.Serial = candidate
	return nil
}

// isValidYYMMDD reports whether s is a 6-character YYMMDD string with a
// valid month and day. Year is any 2-digit value (00-99).
func isValidYYMMDD(s string) bool {
	if len(s) != 6 {
		return false
	}
	_, err := time.Parse("060102", s)
	return err == nil
}

// mustParseUint parses an all-digit string into uint64. Panics on
// malformed input — the caller is responsible for passing only strings
// that were validated as digit-substrings (e.g., a fixed-width slice of
// a YYMMDDNNNN-formatted serial). Using strconv via this thin wrapper
// keeps the panic behavior explicit while sharing stdlib's robust
// parsing (Hamilton L1).
func mustParseUint(s string) uint64 {
	n, err := strconv.ParseUint(s, 10, 64)
	if err != nil {
		// Programmer error if we ever hit this — every caller passes
		// digits-only strings derived from time.Format or a sliced
		// SOA serial. Panic so the bug surfaces in tests/CI rather
		// than silently producing a 0 serial.
		panic(fmt.Sprintf("mustParseUint(%q): %v", s, err))
	}
	return n
}

// writeAtomic serializes rrs to a temp file in the same directory as
// z.Path, then renames over the destination. POSIX guarantees atomic
// rename on local filesystems, so a partial write can never leave a
// corrupt zone file on disk.
//
// Format: one RR per line, tab-separated owner/TTL/class/type/rdata.
// Comments and multi-line SOA formatting from the original file are
// NOT preserved (v1 limitation; sophisticated comment preservation can
// land in v2). A short header line is emitted with the write timestamp
// and the plugin name, so it's obvious in `git log` what touched the
// file.
func (z *zoneFile) writeAtomic(rrs []dns.RR, now time.Time) error {
	dir := filepath.Dir(z.Path)
	tmp, err := os.CreateTemp(dir, ".rfc2136-*.zone")
	if err != nil {
		return fmt.Errorf("create temp: %w", err)
	}
	tmpPath := tmp.Name()
	// Best-effort cleanup if we fail before the rename.
	defer func() {
		if tmpPath != "" {
			_ = os.Remove(tmpPath)
		}
	}()

	header := fmt.Sprintf("; Auto-written by coredns-rfc2136 on %s\n; Zone: %s\n$ORIGIN %s\n",
		now.UTC().Format(time.RFC3339), z.Origin, z.Origin)
	if _, err := tmp.WriteString(header); err != nil {
		_ = tmp.Close()
		return fmt.Errorf("write header: %w", err)
	}

	for _, rr := range rrs {
		if _, err := tmp.WriteString(rr.String() + "\n"); err != nil {
			_ = tmp.Close()
			return fmt.Errorf("write rr: %w", err)
		}
	}

	if err := tmp.Sync(); err != nil {
		_ = tmp.Close()
		return fmt.Errorf("sync: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("close: %w", err)
	}
	if err := os.Rename(tmpPath, z.Path); err != nil {
		return fmt.Errorf("rename %s -> %s: %w", tmpPath, z.Path, err)
	}
	tmpPath = "" // suppress cleanup; rename consumed it
	return nil
}

// commit stages and commits the zone file via git. Runs from the
// repository directory inferred from the zone file's parent. Returns
// nil silently if AutoCommit is false. Returns an error if the commit
// fails; the caller decides whether to roll back the file write.
//
// Both git invocations run under a context with a hard timeout
// (gitCommandTimeout). If git hangs (NFS stall, gpg-sign prompt,
// pre-commit hook waiting on stdin), we kill it rather than block the
// caller's per-zone mutex indefinitely. ACME storms must not be able
// to wedge the plugin via git getting stuck.
func (z *zoneFile) commit(message string) error {
	if !z.AutoCommit {
		return nil
	}
	// We run git from the directory containing the zone file. git will
	// walk upward to find the .git dir.
	dir := filepath.Dir(z.Path)
	ctx, cancel := context.WithTimeout(context.Background(), gitCommandTimeout)
	defer cancel()
	// `git add` first; if file is already in the index, no harm done.
	add := exec.CommandContext(ctx, "git",
		"-C", dir,
		"add", "--", z.Path,
	)
	if out, err := add.CombinedOutput(); err != nil {
		return fmt.Errorf("git add failed: %w: %s", err, strings.TrimSpace(string(out)))
	}
	commit := exec.CommandContext(ctx, "git",
		"-C", dir,
		"-c", "user.name="+z.GitAuthorName,
		"-c", "user.email="+z.GitAuthorEmail,
		"commit", "-q", "-m", message, "--", z.Path,
	)
	if out, err := commit.CombinedOutput(); err != nil {
		return fmt.Errorf("git commit failed: %w: %s", err, strings.TrimSpace(string(out)))
	}
	return nil
}