// Package backup creates and restores SQLite database backups.
//
// Backups use SQLite's VACUUM INTO, which writes a transactionally consistent
// copy of the database while it is being written to. Copying the .db file with
// the filesystem would capture a torn snapshot whose committed data lives in a
// write-ahead log the copy does not include.
package backup

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"

	"github.com/owen/vibedns/internal/database"
)

// pendingSuffix marks a restore staged for the next start.
const pendingSuffix = ".restore-pending"

// Info describes one backup file.
type Info struct {
	Name      string    `json:"name"`
	Path      string    `json:"path"`
	SizeBytes int64     `json:"size_bytes"`
	CreatedAt time.Time `json:"created_at"`
}

// SizeMB renders the size for the UI.
func (i Info) SizeMB() float64 { return float64(i.SizeBytes) / (1024 * 1024) }

// Manager runs manual and scheduled backups.
type Manager struct {
	db  *database.DB
	log *slog.Logger

	mu        sync.RWMutex
	enabled   bool
	dir       string
	interval  time.Duration
	retention int

	running   sync.Mutex // serialises backup runs
	lastRun   time.Time
	lastError string
	wg        sync.WaitGroup
	once      sync.Once
}

// Config controls the backup schedule.
type Config struct {
	Enabled       bool
	Directory     string
	IntervalHours int
	Retention     int
}

// New creates a backup manager.
func New(db *database.DB, log *slog.Logger, cfg Config) *Manager {
	m := &Manager{db: db, log: log}
	m.SetConfig(cfg)
	return m
}

// SetConfig replaces the backup configuration.
func (m *Manager) SetConfig(cfg Config) {
	if cfg.IntervalHours < 1 {
		cfg.IntervalHours = 24
	}
	if cfg.Retention < 1 {
		cfg.Retention = 7
	}
	m.mu.Lock()
	m.enabled = cfg.Enabled
	m.dir = strings.TrimSpace(cfg.Directory)
	m.interval = time.Duration(cfg.IntervalHours) * time.Hour
	m.retention = cfg.Retention
	m.mu.Unlock()
}

// Directory returns the configured backup directory.
func (m *Manager) Directory() string {
	m.mu.RLock()
	defer m.mu.RUnlock()
	return m.dir
}

// Run creates a backup now and prunes old ones.
func (m *Manager) Run(ctx context.Context) (Info, error) {
	m.running.Lock()
	defer m.running.Unlock()

	m.mu.RLock()
	dir, retention := m.dir, m.retention
	m.mu.RUnlock()

	if dir == "" {
		return Info{}, errors.New("no backup directory is configured")
	}
	if err := os.MkdirAll(dir, 0o750); err != nil {
		return Info{}, fmt.Errorf("create backup directory %s: %w", dir, err)
	}

	name := fmt.Sprintf("vibedns-%s.db", time.Now().UTC().Format("20060102-150405"))
	path := filepath.Join(dir, name)

	// VACUUM INTO fails if the target exists, which is exactly the behaviour we
	// want: a backup must never silently overwrite another.
	if _, err := os.Stat(path); err == nil {
		return Info{}, fmt.Errorf("a backup named %s already exists", name)
	}

	// Checkpointing first keeps the WAL small and the copy quick.
	if err := m.db.Checkpoint(ctx); err != nil {
		m.log.Warn("could not checkpoint the write-ahead log before backup", "error", err)
	}

	if _, err := m.db.ExecContext(ctx, `VACUUM INTO ?`, path); err != nil {
		m.recordError(err)
		return Info{}, fmt.Errorf("write backup to %s: %w", path, err)
	}
	if err := os.Chmod(path, 0o600); err != nil {
		m.log.Warn("could not restrict backup file permissions", "path", path, "error", err)
	}

	fi, err := os.Stat(path)
	if err != nil {
		m.recordError(err)
		return Info{}, fmt.Errorf("verify backup %s: %w", path, err)
	}

	m.mu.Lock()
	m.lastRun = time.Now()
	m.lastError = ""
	m.mu.Unlock()

	info := Info{Name: name, Path: path, SizeBytes: fi.Size(), CreatedAt: fi.ModTime()}
	m.log.Info("database backup created", "path", path, "bytes", info.SizeBytes)

	if removed, err := Prune(dir, retention); err != nil {
		m.log.Warn("could not prune old backups", "error", err)
	} else if removed > 0 {
		m.log.Info("pruned old backups", "removed", removed, "retention", retention)
	}
	return info, nil
}

func (m *Manager) recordError(err error) {
	m.mu.Lock()
	m.lastError = err.Error()
	m.mu.Unlock()
}

// List returns the backups in the configured directory, newest first.
func (m *Manager) List() ([]Info, error) {
	return List(m.Directory())
}

// List returns the backups in dir, newest first.
func List(dir string) ([]Info, error) {
	if dir == "" {
		return nil, nil
	}
	entries, err := os.ReadDir(dir)
	if errors.Is(err, os.ErrNotExist) {
		return nil, nil
	}
	if err != nil {
		return nil, fmt.Errorf("read backup directory %s: %w", dir, err)
	}
	var out []Info
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".db") {
			continue
		}
		fi, err := e.Info()
		if err != nil {
			continue
		}
		out = append(out, Info{
			Name:      e.Name(),
			Path:      filepath.Join(dir, e.Name()),
			SizeBytes: fi.Size(),
			CreatedAt: fi.ModTime(),
		})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].CreatedAt.After(out[j].CreatedAt) })
	return out, nil
}

// Prune deletes all but the newest `keep` backups.
func Prune(dir string, keep int) (int, error) {
	if keep < 1 {
		return 0, nil
	}
	backups, err := List(dir)
	if err != nil {
		return 0, err
	}
	if len(backups) <= keep {
		return 0, nil
	}
	removed := 0
	for _, b := range backups[keep:] {
		if err := os.Remove(b.Path); err != nil {
			return removed, fmt.Errorf("remove old backup %s: %w", b.Name, err)
		}
		removed++
	}
	return removed, nil
}

// Resolve validates that name refers to a backup inside dir and returns its
// full path. It exists to keep a crafted name from escaping the directory.
func Resolve(dir, name string) (string, error) {
	if dir == "" {
		return "", errors.New("no backup directory is configured")
	}
	clean := filepath.Base(filepath.Clean("/" + name))
	if clean == "." || clean == "/" || clean == "" {
		return "", fmt.Errorf("%q is not a valid backup name", name)
	}
	if !strings.HasSuffix(clean, ".db") {
		return "", fmt.Errorf("%q is not a backup file", name)
	}
	path := filepath.Join(dir, clean)
	if _, err := os.Stat(path); err != nil {
		return "", fmt.Errorf("backup %s was not found", clean)
	}
	return path, nil
}

// Delete removes one backup by name.
func Delete(dir, name string) error {
	path, err := Resolve(dir, name)
	if err != nil {
		return err
	}
	if err := os.Remove(path); err != nil {
		return fmt.Errorf("delete backup %s: %w", name, err)
	}
	return nil
}

// Start launches the scheduled backup loop.
func (m *Manager) Start(ctx context.Context) {
	m.once.Do(func() {
		m.wg.Add(1)
		go m.loop(ctx)
	})
}

// Stop waits for the scheduler to exit.
func (m *Manager) Stop() { m.wg.Wait() }

func (m *Manager) loop(ctx context.Context) {
	defer m.wg.Done()
	// Check every few minutes rather than sleeping for the whole interval, so
	// a settings change takes effect promptly.
	const tick = 5 * time.Minute
	t := time.NewTicker(tick)
	defer t.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			m.mu.RLock()
			enabled, interval, last := m.enabled, m.interval, m.lastRun
			m.mu.RUnlock()
			if !enabled {
				continue
			}
			if !last.IsZero() && time.Since(last) < interval {
				continue
			}
			if _, err := m.Run(ctx); err != nil {
				m.log.Error("scheduled backup failed", "error", err)
			}
		}
	}
}

// Status describes the backup subsystem for the settings page.
type Status struct {
	Enabled       bool      `json:"enabled"`
	Directory     string    `json:"directory"`
	IntervalHours int       `json:"interval_hours"`
	Retention     int       `json:"retention"`
	LastRun       time.Time `json:"last_run"`
	LastError     string    `json:"last_error,omitempty"`
	Count         int       `json:"count"`
	TotalBytes    int64     `json:"total_bytes"`
}

// Status returns the current backup status.
func (m *Manager) Status() Status {
	m.mu.RLock()
	s := Status{
		Enabled:       m.enabled,
		Directory:     m.dir,
		IntervalHours: int(m.interval / time.Hour),
		Retention:     m.retention,
		LastRun:       m.lastRun,
		LastError:     m.lastError,
	}
	m.mu.RUnlock()

	if backups, err := List(s.Directory); err == nil {
		s.Count = len(backups)
		for _, b := range backups {
			s.TotalBytes += b.SizeBytes
		}
	}
	return s
}

// --- Restore ------------------------------------------------------------

// StageRestore validates a backup and stages it to replace the live database
// on the next start.
//
// Overwriting the database file underneath a running process would leave open
// connections reading a file that no longer exists, so the swap is deferred to
// startup, where nothing is holding the database open.
func StageRestore(dbPath, backupPath string) error {
	if err := Verify(backupPath); err != nil {
		return err
	}
	pending := dbPath + pendingSuffix
	if err := copyFile(backupPath, pending, 0o600); err != nil {
		return fmt.Errorf("stage restore: %w", err)
	}
	return nil
}

// PendingRestore reports whether a restore is staged.
func PendingRestore(dbPath string) (string, bool) {
	p := dbPath + pendingSuffix
	if _, err := os.Stat(p); err == nil {
		return p, true
	}
	return "", false
}

// CancelRestore discards a staged restore.
func CancelRestore(dbPath string) error {
	p := dbPath + pendingSuffix
	if err := os.Remove(p); err != nil && !errors.Is(err, os.ErrNotExist) {
		return fmt.Errorf("cancel staged restore: %w", err)
	}
	return nil
}

// ApplyPendingRestore swaps a staged backup into place. It must be called
// before the database is opened.
//
// The database being replaced is preserved alongside it, so a restore that
// turns out to be the wrong choice is still recoverable.
func ApplyPendingRestore(dbPath string, log *slog.Logger) (bool, error) {
	pending := dbPath + pendingSuffix
	if _, err := os.Stat(pending); err != nil {
		return false, nil
	}

	if _, err := os.Stat(dbPath); err == nil {
		safety := fmt.Sprintf("%s.pre-restore-%s", dbPath, time.Now().UTC().Format("20060102-150405"))
		if err := os.Rename(dbPath, safety); err != nil {
			return false, fmt.Errorf("preserve the current database before restoring: %w", err)
		}
		log.Info("previous database preserved", "path", safety)
	}
	// The WAL and shared-memory sidecars belong to the replaced database and
	// would corrupt the restored one.
	for _, suffix := range []string{"-wal", "-shm"} {
		if err := os.Remove(dbPath + suffix); err != nil && !errors.Is(err, os.ErrNotExist) {
			return false, fmt.Errorf("remove stale %s file: %w", suffix, err)
		}
	}
	if err := os.Rename(pending, dbPath); err != nil {
		return false, fmt.Errorf("move the staged database into place: %w", err)
	}
	if err := os.Chmod(dbPath, 0o600); err != nil {
		log.Warn("could not restrict restored database permissions", "error", err)
	}
	log.Info("database restored from backup", "path", dbPath)
	return true, nil
}

// Verify checks that a file is a usable vibedns database.
func Verify(path string) error {
	fi, err := os.Stat(path)
	if err != nil {
		return fmt.Errorf("backup file %s cannot be read: %w", filepath.Base(path), err)
	}
	if fi.Size() < 512 {
		return fmt.Errorf("backup file %s is too small to be a database", filepath.Base(path))
	}

	db, err := sql.Open("sqlite", "file:"+path+"?mode=ro&_pragma=query_only(1)")
	if err != nil {
		return fmt.Errorf("backup file %s could not be opened: %w", filepath.Base(path), err)
	}
	defer db.Close()

	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	defer cancel()

	var check string
	if err := db.QueryRowContext(ctx, `PRAGMA integrity_check`).Scan(&check); err != nil {
		return fmt.Errorf("backup file %s failed its integrity check: %w", filepath.Base(path), err)
	}
	if check != "ok" {
		return fmt.Errorf("backup file %s failed its integrity check: %s", filepath.Base(path), check)
	}

	var n int
	err = db.QueryRowContext(ctx,
		`SELECT COUNT(*) FROM sqlite_master WHERE type = 'table' AND name = 'schema_migrations'`).Scan(&n)
	if err != nil || n == 0 {
		return fmt.Errorf("%s does not look like a vibedns database: no migration table was found",
			filepath.Base(path))
	}
	return nil
}

func copyFile(src, dst string, mode os.FileMode) error {
	in, err := os.Open(src)
	if err != nil {
		return err
	}
	defer in.Close()

	tmp := dst + ".tmp"
	out, err := os.OpenFile(tmp, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, mode)
	if err != nil {
		return err
	}
	if _, err := io.Copy(out, in); err != nil {
		out.Close()
		os.Remove(tmp)
		return err
	}
	if err := out.Sync(); err != nil {
		out.Close()
		os.Remove(tmp)
		return err
	}
	if err := out.Close(); err != nil {
		os.Remove(tmp)
		return err
	}
	return os.Rename(tmp, dst)
}