// Package backup makes, lists and restores backups of playlink's database
// (settings, library pairs, scan results and history, not music files).
//
// A backup is a zip holding a consistent snapshot of the database (SQLite
// "VACUUM INTO", safe while the app runs) and a small backup.json. Restoring
// stages the database next to the live one; it is swapped in at the next
// start (see ApplyStaged), and the replaced database is kept.
package backup

import (
	"archive/zip"
	"context"
	"database/sql"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/cdnutter/playlink/internal/store"
)

const (
	TypeManual    = "manual"
	TypeScheduled = "scheduled"
	TypeAction    = "action" // taken automatically before an action in live mode

	dbName      = "playlink.db"
	stagedName  = "playlink.db.restore"
	metaName    = "backup.json"
	keepActions = 10
)

var nameRe = regexp.MustCompile(`^playlink_backup_(manual|scheduled|action)_\d{4}\.\d{2}\.\d{2}_\d{2}\.\d{2}\.\d{2}(_\d+)?\.zip$`)

var ErrBadName = errors.New("not a Playlink backup file name")

// Settings stored in the settings table.
const (
	KeyFolder        = "backup_folder"
	KeyIntervalDays  = "backup_interval_days"
	KeyRetentionDays = "backup_retention_days"
	KeyBeforeActions = "backup_before_actions"
)

type Manager struct {
	st      *store.Store
	dataDir string
	version string
	log     *slog.Logger
	mu      sync.Mutex // one backup or restore at a time
}

func New(st *store.Store, dataDir, version string, log *slog.Logger) *Manager {
	return &Manager{st: st, dataDir: dataDir, version: version, log: log}
}

// Config is the user-facing backup configuration.
type Config struct {
	Folder        string `json:"folder"`
	IntervalDays  int    `json:"interval_days"`  // 0 = scheduled backups off
	RetentionDays int    `json:"retention_days"` // scheduled backups older than this are removed
	BeforeActions bool   `json:"before_actions"`
}

func (m *Manager) Config(ctx context.Context) Config {
	c := Config{
		Folder:        m.st.Setting(ctx, KeyFolder, filepath.Join(m.dataDir, "backups")),
		BeforeActions: m.st.Setting(ctx, KeyBeforeActions, "0") == "1",
	}
	c.IntervalDays, _ = strconv.Atoi(m.st.Setting(ctx, KeyIntervalDays, "7"))
	c.RetentionDays, _ = strconv.Atoi(m.st.Setting(ctx, KeyRetentionDays, "28"))
	return c
}

func (m *Manager) SetConfig(ctx context.Context, c Config) error {
	c.Folder = filepath.Clean(strings.TrimSpace(c.Folder))
	if !filepath.IsAbs(c.Folder) {
		return errors.New("the backup folder must be an absolute path")
	}
	if c.IntervalDays < 0 || c.IntervalDays > 365 || c.RetentionDays < 1 || c.RetentionDays > 3650 {
		return errors.New("interval must be 0-365 days and retention 1-3650 days")
	}
	if err := os.MkdirAll(c.Folder, 0o750); err != nil {
		return fmt.Errorf("backup folder: %w", err)
	}
	probe := filepath.Join(c.Folder, ".playlink-write-test")
	if err := os.WriteFile(probe, nil, 0o600); err != nil {
		return fmt.Errorf("backup folder is not writable: %w", err)
	}
	_ = os.Remove(probe)
	before := "0"
	if c.BeforeActions {
		before = "1"
	}
	for k, v := range map[string]string{KeyFolder: c.Folder, KeyIntervalDays: strconv.Itoa(c.IntervalDays),
		KeyRetentionDays: strconv.Itoa(c.RetentionDays), KeyBeforeActions: before} {
		if err := m.st.SetSetting(ctx, k, v); err != nil {
			return err
		}
	}
	return nil
}

// Backup is one backup file.
type Backup struct {
	Name    string `json:"name"`
	Type    string `json:"type"`
	Size    int64  `json:"size"`
	Created int64  `json:"created"`
}

type meta struct {
	App     string `json:"app"`
	Version string `json:"version"`
	Schema  int    `json:"schema"`
	Type    string `json:"type"`
	Reason  string `json:"reason,omitempty"`
	Created int64  `json:"created"`
}

// Create writes a new backup and applies retention.
func (m *Manager) Create(ctx context.Context, typ, reason string) (Backup, error) {
	m.mu.Lock()
	defer m.mu.Unlock()
	cfg := m.Config(ctx)
	if err := os.MkdirAll(cfg.Folder, 0o750); err != nil {
		return Backup{}, err
	}
	now := time.Now()
	name := fmt.Sprintf("playlink_backup_%s_%s.zip", typ, now.Format("2006.01.02_15.04.05"))
	for i := 2; fileExists(filepath.Join(cfg.Folder, name)); i++ {
		name = fmt.Sprintf("playlink_backup_%s_%s_%d.zip", typ, now.Format("2006.01.02_15.04.05"), i)
	}
	snap := filepath.Join(cfg.Folder, ".snapshot-"+strconv.FormatInt(now.UnixNano(), 10)+".db")
	defer os.Remove(snap)
	if _, err := m.st.DB.ExecContext(ctx, `VACUUM INTO ?`, snap); err != nil {
		return Backup{}, fmt.Errorf("snapshot: %w", err)
	}
	var schema int
	_ = m.st.DB.QueryRowContext(ctx, `SELECT COALESCE(MAX(v), 0) FROM schema_version`).Scan(&schema)
	tmp := filepath.Join(cfg.Folder, "."+name+".tmp")
	if err := writeZip(tmp, snap, meta{App: "playlink", Version: m.version, Schema: schema, Type: typ, Reason: reason, Created: now.Unix()}); err != nil {
		_ = os.Remove(tmp)
		return Backup{}, err
	}
	final := filepath.Join(cfg.Folder, name)
	if err := os.Rename(tmp, final); err != nil {
		_ = os.Remove(tmp)
		return Backup{}, err
	}
	fi, _ := os.Stat(final)
	m.log.Info("backup created", "name", name, "reason", reason)
	m.prune(cfg)
	return Backup{Name: name, Type: typ, Size: fi.Size(), Created: now.Unix()}, nil
}

func writeZip(path, dbFile string, md meta) error {
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o640)
	if err != nil {
		return err
	}
	zw := zip.NewWriter(f)
	fail := func(e error) error { zw.Close(); f.Close(); return e }
	w, err := zw.Create(dbName)
	if err != nil {
		return fail(err)
	}
	src, err := os.Open(dbFile)
	if err != nil {
		return fail(err)
	}
	_, err = io.Copy(w, src)
	src.Close()
	if err != nil {
		return fail(err)
	}
	w, err = zw.Create(metaName)
	if err != nil {
		return fail(err)
	}
	if err := json.NewEncoder(w).Encode(md); err != nil {
		return fail(err)
	}
	if err := zw.Close(); err != nil {
		f.Close()
		return err
	}
	if err := f.Sync(); err != nil {
		f.Close()
		return err
	}
	return f.Close()
}

func fileExists(p string) bool { _, err := os.Stat(p); return err == nil }

// List returns backups, newest first.
func (m *Manager) List(ctx context.Context) ([]Backup, error) {
	cfg := m.Config(ctx)
	ents, err := os.ReadDir(cfg.Folder)
	if errors.Is(err, os.ErrNotExist) {
		return []Backup{}, nil
	}
	if err != nil {
		return nil, err
	}
	out := []Backup{}
	for _, en := range ents {
		sub := nameRe.FindStringSubmatch(en.Name())
		if sub == nil || !en.Type().IsRegular() {
			continue
		}
		fi, err := en.Info()
		if err != nil {
			continue
		}
		out = append(out, Backup{Name: en.Name(), Type: sub[1], Size: fi.Size(), Created: createdOf(en.Name(), fi.ModTime())})
	}
	sort.Slice(out, func(i, j int) bool {
		return out[i].Created > out[j].Created || (out[i].Created == out[j].Created && out[i].Name > out[j].Name)
	})
	return out, nil
}

var stampRe = regexp.MustCompile(`\d{4}\.\d{2}\.\d{2}_\d{2}\.\d{2}\.\d{2}`)

func createdOf(name string, fallback time.Time) int64 {
	if t, err := time.ParseInLocation("2006.01.02_15.04.05", stampRe.FindString(name), time.Local); err == nil {
		return t.Unix()
	}
	return fallback.Unix()
}

// Path returns the absolute path of a backup, refusing anything that is not
// a backup file name (no path traversal).
func (m *Manager) Path(ctx context.Context, name string) (string, error) {
	if !nameRe.MatchString(name) {
		return "", ErrBadName
	}
	p := filepath.Join(m.Config(ctx).Folder, name)
	if !fileExists(p) {
		return "", os.ErrNotExist
	}
	return p, nil
}

func (m *Manager) Delete(ctx context.Context, name string) error {
	p, err := m.Path(ctx, name)
	if err != nil {
		return err
	}
	return os.Remove(p)
}

// prune removes scheduled backups older than the retention period and all
// but the newest action backups. Manual backups are kept until deleted.
func (m *Manager) prune(cfg Config) {
	list, err := m.List(context.Background())
	if err != nil {
		return
	}
	cutoff := time.Now().Add(-time.Duration(cfg.RetentionDays) * 24 * time.Hour).Unix()
	actions := 0
	for _, b := range list { // newest first
		remove := false
		switch b.Type {
		case TypeScheduled:
			remove = b.Created < cutoff
		case TypeAction:
			actions++
			remove = actions > keepActions
		}
		if remove {
			if err := os.Remove(filepath.Join(cfg.Folder, b.Name)); err == nil {
				m.log.Info("old backup removed", "name", b.Name)
			}
		}
	}
}

// Due makes a scheduled backup when the interval has passed.
func (m *Manager) Due(ctx context.Context) {
	cfg := m.Config(ctx)
	if cfg.IntervalDays <= 0 {
		return
	}
	list, err := m.List(ctx)
	if err != nil {
		return
	}
	for _, b := range list {
		if b.Type == TypeScheduled {
			if time.Since(time.Unix(b.Created, 0)) < time.Duration(cfg.IntervalDays)*24*time.Hour {
				return
			}
			break
		}
	}
	if _, err := m.Create(ctx, TypeScheduled, "schedule"); err != nil {
		m.log.Warn("scheduled backup failed", "err", err)
	}
}

// BeforeAction makes an action backup when that option is on. An error means
// the action must not run.
func (m *Manager) BeforeAction(ctx context.Context, reason string) error {
	if !m.Config(ctx).BeforeActions {
		return nil
	}
	if _, err := m.Create(ctx, TypeAction, reason); err != nil {
		return fmt.Errorf("the backup before this action failed, so the action was not started: %w", err)
	}
	return nil
}

// ---- restore ----

// Stage validates a backup zip and places its database next to the live one,
// to be swapped in at the next start.
func (m *Manager) Stage(ctx context.Context, zipPath string, maxSchema int) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	zr, err := zip.OpenReader(zipPath)
	if err != nil {
		return fmt.Errorf("not a valid backup zip: %w", err)
	}
	defer zr.Close()
	var dbf *zip.File
	for _, f := range zr.File {
		if f.Name == dbName {
			dbf = f
		}
	}
	if dbf == nil {
		return errors.New("the zip does not contain a Playlink database")
	}
	tmp := filepath.Join(m.dataDir, stagedName+".tmp")
	_ = os.Remove(tmp)
	defer os.Remove(tmp)
	if err := extract(dbf, tmp); err != nil {
		return err
	}
	if err := validate(ctx, tmp, maxSchema); err != nil {
		return err
	}
	if err := os.Rename(tmp, filepath.Join(m.dataDir, stagedName)); err != nil {
		return err
	}
	m.log.Warn("backup staged for restore; restarting to apply it", "from", filepath.Base(zipPath))
	return nil
}

func extract(f *zip.File, dst string) error {
	rc, err := f.Open()
	if err != nil {
		return err
	}
	defer rc.Close()
	out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o640)
	if err != nil {
		return err
	}
	// Cap the size so a hostile zip cannot fill the disk.
	if _, err := io.Copy(out, io.LimitReader(rc, 20<<30)); err != nil {
		out.Close()
		return err
	}
	if err := out.Sync(); err != nil {
		out.Close()
		return err
	}
	return out.Close()
}

// validate opens a database file read-only and checks it is an intact
// playlink database this version can use.
func validate(ctx context.Context, path string, maxSchema int) error {
	db, err := sql.Open("sqlite", "file:"+path+"?mode=ro")
	if err != nil {
		return err
	}
	defer db.Close()
	var ok string
	if err := db.QueryRowContext(ctx, `PRAGMA integrity_check`).Scan(&ok); err != nil || ok != "ok" {
		return fmt.Errorf("the backup database is damaged (%v %s)", err, ok)
	}
	var schema int
	if err := db.QueryRowContext(ctx, `SELECT COALESCE(MAX(v), 0) FROM schema_version`).Scan(&schema); err != nil {
		return errors.New("the backup is not a Playlink database")
	}
	if schema < 1 || schema > maxSchema {
		return fmt.Errorf("the backup is from a newer version of Playlink (schema %d, this version supports up to %d)", schema, maxSchema)
	}
	var n int
	if err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM sqlite_master WHERE type = 'table' AND name IN ('pairs', 'users', 'operations')`).Scan(&n); err != nil || n != 3 {
		return errors.New("the backup is not a Playlink database")
	}
	return nil
}

// ApplyStaged swaps a staged restore in before the database is opened. The
// database it replaces is kept as playlink.db.pre-restore-<time>.
func ApplyStaged(dataDir string, log *slog.Logger) error {
	staged := filepath.Join(dataDir, stagedName)
	if !fileExists(staged) {
		return nil
	}
	live := filepath.Join(dataDir, dbName)
	keep := live + ".pre-restore-" + time.Now().Format("2006.01.02_15.04.05")
	if fileExists(live) {
		if err := os.Rename(live, keep); err != nil {
			return err
		}
		for _, sfx := range []string{"-wal", "-shm"} {
			if fileExists(live + sfx) {
				_ = os.Rename(live+sfx, keep+sfx)
			}
		}
	}
	if err := os.Rename(staged, live); err != nil {
		return err
	}
	log.Warn("database restored from backup", "previous database kept as", filepath.Base(keep))
	return nil
}