// Package backup writes periodic metadata-only snapshots of the database to a
// local directory, on a schedule, so a fresh copy of the receipt metadata always
// survives even if the live DB is lost. Blobs (receipt + attachment images) are
// excluded to keep the backups small — the on-disk files and the full-blob export
// remain the source for the images themselves.
package backup

import (
	"context"
	"fmt"
	"log"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"
)

// Snapshotter writes a point-in-time DB copy. *storage.Store satisfies this.
type Snapshotter interface {
	SnapshotTo(destPath string, includeBlobs bool) error
}

const (
	prefix = "hsa_sqlite_backup_"
	suffix = ".db"
	stamp  = "2006_01_02" // yyyy_mm_dd — one backup per day
)

// Schedule runs metadata-only backups into dir, attempting one on start and then
// every `every`, keeping the most recent `keep`. It blocks until ctx is cancelled,
// so run it in a goroutine. `now` is injected for testability (pass time.Now).
//
// Backups are idempotent per day (date-only filenames): a write whose dated file
// already exists is a no-op, so frequent restarts — or intervals shorter than a
// day — never duplicate or spam, they just settle at one backup per day.
func Schedule(ctx context.Context, s Snapshotter, dir string, every time.Duration, keep int, now func() time.Time) {
	if err := os.MkdirAll(dir, 0o700); err != nil {
		log.Printf("backup: cannot create %s: %v (backups disabled)", dir, err)
		return
	}
	for {
		if err := writeBackup(s, dir, now()); err != nil {
			log.Printf("backup: %v", err)
		} else if err := prune(dir, keep); err != nil {
			log.Printf("backup prune: %v", err)
		}
		timer := time.NewTimer(every)
		select {
		case <-ctx.Done():
			timer.Stop()
			return
		case <-timer.C:
		}
	}
}

func writeBackup(s Snapshotter, dir string, now time.Time) error {
	name := prefix + now.UTC().Format(stamp) + suffix
	dest := filepath.Join(dir, name)
	if _, err := os.Stat(dest); err == nil {
		return nil // today's backup already exists (date-only name) — nothing to do
	}
	if err := s.SnapshotTo(dest, false); err != nil {
		return fmt.Errorf("write %s: %w", name, err)
	}
	log.Printf("backup: wrote %s", name)
	return nil
}

// backupFiles returns the backup filenames in chronological order (the timestamped
// names sort lexically the same as by time).
func backupFiles(dir string) []string {
	ents, err := os.ReadDir(dir)
	if err != nil {
		return nil
	}
	var names []string
	for _, e := range ents {
		if !e.IsDir() && strings.HasPrefix(e.Name(), prefix) && strings.HasSuffix(e.Name(), suffix) {
			names = append(names, e.Name())
		}
	}
	sort.Strings(names)
	return names
}

// prune deletes the oldest backups beyond the most recent `keep`.
func prune(dir string, keep int) error {
	names := backupFiles(dir)
	if keep <= 0 || len(names) <= keep {
		return nil
	}
	for _, name := range names[:len(names)-keep] {
		if err := os.Remove(filepath.Join(dir, name)); err != nil {
			return err
		}
	}
	return nil
}