package cache import ( "bufio" "encoding/gob" "errors" "fmt" "os" "github.com/Necoro/feed2imap-go/internal/log" ) const currentVersion byte = 1 type Cache interface { Version() byte transformToCurrent() (Cache, error) } type feedId struct { Name string Url string } type v1Cache struct { version byte Ids map[feedId]uint64 NextId uint64 } func (cache *v1Cache) Version() byte { return cache.version } func New() Cache { cache := v1Cache{Ids: map[feedId]uint64{}} cache.version = currentVersion return &cache } func cacheForVersion(version byte) (Cache, error) { switch version { case 1: return New(), nil default: return nil, fmt.Errorf("unknown cache version '%d'", version) } } func (cache *v1Cache) transformToCurrent() (Cache, error) { return cache, nil } func Store(fileName string, cache Cache) error { if cache == nil { return fmt.Errorf("trying to store nil cache") } if cache.Version() != currentVersion { return fmt.Errorf("trying to store cache with unsupported version '%d' (current: '%d')", cache.Version(), currentVersion) } f, err := os.Create(fileName) if err != nil { return fmt.Errorf("trying to store cache to '%s': %w", fileName, err) } defer f.Close() writer := bufio.NewWriter(f) if err = writer.WriteByte(currentVersion); err != nil { return fmt.Errorf("writing to '%s': %w", fileName, err) } encoder := gob.NewEncoder(writer) if err = encoder.Encode(cache); err != nil { return fmt.Errorf("encoding cache: %w", err) } writer.Flush() log.Printf("Stored cache to '%s'.", fileName) return nil } func Read(fileName string) (Cache, error) { f, err := os.Open(fileName) if err != nil { if errors.Is(err, os.ErrNotExist) { // no cache there yet -- make new return New(), nil } return nil, fmt.Errorf("opening cache at '%s': %w", fileName, err) } defer f.Close() log.Printf("Loading cache from '%s'", fileName) reader := bufio.NewReader(f) version, err := reader.ReadByte() if err != nil { return nil, fmt.Errorf("reading from '%s': %w", fileName, err) } cache, err := cacheForVersion(version) if err != nil { return nil, err } decoder := gob.NewDecoder(reader) if err = decoder.Decode(cache); err != nil { return nil, fmt.Errorf("decoding for version '%d' from '%s': %w", version, fileName, err) } if cache, err = cache.transformToCurrent(); err != nil { return nil, fmt.Errorf("cannot transform from version %d to %d: %w", version, currentVersion, err) } log.Printf("Loaded cache (version %d), transformed to version %d.", version, currentVersion) return cache, nil }