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|
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
}
|
