Started caching

1 files changed, 120 insertions, 0 deletions

diff --git a/internal/cache/cache.go b/internal/cache/cache.go
new file mode 100644
index 0000000..979d661
--- /dev/null
+++ b/internal/cache/cache.go
@@ -0,0 +1,120 @@
+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
+}