better usage prompt + hash opti on server

cmd/server/data/data.go

@@ -9,12 +9,50 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"sync"
+)
+
+type (
+	cache map[string]cachedInfo
+
+	cachedInfo struct {
+		MD5     string
+		Version int
+	}
 )
 
 var (
 	ErrBackupNotExists error = errors.New("backup not found")
+
+	// singleton
+	hashCacheMu sync.RWMutex
+	hashCache   cache = make(map[string]cachedInfo)
 )
 
+func (c cache) Get(gameID string) (cachedInfo, bool) {
+	hashCacheMu.RLock()
+	defer hashCacheMu.RUnlock()
+
+	if v, ok := c[gameID]; ok {
+		return v, true
+	}
+	return cachedInfo{}, false
+}
+
+func (c cache) Register(gameID string, v cachedInfo) {
+	hashCacheMu.Lock()
+	defer hashCacheMu.Unlock()
+
+	c[gameID] = v
+}
+
+func (c cache) Remove(gameID string) {
+	hashCacheMu.Lock()
+	defer hashCacheMu.Unlock()
+
+	delete(c, gameID)
+}
+
 func Write(gameID, documentRoot string, r io.Reader) error {
 	dataFolderPath := filepath.Join(documentRoot, "data", gameID)
 	partPath := filepath.Join(dataFolderPath, "data.tar.gz.part")
@@ -42,6 +80,7 @@ func Write(gameID, documentRoot string, r io.Reader) error {
 		return err
 	}
 
+	hashCache.Remove(gameID)
 	return nil
 }
 
@@ -97,6 +136,7 @@ func UpdateMetadata(gameID, documentRoot string, m repository.Metadata) error {
 
 func ArchiveInfo(gameID, documentRoot, uuid string) (repository.Backup, error) {
 	dataFolderPath := filepath.Join(documentRoot, "data", gameID, "hist", uuid, "data.tar.gz")
+	cacheID := gameID + ":" + uuid
 
 	finfo, err := os.Stat(dataFolderPath)
 	if err != nil {
@@ -106,11 +146,29 @@ func ArchiveInfo(gameID, documentRoot, uuid string) (repository.Backup, error) {
 		return repository.Backup{}, err
 	}
 
+	v, err := getVersion(gameID, documentRoot)
+	if err != nil {
+		return repository.Backup{}, fmt.Errorf("failed to read game metadata: %w", err)
+	}
+
+	if m, ok := hashCache.Get(cacheID); ok {
+		return repository.Backup{
+			CreatedAt: finfo.ModTime(),
+			UUID:      uuid,
+			MD5:       m.MD5,
+		}, nil
+	}
+
 	h, err := hash.FileMD5(dataFolderPath)
 	if err != nil {
 		return repository.Backup{}, fmt.Errorf("failed to calculate file md5: %w", err)
 	}
 
+	hashCache.Register(cacheID, cachedInfo{
+		Version: v,
+		MD5:     h,
+	})
+
 	return repository.Backup{
 		CreatedAt: finfo.ModTime(),
 		UUID:      uuid,
@@ -118,6 +176,62 @@ func ArchiveInfo(gameID, documentRoot, uuid string) (repository.Backup, error) {
 	}, nil
 }
 
+func Hash(gameID, documentRoot string) (string, error) {
+	path := filepath.Clean(filepath.Join(documentRoot, "data", gameID))
+
+	sdir, err := os.Stat(path)
+	if err != nil {
+		return "", err
+	}
+
+	if !sdir.IsDir() {
+		return "", err
+	}
+
+	v, err := getVersion(gameID, documentRoot)
+	if err != nil {
+		return "", fmt.Errorf("failed to read game metadata: %w", err)
+	}
+
+	if m, ok := hashCache.Get(gameID); ok {
+		if v == m.Version {
+			return m.MD5, nil
+		}
+	}
+
+	path = filepath.Join(path, "data.tar.gz")
+
+	h, err := hash.FileMD5(path)
+	if err != nil {
+		return "", err
+	}
+
+	hashCache.Register(gameID, cachedInfo{
+		Version: v,
+		MD5:     h,
+	})
+
+	return h, nil
+}
+
+func getVersion(gameID, documentRoot string) (int, error) {
+	path := filepath.Join(documentRoot, "data", gameID, "metadata.json")
+
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0740)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	d := json.NewDecoder(f)
+	var m repository.Metadata
+	if err := d.Decode(&m); err != nil {
+		return 0, err
+	}
+
+	return m.Version, nil
+}
+
 func makeDataFolder(gameID, documentRoot string) error {
 	if err := os.MkdirAll(filepath.Join(documentRoot, "data", gameID), 0740); err != nil {
 		return err