pkg/tarsum: use strings.Cut() and minor refactor

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/tarsum/tarsum.go

@@ -62,13 +62,11 @@ func NewTarSumHash(r io.Reader, dc bool, v Version, tHash THash) (TarSum, error)
 
 // NewTarSumForLabel creates a new TarSum using the provided TarSum version+hash label.
 func NewTarSumForLabel(r io.Reader, disableCompression bool, label string) (TarSum, error) {
-	parts := strings.SplitN(label, "+", 2)
-	if len(parts) != 2 {
+	versionName, hashName, ok := strings.Cut(label, "+")
+	if !ok {
 		return nil, errors.New("tarsum label string should be of the form: {tarsum_version}+{hash_name}")
 	}
 
-	versionName, hashName := parts[0], parts[1]
-
 	version, ok := tarSumVersionsByName[versionName]
 	if !ok {
 		return nil, fmt.Errorf("unknown TarSum version name: %q", versionName)

pkg/tarsum/versioning.go

@@ -69,16 +69,12 @@ func (tsv Version) String() string {
 
 // GetVersionFromTarsum returns the Version from the provided string.
 func GetVersionFromTarsum(tarsum string) (Version, error) {
-	tsv := tarsum
-	if strings.Contains(tarsum, "+") {
-		tsv = strings.SplitN(tarsum, "+", 2)[0]
-	}
-	for v, s := range tarSumVersions {
-		if s == tsv {
-			return v, nil
-		}
+	versionName, _, _ := strings.Cut(tarsum, "+")
+	version, ok := tarSumVersionsByName[versionName]
+	if !ok {
+		return -1, ErrNotVersion
 	}
-	return -1, ErrNotVersion
+	return version, nil
 }
 
 // Errors that may be returned by functions in this package