move the id type into spec

server/ctrlsubsonic/specid/ids.go

@@ -0,0 +1,59 @@
+package specid
+
+// this package is at such a high level in the hierarchy because
+// it's used by both `server/db` (for now) and `server/ctrlsubsonic`
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+var (
+	ErrBadSeparator = errors.New("bad separator")
+	ErrNotAnInt     = errors.New("not an int")
+	ErrBadPrefix    = errors.New("bad prefix")
+)
+
+type IDT string
+
+const (
+	Artist    IDT = "ar"
+	Album     IDT = "al"
+	Track     IDT = "tr"
+	separator     = "-"
+)
+
+type ID struct {
+	Type  IDT
+	Value int
+}
+
+func New(in string) (ID, error) {
+	parts := strings.Split(in, separator)
+	if len(parts) != 2 {
+		return ID{}, ErrBadSeparator
+	}
+	partType := parts[0]
+	partValue := parts[1]
+	val, err := strconv.Atoi(partValue)
+	if err != nil {
+		return ID{}, fmt.Errorf("%q: %w", partValue, ErrNotAnInt)
+	}
+	for _, acc := range []IDT{Artist, Album, Track} {
+		if partType == string(acc) {
+			return ID{Type: acc, Value: val}, nil
+		}
+	}
+	return ID{}, fmt.Errorf("%q: %w", partType, ErrBadPrefix)
+}
+
+func (i ID) String() string {
+	return fmt.Sprintf("%s%s%d", i.Type, separator, i.Value)
+}
+
+func (i ID) MarshalJSON() ([]byte, error) {
+	return json.Marshal(i.String())
+}