added "default playlist"/ favorites system

backend/favorites/config.go

@@ -0,0 +1,61 @@
+// Package favorites manages the default playlist configuration.
+package favorites
+
+import (
+	"errors"
+	"fmt"
+)
+
+var errUnknownIconStyle = errors.New(
+	"unknown favorites icon style",
+)
+
+// IconStyle controls the icon used to indicate favourited tracks.
+type IconStyle string
+
+// Valid IconStyle values.
+const (
+	// IconHeart uses a heart icon.
+	IconHeart IconStyle = "heart"
+
+	// IconStar uses a star icon.
+	IconStar IconStyle = "star"
+)
+
+// DefaultIconStyle is applied when no value has been set.
+const DefaultIconStyle = IconHeart
+
+// DefaultPlaylistName is the name given to the auto-created
+// default playlist.
+const DefaultPlaylistName = "Favorites"
+
+// Config holds favourites preferences.
+type Config struct {
+	PlaylistID int64     `toml:"PlaylistID"`
+	IconStyle  IconStyle `toml:"IconStyle"`
+}
+
+// ApplyDefaults fills zero-value fields with sensible defaults.
+func (c *Config) ApplyDefaults() {
+	if c.IconStyle == "" {
+		c.IconStyle = DefaultIconStyle
+	}
+}
+
+// Validate checks that all values are well-formed.
+func (c *Config) Validate() error {
+	c.ApplyDefaults()
+
+	switch c.IconStyle {
+	case IconHeart, IconStar:
+		// Valid.
+	default:
+		return fmt.Errorf(
+			"%w: %q",
+			errUnknownIconStyle,
+			c.IconStyle,
+		)
+	}
+
+	return nil
+}