save and load play queue on exit/startup (partial impl)

backend/savedplayqueue.go

@@ -0,0 +1,83 @@
+package backend
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+
+	"github.com/dweymouth/supersonic/backend/mediaprovider"
+)
+
+type SavedPlayQueue struct {
+	Tracks     []*mediaprovider.Track
+	TrackIndex int
+	TimePos    float64
+}
+
+type serializedSavedPlayQueue struct {
+	ServerID   string   `json:"serverID"`
+	TrackIDs   []string `json:"trackIDs"`
+	TrackIndex int      `json:"trackIndex"`
+	TimePos    float64  `json:"timePos"`
+}
+
+// SavePlayQueue saves the current play queue and playback position to a JSON file.
+func SavePlayQueue(serverID string, pm *PlaybackManager, filepath string) error {
+	queue := pm.GetPlayQueue()
+	stats := pm.PlayerStatus()
+	trackIdx := pm.NowPlayingIndex()
+
+	trackIDs := make([]string, len(queue))
+	for i, tr := range queue {
+		trackIDs[i] = tr.ID
+	}
+
+	saved := serializedSavedPlayQueue{
+		ServerID:   serverID,
+		TrackIDs:   trackIDs,
+		TrackIndex: trackIdx,
+		TimePos:    stats.TimePos,
+	}
+	b, err := json.Marshal(saved)
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(filepath, b, 0644)
+}
+
+// Loads the saved play queue from the given filepath using the current server.
+// Returns an error if the queue could not be loaded for any reason, including the
+// currently logged in server being different than the server from which the queue was saved.
+func LoadPlayQueue(filepath string, sm *ServerManager) (*SavedPlayQueue, error) {
+	b, err := os.ReadFile(filepath)
+	if err != nil {
+		return nil, err
+	}
+
+	var savedData serializedSavedPlayQueue
+	if err := json.Unmarshal(b, &savedData); err != nil {
+		return nil, err
+	}
+
+	if sm.ServerID.String() != savedData.ServerID {
+		return nil, errors.New("saved play queue was from a different server")
+	}
+
+	tracks := make([]*mediaprovider.Track, len(savedData.TrackIDs))
+	mp := sm.Server
+	for i, id := range savedData.TrackIDs {
+		if tr, err := mp.GetTrack(id); err != nil {
+			return nil, err
+		} else {
+			tracks[i] = tr
+		}
+	}
+
+	savedQueue := &SavedPlayQueue{
+		Tracks:     tracks,
+		TrackIndex: savedData.TrackIndex,
+		TimePos:    savedData.TimePos,
+	}
+	return savedQueue, nil
+}