add TrackDAO::getTrackDurationsTotal(QList<TrackId>)

src/library/dao/trackdao.cpp

@@ -163,6 +163,38 @@ TrackId TrackDAO::getTrackIdByLocation(const QString& location) const {
     return trackId;
 }
 
+double TrackDAO::getTrackDurationsTotal(const QList<TrackId>& trackIds) const {
+    double duration = 0.0;
+    if (trackIds.isEmpty()) {
+        return duration;
+    }
+
+    // Create a deduplicated joint string for the query
+    const QString idListJoined = joinTrackIdList(QSet<TrackId>::fromList(trackIds));
+
+    FwdSqlQuery query(m_database, QString("SELECT id, duration "
+                                          "FROM library "
+                                          "WHERE library.id in (%1)")
+                                          .arg(idListJoined));
+    if (query.hasError() || !query.execPrepared()) {
+        return duration;
+    }
+
+    QHash<TrackId, double> idDurationMap;
+    const int idIndex = query.fieldIndex("id");
+    const int durationIndex = query.fieldIndex("duration");
+    while (query.next()) {
+        idDurationMap.insert(TrackId(query.fieldValue(idIndex)),
+                query.fieldValue(durationIndex).toDouble());
+    }
+
+    for (const TrackId id : trackIds) {
+        duration += idDurationMap.value(id);
+    }
+
+    return duration;
+}
+
 QList<TrackId> TrackDAO::resolveTrackIds(
         const QList<mixxx::FileInfo>& fileInfos,
         ResolveTrackIdFlags flags) {

src/library/dao/trackdao.h

@@ -63,6 +63,10 @@ class TrackDAO : public QObject, public virtual DAO, public virtual GlobalTrackC
     QSet<QString> getAllTrackLocations() const;
     QString getTrackLocation(TrackId trackId) const;
 
+    // Returns the total of all track's durations.
+    // Considers duplicates so it can be used for playlists.
+    double getTrackDurationsTotal(const QList<TrackId>& trackIds) const;
+
     // Only used by friend class LibraryScanner, but public for testing!
     bool detectMovedTracks(
             QList<RelocatedTrack>* pRelocatedTracks,