private WorkingBeatmap addDifficultyToSet(BeatmapSetInfo targetBeatmapSet, IBeatmap newBeatmap, ISkin beatmapSkin)
{
// populate circular beatmap set info <-> beatmap info references manually.
// several places like `Save()` or `GetWorkingBeatmap()`
// rely on them being freely traversable in both directions for correct operation.
targetBeatmapSet.Beatmaps.Add(newBeatmap.BeatmapInfo);
newBeatmap.BeatmapInfo.BeatmapSet = targetBeatmapSet;
Save(newBeatmap.BeatmapInfo, newBeatmap, beatmapSkin);
workingBeatmapCache.Invalidate(targetBeatmapSet);
return(GetWorkingBeatmap(newBeatmap.BeatmapInfo));
}
void IWorkingBeatmapCache.Invalidate(BeatmapSetInfo beatmapSetInfo) => workingBeatmapCache.Invalidate(beatmapSetInfo);