db/simple: add option to hide CUE target songs

This reduces duplicates in the music database by hiding the original
song file when it is referenced by a CUE sheet.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1275

src/db/plugins/simple/Directory.cxx

@@ -109,21 +109,21 @@ Directory::FindChild(std::string_view name) const noexcept
 	return nullptr;
 }
 
-bool
-Directory::TargetExists(std::string_view _target) const noexcept
+Song *
+Directory::LookupTargetSong(std::string_view _target) noexcept
 {
 	StringView target{_target};
 
 	if (target.SkipPrefix("../")) {
 		if (parent == nullptr)
-			return false;
+			return nullptr;
 
-		return parent->TargetExists(target);
+		return parent->LookupTargetSong(target);
 	}
 
 	/* sorry for the const_cast ... */
-	const auto lr = const_cast<Directory *>(this)->LookupDirectory(target);
-	return lr.directory->FindSong(lr.rest) != nullptr;
+	const auto lr = LookupDirectory(target);
+	return lr.directory->FindSong(lr.rest);
 }
 
 void
@@ -229,6 +229,7 @@ Directory::Sort() noexcept
 
 void
 Directory::Walk(bool recursive, const SongFilter *filter,
+		bool hide_playlist_targets,
 		const VisitDirectory& visit_directory, const VisitSong& visit_song,
 		const VisitPlaylist& visit_playlist) const
 {
@@ -247,7 +248,10 @@ Directory::Walk(bool recursive, const SongFilter *filter,
 	}
 
 	if (visit_song) {
-		for (auto &song : songs){
+		for (auto &song : songs) {
+			if (hide_playlist_targets && song.in_playlist)
+				continue;
+
 			const auto song2 = song.Export();
 			if (filter == nullptr || filter->Match(song2))
 				visit_song(song2);
@@ -265,6 +269,7 @@ Directory::Walk(bool recursive, const SongFilter *filter,
 
 		if (recursive)
 			child.Walk(recursive, filter,
+				   hide_playlist_targets,
 				   visit_directory, visit_song,
 				   visit_playlist);
 	}

src/db/plugins/simple/Directory.hxx

@@ -214,7 +214,7 @@ public:
 	LookupResult LookupDirectory(std::string_view uri) noexcept;
 
 	[[gnu::pure]]
-	bool TargetExists(std::string_view target) const noexcept;
+	Song *LookupTargetSong(std::string_view target) noexcept;
 
 	[[gnu::pure]]
 	bool IsEmpty() const noexcept {
@@ -303,6 +303,7 @@ public:
 	 * Caller must lock #db_mutex.
 	 */
 	void Walk(bool recursive, const SongFilter *match,
+		  bool hide_playlist_targets,
 		  const VisitDirectory& visit_directory, const VisitSong& visit_song,
 		  const VisitPlaylist& visit_playlist) const;
 

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -60,6 +60,7 @@ inline SimpleDatabase::SimpleDatabase(const ConfigBlock &block)
 #ifdef ENABLE_ZLIB
 	 compress(block.GetBlockValue("compress", true)),
 #endif
+	 hide_playlist_targets(block.GetBlockValue("hide_playlist_targets", true)),
 	 cache_path(block.GetPath("cache_directory"))
 {
 	if (path.IsNull())
@@ -306,6 +307,7 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 			visit_directory(r.directory->Export());
 
 		r.directory->Walk(selection.recursive, selection.filter,
+				  hide_playlist_targets,
 				  visit_directory, visit_song,
 				  visit_playlist);
 		helper.Commit();

src/db/plugins/simple/SimpleDatabasePlugin.hxx

@@ -44,6 +44,8 @@ class SimpleDatabase : public Database {
 	bool compress;
 #endif
 
+	bool hide_playlist_targets;
+
 	/**
 	 * The path where cache files for Mount() are located.
 	 */

src/db/plugins/simple/Song.hxx

@@ -101,6 +101,12 @@ struct Song {
 	 */
 	AudioFormat audio_format = AudioFormat::Undefined();
 
+	/**
+	 * Is this song referenced by at least one playlist file that
+	 * is part of the database?
+	 */
+	bool in_playlist = false;
+
 	template<typename F>
 	Song(F &&_filename, Directory &_parent) noexcept
 		:parent(_parent), filename(std::forward<F>(_filename)) {}