db/DatabasePrint: support descending sort

doc/protocol.xml

@@ -1693,7 +1693,9 @@ OK
 
             <para>
               <varname>sort</varname> sorts the result by the
-              specified tag.  Without <varname>sort</varname>, the
+              specified tag.  The sort is descending if the tag is
+              prefixed with a minus ('-').
+              Without <varname>sort</varname>, the
               order is undefined.  Only the first tag value will be
               used, if multiple of the same type exist.  To sort by
               "Artist", "Album" or "AlbumArtist", you should specify

src/command/DatabaseCommands.cxx

@@ -68,8 +68,15 @@ handle_match(Client &client, Request args, Response &r, bool fold_case)
 		window.SetAll();
 
 	TagType sort = TAG_NUM_OF_ITEM_TYPES;
+	bool descending = false;
 	if (args.size >= 2 && StringIsEqual(args[args.size - 2], "sort")) {
-		sort = tag_name_parse_i(args.back());
+		const char *s = args.back();
+		if (*s == '-') {
+			descending = true;
+			++s;
+		}
+
+		sort = tag_name_parse_i(s);
 		if (sort == TAG_NUM_OF_ITEM_TYPES)
 			throw ProtocolError(ACK_ERROR_ARG, "Unknown sort tag");
 
@@ -87,7 +94,7 @@ handle_match(Client &client, Request args, Response &r, bool fold_case)
 
 	db_selection_print(r, client.GetPartition(),
 			   selection, true, false,
-			   sort,
+			   sort, descending,
 			   window.start, window.end);
 	return CommandResult::OK;
 }

src/db/DatabasePrint.cxx

@@ -149,11 +149,16 @@ CompareNumeric(const char *a, const char *b)
 }
 
 static bool
-CompareTags(TagType type, const Tag &a, const Tag &b)
+CompareTags(TagType type, bool descending, const Tag &a, const Tag &b)
 {
 	const char *a_value = a.GetSortValue(type);
 	const char *b_value = b.GetSortValue(type);
 
+	if (descending) {
+		using std::swap;
+		swap(a_value, b_value);
+	}
+
 	switch (type) {
 	case TAG_DISC:
 	case TAG_TRACK:
@@ -168,7 +173,7 @@ void
 db_selection_print(Response &r, Partition &partition,
 		   const DatabaseSelection &selection,
 		   bool full, bool base,
-		   TagType sort,
+		   TagType sort, bool descending,
 		   unsigned window_start, unsigned window_end)
 {
 	const Database &db = partition.GetDatabaseOrThrow();
@@ -216,8 +221,10 @@ db_selection_print(Response &r, Partition &partition,
 		}
 
 		std::stable_sort(songs.begin(), songs.end(),
-				 [sort](const DetachedSong &a, const DetachedSong &b){
+				 [sort, descending](const DetachedSong &a,
-					 return CompareTags(sort, a.GetTag(),
+						    const DetachedSong &b){
+					 return CompareTags(sort, descending,
+							    a.GetTag(),
 							    b.GetTag());
 				 });
 
@@ -242,7 +249,7 @@ db_selection_print(Response &r, Partition &partition,
 		   bool full, bool base)
 {
 	db_selection_print(r, partition, selection, full, base,
-			   TAG_NUM_OF_ITEM_TYPES,
+			   TAG_NUM_OF_ITEM_TYPES, false,
 			   0, std::numeric_limits<int>::max());
 }
 

src/db/DatabasePrint.hxx

@@ -42,7 +42,7 @@ void
 db_selection_print(Response &r, Partition &partition,
 		   const DatabaseSelection &selection,
 		   bool full, bool base,
-		   TagType sort,
+		   TagType sort, bool descending,
 		   unsigned window_start, unsigned window_end);
 
 void