db/DatabasePrint: support descending sort

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){
-					 return CompareTags(sort, a.GetTag(),
+				 [sort, descending](const DetachedSong &a,
+						    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