queue/Print: support sorting by priority

doc/protocol.rst

@@ -787,7 +787,8 @@ Whenever possible, ids should be used.
     "ArtistSort", "AlbumSort" or "AlbumArtistSort" instead.  These
     will automatically fall back to the former if "\*Sort" doesn't
     exist.  "AlbumArtist" falls back to just "Artist".  The type
-    "Last-Modified" can sort by file modification time.
+    "Last-Modified" can sort by file modification time, and "prio"
+    sorts by queue priority.
 
     ``window`` can be used to query only a portion of the real
     response.  The parameter is two zero-based queue positions; a

src/command/QueueCommands.cxx

@@ -293,6 +293,9 @@ ParseSortTag(const char *s)
 	if (StringIsEqualIgnoreCase(s, "Last-Modified"))
 		return TagType(SORT_TAG_LAST_MODIFIED);
 
+	if (StringIsEqualIgnoreCase(s, "prio"))
+		return TagType(SORT_TAG_PRIO);
+
 	TagType tag = tag_name_parse_i(s);
 	if (tag == TAG_NUM_OF_ITEM_TYPES)
 		throw ProtocolError(ACK_ERROR_ARG, "Unknown sort tag");

src/queue/Print.cxx

@@ -143,6 +143,15 @@ PrintSortedQueue(Response &r, const Queue &queue,
 
 					 return a.GetLastModified() < b.GetLastModified();
 				 });
+	else if (sort == TagType(SORT_TAG_PRIO))
+		std::stable_sort(v.begin(), v.end(),
+				 [&queue, descending](unsigned a_pos, unsigned b_pos){
+					 if (descending)
+						 std::swap(a_pos, b_pos);
+
+					 return queue.GetPriorityAtPosition(a_pos) <
+						 queue.GetPriorityAtPosition(b_pos);
+				 });
 	else
 		std::stable_sort(v.begin(), v.end(),
 				 [&queue, sort, descending](unsigned a_pos,

src/song/Filter.hxx

@@ -31,6 +31,11 @@
  */
 #define SORT_TAG_LAST_MODIFIED (TAG_NUM_OF_ITEM_TYPES + 3)
 
+/**
+ * Special value for QueueSelection::sort
+ */
+#define SORT_TAG_PRIO (TAG_NUM_OF_ITEM_TYPES + 4)
+
 template<typename T> struct ConstBuffer;
 enum TagType : uint8_t;
 struct LightSong;