command/QueueCommands: offset relative "addid" positions by one

doc/protocol.rst

@@ -711,9 +711,10 @@ Whenever possible, ids should be used.
 
     If the second parameter is given, then the song is inserted at the
     specified position.  If the parameter starts with ``+`` or ``-``,
-    then it is relative to the current song; e.g. ``+1`` inserts right
-    after the current song and ``-1`` inserts right before the current
-    song.  (``±0`` is not a legal value.)
+    then it is relative to the current song; e.g. ``+0`` inserts right
+    after the current song and ``-0`` inserts right before the current
+    song (i.e. zero songs between the current song and the newly added
+    song).
 
 .. _command_clear:
 

src/command/QueueCommands.cxx

@@ -120,8 +120,8 @@ handle_addid(Client &client, Request args, Response &r)
 		const auto queue_length = partition.playlist.queue.GetLength();
 
 		const char *const s = args[1];
-		if (*s == '+' || *s == '-') {
-			/* relative to the current song */
+		if (*s == '+') {
+			/* after the current song */
 
 			const int current =
 				partition.playlist.GetCurrentPosition();
@@ -129,23 +129,23 @@ handle_addid(Client &client, Request args, Response &r)
 				throw ProtocolError(ACK_ERROR_PLAYER_SYNC,
 						    "No current song");
 
-			to = args.ParseInt(1, -current - 1,
-					   queue_length - current);
-			if (to == 0)
-				throw ProtocolError(ACK_ERROR_ARG,
-						    "Zero is not a legal relative position");
-
-			/* special case for negative offsets: the
-			   offset "-1" shall insert the new song right
-			   before the current song (just like "+1"
-			   inserts right after the current song);
-			   computationally, that would be a zero
-			   offset, but that's not intuitive, so we
-			   need to add one here */
-			if (to < 0)
-				++to;
-
-			to += current;
+			assert(unsigned(current) < queue_length);
+
+			to = current + 1 +
+				ParseCommandArgUnsigned(s + 1,
+							queue_length - current - 1);
+		} else if (*s == '-') {
+			/* before the current song */
+
+			const int current =
+				partition.playlist.GetCurrentPosition();
+			if (current < 0)
+				throw ProtocolError(ACK_ERROR_PLAYER_SYNC,
+						    "No current song");
+
+			assert(unsigned(current) < queue_length);
+
+			to = current - ParseCommandArgUnsigned(s + 1, current);
 		} else
 			/* absolute position */
 			to = args.ParseUnsigned(1, queue_length);