lib: Give Tags a stacked() function

This will be called when a tag is added to the tag stack to check if the
current track pointer needs to be reset.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

lib/tag.py

@@ -98,6 +98,11 @@ class Tag:
             self.tracks.remove(track)
         self.TrackRemoved.publish(self, track)
 
+    def stacked(self):
+        with self.lock:
+            if self.current >= len(self.tracks):
+                self.current = -1
+
 
 class SuperTag(Tag):
     def __init__(self, parent, name, sort=None):

lib/test_tag.py

@@ -167,6 +167,18 @@ class TestTag(unittest.TestCase):
         self.assertEqual(t.next(), 0)
         self.assertEqual(t.next(), 1)
 
+    def test_tag_stacked(self):
+        t = tag.Tag("test")
+        t.tracks = [ 0, 1, 2 ]
+
+        t.current = 3
+        t.stacked()
+        self.assertEqual(t.current, -1)
+
+        t.current = 1
+        t.stacked()
+        self.assertEqual(t.current, 1)
+
 
 class TestSuperTag(unittest.TestCase):
     def test_super_tag(self):