db: Give the Previous playlist special next and previous functions

previous_track() moves deeper into the previous tracks list while
next_track() moves us closer to the start. I also make sure we reset
current track when new tracks are added to the playlist.

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

db/test_user.py

@@ -204,6 +204,38 @@ class TestPrevious(unittest.TestCase):
         self.assertEqual(previous.get_track_index(track2), 0)
         self.assertEqual(self.removed, track1)
 
+    def test_previous_track(self):
+        previous = db.user.Table.find("Previous")
+        self.assertEqual(previous.get_property("current"), -1)
+
+        previous.add_track(db.make_fake_track(1, 1, "Track 1", "/a/b/c/1.ogg"))
+        self.assertEqual(previous.get_property("current"), 0)
+        previous.add_track(db.make_fake_track(2, 2, "Track 2", "/a/b/c/2.ogg"))
+        self.assertEqual(previous.get_property("current"), 0)
+        previous.add_track(db.make_fake_track(3, 3, "Track 3", "/a/b/c/3.ogg"))
+        self.assertEqual(previous.get_property("current"), 0)
+
+        self.assertEqual(previous.previous_track(), previous.get_track(1))
+        self.assertEqual(previous.previous_track(), previous.get_track(2))
+        self.assertIsNone(previous.previous_track())
+        self.assertIsNone(previous.previous_track())
+
+    def test_next_track(self):
+        previous = db.user.Table.find("Previous")
+        previous.add_track(db.make_fake_track(1, 1, "Track 1", "/a/b/c/1.ogg"))
+        previous.add_track(db.make_fake_track(2, 2, "Track 2", "/a/b/c/2.ogg"))
+        previous.add_track(db.make_fake_track(3, 3, "Track 3", "/a/b/c/3.ogg"))
+        previous.current = 2
+
+        self.assertEqual(previous.next_track(), previous.get_track(1))
+        self.assertEqual(previous.current, 1)
+        self.assertEqual(previous.next_track(), previous.get_track(0))
+        self.assertEqual(previous.current, 0)
+        self.assertIsNone(previous.next_track())
+        self.assertEqual(previous.current, -1)
+        self.assertIsNone(previous.next_track())
+        self.assertEqual(previous.current, -1)
+
 
 class TestQueuedTracks(unittest.TestCase):
     def track_added(self, plist, added):

db/user.py

@@ -88,10 +88,18 @@ class Previous(UserPlaylist):
         UserPlaylist.__init__(self, row, "edit-undo", "temp_playlist_map")
 
     def add_track(self, track):
-        if super().add_track(track):
-            return True
-        self.remove_track(track)
-        return super().add_track(track)
+        if self.get_track_index(track):
+            self.remove_track(track)
+        super().add_track(track)
+        self.current = 0
+        return True
+
+    def next_track(self):
+        self.current = max(-1, self.current - 1)
+        return self.get_current_track()
+
+    def previous_track(self):
+        return super().next_track()
 
 
 class UserTable(playlist.Model):