trackdb: Give TagStacks next() and previous() operations

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

trackdb/stack.py

@@ -5,9 +5,25 @@ class TagStack:
     def __init__(self):
         self.tags = [ ]
 
+    def __do_next__(self, tag):
+        if track := tag.next():
+            track.add_to_playlist("Previous")
+        return track
+
+    def next(self):
+        if len(self.tags) == 0:
+            return self.__do_next__(tags.User["Collection"])
+        if track := self.__do_next__(self.tags[0]):
+            return track
+        self.pop()
+        return self.next()
+
     def pop(self):
         self.tags.pop(0)
 
+    def previous(self):
+        return tags.User["Previous"].next()
+
     def push(self, tag):
         if tag == tags.User["Previous"]:
             return

trackdb/test_stack.py

@@ -4,11 +4,42 @@ from . import stack
 from . import tags
 import unittest
 
+class FakeTrack:
+    def __init__(self, n):
+        self.n = n
+
+    def add_to_playlist(self, name):
+        tags.User.add(name, self)
+
 class TestTagStack(unittest.TestCase):
+    def tearDown(self):
+        tags.reset()
+
     def test_tag_stack_init(self):
         s = stack.TagStack()
         self.assertEqual(s.tags, [ ])
 
+    def test_tag_stack_next(self):
+        s = stack.TagStack()
+        t = tag.Tag("Test")
+        s.push(t)
+
+        t.tracks = [ FakeTrack(1), FakeTrack(2), FakeTrack(3) ]
+        tags.User["Collection"].tracks = [ FakeTrack(4), FakeTrack(5) ]
+
+        self.assertEqual(s.next().n, 1)
+        self.assertEqual(s.tags, [ t ])
+        self.assertEqual(s.next().n, 2)
+        self.assertEqual(s.tags, [ t ])
+        self.assertEqual(s.next().n, 3)
+        self.assertEqual(s.tags, [ t ])
+        self.assertEqual(s.next().n, 4)
+        self.assertEqual(s.tags, [ ])
+        self.assertEqual(s.next().n, 5)
+
+        self.assertEqual([ t.n for t in tags.User["Previous"].tracks ],
+                         [ 5, 4, 3, 2, 1 ])
+
     def test_tag_stack_pop(self):
         s = stack.TagStack()
         t1 = tag.Tag("Test")
@@ -20,6 +51,15 @@ class TestTagStack(unittest.TestCase):
         s.pop()
         self.assertEqual(s.tags, [ ])
 
+    def test_tag_stack_previous(self):
+        s = stack.TagStack()
+        for i in [ 1, 2, 3 ]:
+            tags.User["Previous"].add_track(i)
+
+        self.assertEqual(s.previous(), 2)
+        self.assertEqual(s.previous(), 1)
+        self.assertIsNone(s.previous())
+
     def test_tag_stack_push(self):
         s = stack.TagStack()
         t1 = tag.Tag("Test")