discogs: revise subtrack merging based on subindex

Modify the coalescing of subtracks by taking into account the subindices
of the subtracks for deciding if they represent "logical" tracks
(merging them into one single track, as previously) or if they represent
physical tracks (treating them as individual tracks).
Add unit tests for nested logical/physical subtracks, and disc titles.

beetsplug/discogs.py

@@ -354,15 +354,27 @@ class DiscogsPlugin(BeetsPlugin):
             """Modify `tracklist` in place, merging a list of `subtracks` into
             a single track into `tracklist`."""
             # Calculate position based on first subtrack, without subindex.
-            idx, medium_idx, _ = self.get_track_index(subtracks[0]['position'])
+            idx, medium_idx, sub_idx = \
+                self.get_track_index(subtracks[0]['position'])
             position = '%s%s' % (idx or '', medium_idx or '')
 
-            if len(tracklist) > 1 and not tracklist[-1]['position']:
-                # Assume the previous index track contains the track title, and
-                # "convert" it to a real track. The only exception is if the
-                # index track is the only one on the tracklist, as it probably
-                # is a medium title.
-                tracklist[-1]['position'] = position
+            if tracklist and not tracklist[-1]['position']:
+                # Assume the previous index track contains the track title.
+                if sub_idx:
+                    # "Convert" the track title to a real track, discarding the
+                    # subtracks assuming they are logical divisions of a
+                    # physical track (12.2.9 Subtracks).
+                    tracklist[-1]['position'] = position
+                else:
+                    # Promote the subtracks to real tracks, discarding the
+                    # index track, assumeing the subtracks are physical tracks.
+                    index_track = tracklist.pop()
+                    # Fix artists when they are specified on the index track.
+                    if index_track.get('artists'):
+                        for subtrack in subtracks:
+                            if not subtrack.get('artists'):
+                                subtrack['artists'] = index_track['artists']
+                    tracklist.extend(subtracks)
             else:
                 # Merge the subtracks, pick a title, and append the new track.
                 track = subtracks[0].copy()

test/test_discogs.py

@@ -272,11 +272,14 @@ class DGAlbumInfoTest(_common.TestCase):
 
         d = DiscogsPlugin().get_album_info(release)
         self.assertEqual(d.mediums, 1)
+        self.assertEqual(d.tracks[0].disctitle, 'MEDIUM TITLE')
         self.assertEqual(len(d.tracks), 1)
         self.assertEqual(d.tracks[0].title, 'TRACK GROUP TITLE')
 
-    def test_parse_tracklist_subtracks_nested(self):
-        """Test parsing of subtracks defined inside a index track."""
+    def test_parse_tracklist_subtracks_nested_logical(self):
+        """Test parsing of subtracks defined inside a index track that are
+        logical subtracks (ie. should be grouped together into a single track).
+        """
         release = self._make_release_from_positions(['1', '', '3'])
         # Track 2: Index track with track group title, and sub_tracks
         release.data['tracklist'][1]['title'] = 'TRACK GROUP TITLE'
@@ -290,6 +293,40 @@ class DGAlbumInfoTest(_common.TestCase):
         self.assertEqual(len(d.tracks), 3)
         self.assertEqual(d.tracks[1].title, 'TRACK GROUP TITLE')
 
+    def test_parse_tracklist_subtracks_nested_physical(self):
+        """Test parsing of subtracks defined inside a index track that are
+        physical subtracks (ie. should not be grouped together).
+        """
+        release = self._make_release_from_positions(['1', '', '4'])
+        # Track 2: Index track with track group title, and sub_tracks
+        release.data['tracklist'][1]['title'] = 'TRACK GROUP TITLE'
+        release.data['tracklist'][1]['sub_tracks'] = [
+            self._make_track('TITLE ONE', '2', '01:01'),
+            self._make_track('TITLE TWO', '3', '02:02')
+        ]
+
+        d = DiscogsPlugin().get_album_info(release)
+        self.assertEqual(d.mediums, 1)
+        self.assertEqual(len(d.tracks), 4)
+        self.assertEqual(d.tracks[1].title, 'TITLE ONE')
+        self.assertEqual(d.tracks[2].title, 'TITLE TWO')
+
+    def test_parse_tracklist_disctitles(self):
+        """Test parsing of index tracks that act as disc titles."""
+        release = self._make_release_from_positions(['', '1-1', '1-2', '',
+                                                     '2-1'])
+        # Track 1: Index track with medium title (Cd1)
+        release.data['tracklist'][0]['title'] = 'MEDIUM TITLE CD1'
+        # Track 4: Index track with medium title (Cd2)
+        release.data['tracklist'][3]['title'] = 'MEDIUM TITLE CD2'
+
+        d = DiscogsPlugin().get_album_info(release)
+        self.assertEqual(d.mediums, 2)
+        self.assertEqual(d.tracks[0].disctitle, 'MEDIUM TITLE CD1')
+        self.assertEqual(d.tracks[1].disctitle, 'MEDIUM TITLE CD1')
+        self.assertEqual(d.tracks[2].disctitle, 'MEDIUM TITLE CD2')
+        self.assertEqual(len(d.tracks), 3)
+
 
 def suite():
     return unittest.TestLoader().loadTestsFromName(__name__)