Add prune logic

karaokatalog/deduplicate/prune.py

@@ -1,12 +1,34 @@
+import re
 from collections.abc import Sequence
 
 from karaokatalog.deduplicate.instructions.DeleteInstruction import DeleteInstruction
 from karaokatalog.Song import Song
 
+DISCOURAGED_DIR_PATTERN = re.compile(r"/UltrastarDX/Ultrastar DX/(Unsortiert/)?")
+
 
 def prune(equivalent_songs: Sequence[Song]) -> Sequence[DeleteInstruction]:
     """
     Prune a sequence of equivalent songs, by returning a sequence of instructions
     that delete all but one song in this sequence.
     """
-    pass
+    preferred_songs = tuple(
+        song
+        for song in equivalent_songs
+        if not DISCOURAGED_DIR_PATTERN.search(str(song.dir))
+    )
+    discouraged_songs = tuple(
+        song
+        for song in equivalent_songs
+        if DISCOURAGED_DIR_PATTERN.search(str(song.dir))
+    )
+
+    # Keep an arbitrarily chosen preferred song, if there is at least one, otherwise, just keep an arbitrarily chosen
+    # discouraged song.
+    song_to_keep = preferred_songs[0] if preferred_songs else discouraged_songs[0]
+
+    return tuple(
+        DeleteInstruction(song.dir)
+        for song in equivalent_songs
+        if song is not song_to_keep
+    )