playlist_manager: Sort first/last videos correctly

In addition to sorting the videos themselves into the correct spots,
we need to special case them when scanning for the correct place to insert other videos.

Note this only places them in the correct place on insert,
which requires they be set in the playlist config BEFORE being inserted.

A follow-up commit will handle the case of needing to re-order them post insert.

playlist_manager/playlist_manager/main.py

@@ -137,7 +137,7 @@ class PlaylistManager(object):
 		# Insert each new video one at a time
 		logging.debug(f"Inserting new videos for playlist {playlist_id}: {new_videos}")
 		for video in new_videos:
-			index = self.find_insert_index(videos, self.get_playlist(playlist_id), video)
+			index = self.find_insert_index(videos, playlist_config, self.get_playlist(playlist_id), video)
 			self.insert_into_playlist(playlist_id, video.video_id, index)
 
 	def refresh_playlist(self, playlist_id):
@@ -171,11 +171,17 @@ class PlaylistManager(object):
 			) for item in query.items
 		]
 
-	def find_insert_index(self, videos, playlist, new_video):
+	def find_insert_index(self, videos, playlist_config, playlist, new_video):
 		"""Find the index at which to insert new_video into playlist such that
 		playlist remains sorted (it is assumed to already be sorted).
 		videos should be a mapping from video ids to video info.
 		"""
+		# Handle special cases first.
+		if playlist_config.first_event_id == new_video.id:
+			return 0
+		if playlist_config.last_event_id == new_video.id:
+			return len(playlist)
+
 		# To try to behave as best we can in the presence of mis-sorted playlists,
 		# we walk through the list linearly. We insert immediately before the first
 		# item that should be after us in sort order.
@@ -186,6 +192,18 @@ class PlaylistManager(object):
 				# ignore unknowns
 				continue
 			video = videos[video_id]
+
+			# The starting video of a playlist can have its own times, unaffiliated with other videos
+			if video.id == playlist_config.first_event_id:
+				continue
+			# The new video needs to go before the last video in the playlist.
+			# This will produce incorrect results if the last video is in the wrong position
+			# so we only accept this if the last video is actually in the last position,
+			# otherwise we ignore it.
+			if video.id == playlist_config.last_event_id:
+				if n == len(playlist) - 1:
+					return n
+				continue
 			# if this video is after new video, return this index
 			if new_video.start_time < video.start_time:
 				return n