Add note array and view.

rhythm_game/note/note_subset.gd

@@ -0,0 +1,25 @@
+## Contain a subset of notes in a [NoteArray].
+## Notes are expected to be sorted in ascending order by beat.
+## The original index of a note can be found with [method id_at].
+class_name NoteSubset extends NoteArray
+
+## The array where notes in this subset come from.
+var full_set: NoteArray
+
+## The ID (index of note in full_set) of the note at [param index].
+func id_at(index) -> int:
+	return _ids.get(index)
+
+## Parse the given [param notes] and return a [class NoteSubset]
+## containing only notes in [param lane].
+static func get_notes_in_lane(notes: NoteArray, lane: int) -> NoteSubset:
+	var lane_notes: NoteSubset = NoteSubset.new()
+	lane_notes.full_set = notes
+	for i in range(notes.size( )):
+		if notes.lane_at(i) == lane:
+			lane_notes._ids.append(i)
+			lane_notes.append_note_data(notes.beat_at(i), notes.type_at(i), lane)
+	return lane_notes
+
+## ====== IMPLEMENTATION ====== #
+var _ids: Array[int] = []