docs: private void splitAt(long t) {
          if (data.containsKey(t)) return; // already split at this timestamp
  
          SortedMap<Long, int[]> head = data.headMap(t);
  
          int v = head.isEmpty() ? 0 : data.get(head.lastKey())[0];
          data.put(t, new int[]{v});
      }