final sclass ChunkedLongList extends RandomAccessAbstractList<Long> {
  int chunkShift = 16; // => chunkSize = 65536
  new ArrayList<long[]> chunks;
  int actualChunks; // roughly keep track of chunk list's internal capacity so we can shrink in time
  int size;
  
  public void add(long i) {
    ensureCapacityInternal(size+1);
    set(size, i);
    ++size;
  }
  
  public Long set(int i, Long value) {
    ret set(i, (long) value);
  }
  
  public long set(int i, long value) {
    long[] chunk = chunks.get(i >> chunkShift);
    int idx = i & (chunkSize()-1);
    long old = chunk[idx];
    chunk[idx] = value;
    ret old;
  }
  
  public bool add(Long i) {
    add((long) i);
    true;
  }
  
  public int size() { ret size; }
  
  public Long get(int i) {
    ret getPrimitive(i);
  }
  
  public long getPrimitive(int i) {
    if (i >= 0 && i < size) ret chunks.get(i >> chunkShift)[i & (chunkSize()-1)];
    throw new IndexOutOfBoundsException(i + "/" + size);
  }
  
  void ensureCapacityInternal(int minCapacity) {
    while (l(chunks) << chunkShift < minCapacity) {
      chunks.add(new long[chunkSize()]);
      actualChunks = max(actualChunks, l(chunks));
    }
  }
  
  long[] toLongArray() {
    long[] a = new long[size];
    int i = 0;
    for (long[] chunk : chunks) {
      System.arraycopy(chunk, 0, a, i, min(chunk.length, size-i));
      i += chunk.length;
    }
    ret a;
  }
  
  int chunkSize() { ret 1 << chunkShift; }
  
  void truncateAt(int idx) {
    size = idx;
    while (size <= (l(chunks)-1) << chunkShift)
      popLast(chunks);
    if (l(chunks)*2 < actualChunks) {
      chunks.trimToSize();
      actualChunks = l(chunks);
    }
  }
}

Began life as a copy of #1028525