A default set of length-bucket boundaries.

def _bucket_boundaries(max_length, min_length=8, length_bucket_step=1.1):
  """A default set of length-bucket boundaries."""
  assert length_bucket_step > 1.0
  x = min_length
  boundaries = []
  while x < max_length:
    boundaries.append(x)
    x = max(x + 1, int(x * length_bucket_step))
  return boundaries