let nulls = // align bitmap to 0 offset, copying if already aligned
let mut vals = BufferBuilder::<u8>::new(array.value_data().len());
let mut indexes = BufferBuilder::<OffsetSize>::new(array.len() + 1);
indexes.append(0);

for val in array.iter() {
  let char_count = val.chars().count();
  let start = if start >= 0 {
      start.to_usize().unwrap().min(char_count)
  } else {
      char_count - (-start).to_usize().unwrap().min(char_count)
  };
  let length = length.map_or(char_count - start, |length| {
      length.to_usize().unwrap().min(char_count - start)
  });

  let mut start_byte = 0;
  let mut end_byte = val.len();
  for ((idx, (byte_idx, _)) in val.char_indices().enumerate() {
    if idx == start {
      start_byte = byte_idx;
    } else if idx == start + length {
      end_byte = byte_idx;
      break
    }
  }
  // Could even be unchecked
  vals.append_slice(&val[start_byte..end_byte]);
  indexes.append(vals.len() as _);
}

let data = ArrayDataBuilder::new(array.data_type()).len(array.len()).add_buffer(vals.finish())
  .add_buffer(indexes.finish()).add_buffer(vals.finish());

Ok(GenericStringArray::<OffsetSize>::from(unsafe {data.build_unchecked()}))