Function ReverseBlockOffsets

cpp/src/arrow/util/bitmap_ops.cc:178–219

}

void ReverseBlockOffsets(const uint8_t* data, int64_t offset, int64_t length,
                         int64_t dest_offset, uint8_t* dest) {
  int64_t num_bytes = bit_util::BytesForBits(offset % 8 + length);
  // Shift by its byte offset
  data += offset / 8;
  dest += dest_offset / 8;

  int64_t j_src = num_bytes - 1;
  int64_t i_dest = 0;

  while (length > 0) {
    uint8_t right_trailing_bits_src = (length + offset) % 8;
    right_trailing_bits_src = !right_trailing_bits_src ? 8 : right_trailing_bits_src;

    uint8_t left_trailing_bits_dest = 8 - dest_offset % 8;
    uint8_t left_trailing_mask_dest = 0xFF << (8 - left_trailing_bits_dest);
    if (length <= 8 && (dest_offset % 8) + length < 8) {
      uint8_t extra_bits = static_cast<uint8_t>(8 - ((dest_offset % 8) + length));
      left_trailing_mask_dest <<= extra_bits;
      left_trailing_mask_dest >>= extra_bits;
    }

    uint8_t right_reversed_block;
    if (j_src == 0) {
      right_reversed_block = static_cast<uint8_t>(
          GetReversedBlock(data[0], data[0], right_trailing_bits_src));
    } else {
      right_reversed_block = static_cast<uint8_t>(
          GetReversedBlock(data[j_src - 1], data[j_src], right_trailing_bits_src));
    }

    dest[i_dest] &= ~left_trailing_mask_dest;
    dest[i_dest] |=
        (right_reversed_block << (8 - left_trailing_bits_dest)) & left_trailing_mask_dest;

    dest_offset += left_trailing_bits_dest;
    length -= left_trailing_bits_dest;

    if (left_trailing_bits_dest >= right_trailing_bits_src) j_src--;
    i_dest++;
  }
}

}  // namespace