Function BitmapWordAlign

cpp/src/arrow/util/align_util.h:45–76

// - a trailing area of size `trailing_bits` after the aligned words
template <uint64_t ALIGN_IN_BYTES>
inline BitmapWordAlignParams BitmapWordAlign(const uint8_t* data, int64_t bit_offset,
                                             int64_t length) {
  // TODO: We can remove this condition once CRAN upgrades its macOS
  // SDK from 11.3.
  // __apple_build_version__ should be defined only on Apple clang
#if defined(__apple_build_version__) && !defined(__cpp_lib_bitops)
  static_assert((ALIGN_IN_BYTES != 0) && ((ALIGN_IN_BYTES & (ALIGN_IN_BYTES - 1)) == 0),
                "ALIGN_IN_BYTES should be a positive power of two");
#else
  static_assert(std::has_single_bit(ALIGN_IN_BYTES),
                "ALIGN_IN_BYTES should be a positive power of two");
#endif
  constexpr uint64_t ALIGN_IN_BITS = ALIGN_IN_BYTES * 8;

  BitmapWordAlignParams p;

  // Compute a "bit address" that we can align up to ALIGN_IN_BITS.
  // We don't care about losing the upper bits since we are only interested in the
  // difference between both addresses.
  const uint64_t bit_addr =
      reinterpret_cast<size_t>(data) * 8 + static_cast<uint64_t>(bit_offset);
  const uint64_t aligned_bit_addr = bit_util::RoundUpToPowerOf2(bit_addr, ALIGN_IN_BITS);

  p.leading_bits = std::min<int64_t>(length, aligned_bit_addr - bit_addr);
  p.aligned_words = (length - p.leading_bits) / ALIGN_IN_BITS;
  p.aligned_bits = p.aligned_words * ALIGN_IN_BITS;
  p.trailing_bits = length - p.leading_bits - p.aligned_bits;
  p.trailing_bit_offset = bit_offset + p.leading_bits + p.aligned_bits;

  p.aligned_start = data + (bit_offset + p.leading_bits) / 8;
  return p;
}
}  // namespace internal

namespace util {