Function NextFourWords

cpp/src/arrow/util/bit_block_counter.h:122–160

\brief Return the next run of available bits, usually 256. The returned pair contains the size of run and the number of true values. The last block will have a length less than 256 if the bitmap length is not a multiple of 256, and will return 0-length blocks in subsequent invocations.

  /// multiple of 256, and will return 0-length blocks in subsequent
  /// invocations.
  BitBlockCount NextFourWords() {
    using detail::LoadWord;
    using detail::ShiftWord;

    if (!bits_remaining_) {
      return {0, 0};
    }
    int64_t total_popcount = 0;
    if (offset_ == 0) {
      if (bits_remaining_ < kFourWordsBits) {
        return GetBlockSlow(kFourWordsBits);
      }
      total_popcount += std::popcount(LoadWord(bitmap_));
      total_popcount += std::popcount(LoadWord(bitmap_ + 8));
      total_popcount += std::popcount(LoadWord(bitmap_ + 16));
      total_popcount += std::popcount(LoadWord(bitmap_ + 24));
    } else {
      // When the offset is > 0, we need there to be a word beyond the last
      // aligned word in the bitmap for the bit shifting logic.
      if (bits_remaining_ < 5 * kFourWordsBits - offset_) {
        return GetBlockSlow(kFourWordsBits);
      }
      auto current = LoadWord(bitmap_);
      auto next = LoadWord(bitmap_ + 8);
      total_popcount += std::popcount(ShiftWord(current, next, offset_));
      current = next;
      next = LoadWord(bitmap_ + 16);
      total_popcount += std::popcount(ShiftWord(current, next, offset_));
      current = next;
      next = LoadWord(bitmap_ + 24);
      total_popcount += std::popcount(ShiftWord(current, next, offset_));
      current = next;
      next = LoadWord(bitmap_ + 32);
      total_popcount += std::popcount(ShiftWord(current, next, offset_));
    }
    bitmap_ += bit_util::BytesForBits(kFourWordsBits);
    bits_remaining_ -= kFourWordsBits;
    return {256, static_cast<int16_t>(total_popcount)};
  }

  /// \brief Return the next run of available bits, usually 64. The returned
  /// pair contains the size of run and the number of true values. The last