Method FlushBlock

cpp/src/parquet/encoder.cc:1141–1207

template <typename DType>
void DeltaBitPackEncoder<DType>::FlushBlock() {
  if (values_current_block_ == 0) {
    return;
  }

  // Calculate the frame of reference for this miniblock. This value will be subtracted
  // from all deltas to guarantee all deltas are positive for encoding.
  const T min_delta =
      *std::min_element(deltas_.begin(), deltas_.begin() + values_current_block_);
  bit_writer_.PutZigZagVlqInt(min_delta);

  // Call to GetNextBytePtr reserves mini_blocks_per_block_ bytes of space to write
  // bit widths of miniblocks as they become known during the encoding.
  uint8_t* bit_width_data = bit_writer_.GetNextBytePtr(mini_blocks_per_block_);
  DCHECK(bit_width_data != nullptr);

  const uint32_t num_miniblocks =
      static_cast<uint32_t>(std::ceil(static_cast<double>(values_current_block_) /
                                      static_cast<double>(values_per_mini_block_)));
  for (uint32_t i = 0; i < num_miniblocks; i++) {
    const uint32_t values_current_mini_block =
        std::min(values_per_mini_block_, values_current_block_);

    const uint32_t start = i * values_per_mini_block_;
    const T max_delta = *std::max_element(
        deltas_.begin() + start, deltas_.begin() + start + values_current_mini_block);

    // The minimum number of bits required to write any of values in deltas_ vector.
    // See overflow comment above.
    // 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)
    const auto bit_width = bit_width_data[i] =
        std::log2p1(static_cast<UT>(max_delta) - static_cast<UT>(min_delta));
#else
    const auto bit_width = bit_width_data[i] =
        std::bit_width(static_cast<UT>(max_delta) - static_cast<UT>(min_delta));
#endif

    for (uint32_t j = start; j < start + values_current_mini_block; j++) {
      // Convert delta to frame of reference. See overflow comment above.
      const UT value = static_cast<UT>(deltas_[j]) - static_cast<UT>(min_delta);
      bit_writer_.PutValue(value, bit_width);
    }
    // If there are not enough values to fill the last mini block, we pad the mini block
    // with zeroes so that its length is the number of values in a full mini block
    // multiplied by the bit width.
    for (uint32_t j = values_current_mini_block; j < values_per_mini_block_; j++) {
      bit_writer_.PutValue(0, bit_width);
    }
    values_current_block_ -= values_current_mini_block;
  }

  // If, in the last block, less than <number of miniblocks in a block> miniblocks are
  // needed to store the values, the bytes storing the bit widths of the unneeded
  // miniblocks are still present, their value should be zero, but readers must accept
  // arbitrary values as well.