Function DeltaEncode

be/src/util/bit-packing-test.cc:339–376

template <typename ParquetType>
DeltaData<ParquetType> DeltaEncode(const std::vector<ParquetType>& input, int bit_width) {
  using UnsignedParquetType = typename std::make_unsigned<ParquetType>::type;
  const ParquetType base_value = input.empty() ? 0 : input[0];
  std::vector<ParquetType> deltas;

  /// Calculate the deltas.
  for (int i = 1; i < input.size(); i++) {
    UnsignedParquetType current = input[i];
    UnsignedParquetType previous = input[i - 1];
    ParquetType delta = current - previous;
    deltas.push_back(delta);
  }

  /// Offset the deltas.
  const auto it = std::min_element(deltas.begin(), deltas.end());
  const ParquetType min_delta = (it == deltas.end()) ? 0 : *it;

  std::for_each(deltas.begin(), deltas.end(), [min_delta](ParquetType& element) {
      element = ArithmeticUtil::AsUnsigned<std::minus>(element, min_delta);
      });

  // Bit pack the deltas.
  const int bytes_required = BitUtil::RoundUpNumBytes(bit_width * deltas.size());
  std::vector<uint8_t> out_data(bytes_required);

  if (bytes_required > 0) {
    BitWriter writer(out_data.data(), bytes_required);
    if (bit_width > 0) {
      for (const ParquetType delta : deltas) {
        const UnsignedParquetType delta_unsigned = delta;
        EXPECT_TRUE(writer.PutValue(delta_unsigned, bit_width));
      }
    }
    writer.Flush();
  }

  return DeltaData<ParquetType>(base_value, min_delta, deltas.size(), out_data);
}

template <typename INT_T, typename ParquetType>
void RandomUnpackAndDeltaDecodeTest() {