Function GenerateOffsets

cpp/src/arrow/testing/random.cc:610–663

template <typename OffsetArrayType>
std::shared_ptr<Array> GenerateOffsets(SeedType seed, int64_t size,
                                       typename OffsetArrayType::value_type first_offset,
                                       typename OffsetArrayType::value_type last_offset,
                                       double null_probability, bool force_empty_nulls,
                                       int64_t alignment, MemoryPool* memory_pool) {
  using GenOpt = GenerateOptions<
      typename OffsetArrayType::value_type,
      std::uniform_int_distribution<typename OffsetArrayType::value_type>>;
  GenOpt options(seed, first_offset, last_offset, null_probability);

  BufferVector buffers{2};

  int64_t null_count = 0;

  buffers[0] = *AllocateEmptyBitmap(size, alignment, memory_pool);
  uint8_t* null_bitmap = buffers[0]->mutable_data();
  options.GenerateBitmap(null_bitmap, size, &null_count);
  // Make sure the first and last entry are non-null
  for (const int64_t offset : std::vector<int64_t>{0, size - 1}) {
    if (!arrow::bit_util::GetBit(null_bitmap, offset)) {
      arrow::bit_util::SetBit(null_bitmap, offset);
      --null_count;
    }
  }

  buffers[1] = *AllocateBuffer(sizeof(typename OffsetArrayType::value_type) * size,
                               alignment, memory_pool);
  auto data =
      reinterpret_cast<typename OffsetArrayType::value_type*>(buffers[1]->mutable_data());
  options.GenerateTypedData(data, size);
  // Ensure offsets are in increasing order
  std::sort(data, data + size);
  // Ensure first and last offsets are as required
  DCHECK_GE(data[0], first_offset);
  DCHECK_LE(data[size - 1], last_offset);
  data[0] = first_offset;
  data[size - 1] = last_offset;

  if (force_empty_nulls) {
    arrow::internal::BitmapReader reader(null_bitmap, 0, size);
    for (int64_t i = 0; i < size; ++i) {
      if (reader.IsNotSet()) {
        // Ensure a null entry corresponds to a 0-sized list extent
        // (note this can be neither the first nor the last list entry, see above)
        data[i + 1] = data[i];
      }
      reader.Next();
    }
  }

  auto array_data = ArrayData::Make(
      std::make_shared<typename OffsetArrayType::TypeClass>(), size, buffers, null_count);
  return std::make_shared<OffsetArrayType>(array_data);
}

template <typename OffsetArrayType>
std::shared_ptr<Array> OffsetsFromLengthsArray(OffsetArrayType* lengths,