Function FlattenListArray

cpp/src/arrow/array/array_nested.cc:221–265

template <typename ListArrayT>
Result<std::shared_ptr<Array>> FlattenListArray(const ListArrayT& list_array,
                                                MemoryPool* memory_pool) {
  const int64_t list_array_length = list_array.length();
  std::shared_ptr<arrow::Array> value_array = list_array.values();

  // Shortcut: if a ListArray does not contain nulls, then simply slice its
  // value array with the first and the last offsets.
  if (list_array.null_count() == 0) {
    return SliceArrayWithOffsets(*value_array, list_array.value_offset(0),
                                 list_array.value_offset(list_array_length));
  }

  // Second shortcut: if the list array is *all* nulls, then just return
  // an empty array.
  if (list_array.null_count() == list_array.length()) {
    return MakeEmptyArray(value_array->type(), memory_pool);
  }

  // The ListArray contains nulls: there may be a non-empty sub-list behind
  // a null and it must not be contained in the result.
  std::vector<std::shared_ptr<Array>> non_null_fragments;
  int64_t valid_begin = 0;
  while (valid_begin < list_array_length) {
    int64_t valid_end = valid_begin;
    while (valid_end < list_array_length &&
           (list_array.IsValid(valid_end) || list_array.value_length(valid_end) == 0)) {
      ++valid_end;
    }
    if (valid_begin < valid_end) {
      non_null_fragments.push_back(
          SliceArrayWithOffsets(*value_array, list_array.value_offset(valid_begin),
                                list_array.value_offset(valid_end)));
    }
    valid_begin = valid_end + 1;  // skip null entry
  }

  // Final attempt to avoid invoking Concatenate().
  if (non_null_fragments.size() == 1) {
    return non_null_fragments[0];
  } else if (non_null_fragments.size() == 0) {
    return MakeEmptyArray(value_array->type(), memory_pool);
  }

  return Concatenate(non_null_fragments, memory_pool);
}

template <typename ListViewArrayT, bool HasNulls>
Result<std::shared_ptr<Array>> FlattenListViewArray(const ListViewArrayT& list_view_array,