Method Get_region

r/src/altrep.cpp:290–330

R calls this when it wants data from position `i` to `i + n` copied into `buf` The returned value is the number of values that were really copied (this can be lower than n)

  // The returned value is the number of values that were really copied
  // (this can be lower than n)
  static R_xlen_t Get_region(SEXP alt, R_xlen_t i, R_xlen_t n, c_type* buf) {
    // If we have data2, we can just copy the region into buf
    // using the standard Get_region for this R type
    if (IsMaterialized(alt)) {
      return Standard_Get_region<c_type>(Representation(alt), i, n, buf);
    }

    // The vector was not materialized, aka we don't have data2
    //
    // In that case, we copy the data from the Array, and then
    // do a second pass to force the R sentinels for where the
    // array has nulls
    //
    // This only materializes the region into buf (not the entire vector).
    auto slice = GetChunkedArray(alt)->Slice(i, n);
    R_xlen_t ncopy = slice->length();

    c_type* out = buf;
    for (const auto& array : slice->chunks()) {
      auto n_i = array->length();

      // first copy the data buffer
      memcpy(out, array->data()->template GetValues<c_type>(1), n_i * sizeof(c_type));

      // then set the R NA sentinels if needed
      if (array->null_count() > 0) {
        internal::BitmapReader bitmap_reader(array->null_bitmap()->data(),
                                             array->offset(), n_i);

        for (R_xlen_t j = 0; j < n_i; j++, bitmap_reader.Next()) {
          if (bitmap_reader.IsNotSet()) {
            out[j] = cpp11::na<c_type>();
          }
        }
      }

      out += n_i;
    }

    return ncopy;
  }

  static std::shared_ptr<arrow::compute::ScalarAggregateOptions> NaRmOptions(bool na_rm) {
    auto options = std::make_shared<arrow::compute::ScalarAggregateOptions>(