Function GetInclusionMaskAndOutSchema

cpp/src/arrow/ipc/reader.cc:824–855

If we are selecting only certain fields, populate an inclusion mask for fast lookups. Additionally, drop deselected fields from the reader's schema.

// If we are selecting only certain fields, populate an inclusion mask for fast lookups.
// Additionally, drop deselected fields from the reader's schema.
Status GetInclusionMaskAndOutSchema(const std::shared_ptr<Schema>& full_schema,
                                    const std::vector<int>& included_indices,
                                    std::vector<bool>* inclusion_mask,
                                    std::shared_ptr<Schema>* out_schema) {
  inclusion_mask->clear();
  if (included_indices.empty()) {
    *out_schema = full_schema;
    return Status::OK();
  }

  inclusion_mask->resize(full_schema->num_fields(), false);

  auto included_indices_sorted = included_indices;
  std::sort(included_indices_sorted.begin(), included_indices_sorted.end());

  FieldVector included_fields;
  for (int i : included_indices_sorted) {
    // Ignore out of bounds indices
    if (i < 0 || i >= full_schema->num_fields()) {
      return Status::Invalid("Out of bounds field index: ", i);
    }

    if (inclusion_mask->at(i)) continue;

    inclusion_mask->at(i) = true;
    included_fields.push_back(full_schema->field(i));
  }

  *out_schema = schema(std::move(included_fields), full_schema->endianness(),
                       full_schema->metadata());
  return Status::OK();
}

Status UnpackSchemaMessage(const void* opaque_schema, const IpcReadOptions& options,
                           DictionaryMemo* dictionary_memo,