Function GetPromotionGraph

cpp/src/arrow/json/converter.cc:321–380

}

const PromotionGraph* GetPromotionGraph() {
  static struct : PromotionGraph {
    std::shared_ptr<Field> Null(const std::string& name) const override {
      return field(name, null(), true, Kind::Tag(Kind::kNull));
    }

    std::shared_ptr<DataType> Infer(
        const std::shared_ptr<Field>& unexpected_field) const override {
      auto kind = Kind::FromTag(unexpected_field->metadata());
      switch (kind) {
        case Kind::kNull:
          return null();

        case Kind::kBoolean:
          return boolean();

        case Kind::kNumber:
          return int64();

        case Kind::kString:
          return timestamp(TimeUnit::SECOND);

        case Kind::kArray: {
          const auto& type = checked_cast<const ListType&>(*unexpected_field->type());
          auto value_field = type.value_field();
          return list(value_field->WithType(Infer(value_field)));
        }
        case Kind::kObject: {
          auto fields = unexpected_field->type()->fields();
          for (auto& field : fields) {
            field = field->WithType(Infer(field));
          }
          return struct_(std::move(fields));
        }
        default:
          return nullptr;
      }
    }

    std::shared_ptr<DataType> Promote(
        const std::shared_ptr<DataType>& failed,
        const std::shared_ptr<Field>& unexpected_field) const override {
      switch (failed->id()) {
        case Type::NA:
          return Infer(unexpected_field);

        case Type::TIMESTAMP:
          return utf8();

        case Type::INT64:
          return float64();

        default:
          return nullptr;
      }
    }
  } impl;