Method flatten

src/frontend/spreadsheet/FlattenColumnsDialog.cpp:166–286

! * unit-testable helper function that is creating the new target spreadsheet and doing the actual flattening */

 * unit-testable helper function that is creating the new target spreadsheet and doing the actual flattening
 */
void FlattenColumnsDialog::flatten(const Spreadsheet* sourceSpreadsheet, const QVector<Column*>& valueColumns, const QVector<Column*>& referenceColumns) const {
	// create target spreadsheet
	const int referenceColumnCount = referenceColumns.count();
	auto* targetSpreadsheet = new Spreadsheet(i18n("Flatten of %1", m_spreadsheet->name()));
	targetSpreadsheet->setColumnCount(referenceColumnCount + 2);
	const auto& targetColumns = targetSpreadsheet->children<Column>();

	// set the names and modes for the reference columns in the target spreadsheet
	for (int i = 0; i < referenceColumnCount; ++i) {
		auto* source = referenceColumns.at(i);
		auto* target = targetSpreadsheet->column(i);
		target->setName(source->name());
		target->setColumnMode(source->columnMode());
	}

	// add "Category" and "Value" columns which will contain the flattened data
	auto* categoryColumn = targetSpreadsheet->column(referenceColumnCount);
	categoryColumn->setName(i18n("Category"));
	categoryColumn->setColumnMode(AbstractColumn::ColumnMode::Text);
	categoryColumn->setPlotDesignation(AbstractColumn::PlotDesignation::X);

	auto* valueColumn = targetSpreadsheet->column(referenceColumnCount + 1);
	valueColumn->setName(i18n("Value"));
	auto valueColumnMode = valueColumns.at(0)->columnMode();
	valueColumn->setColumnMode(valueColumnMode);

	// flatten
	int row = 0; // current row in the target spreadsheet
	for (int i = 0; i < sourceSpreadsheet->rowCount(); ++i) {
		for (int j = 0; j < valueColumns.count(); ++j) {
			auto* sourceColumn = valueColumns.at(j);

			// skip the current row if there are now source values to be flattened
			if (sourceColumn->asStringColumn()->textAt(i).isEmpty())
				continue;

			// skip the current row if there is not a single reference value
			bool hasReferenceValues = false;
			for (auto* col : referenceColumns) {
				if (!col->asStringColumn()->textAt(i).isEmpty()) {
					hasReferenceValues = true;
					break;
				}
			}

			if (referenceColumnCount > 0 && !hasReferenceValues)
				continue;

			// add reference values for every source column to be flattened
			int refIndex = 0;
			for (auto* col : referenceColumns) {
				switch (col->columnMode()) {
				case AbstractColumn::ColumnMode::Double: {
					auto value = col->valueAt(i);
					targetColumns.at(refIndex)->setValueAt(row, value);
					break;
				}
				case AbstractColumn::ColumnMode::Integer: {