perf: improve OdfTableRow cell count performance

* Issue was mentioned in #25
* Do not iterate over all cells which is slow for Excel generated spreadsheets which have 16384 columns by default.
* Rely on `number-columns-repeated` attribute to count the _real_ cells.
* ATTN: this gives a different result for merged cells over multiple rows compared to the previous implementation. However, this count seems more in line with the actual content XML.

Author: driesva

odfdom/src/main/java/org/odftoolkit/odfdom/doc/table/OdfTableRow.java

@@ -287,16 +287,18 @@ public OdfTableCell getCellByIndex(int index) {
    * @return the cell count
    */
   public int getCellCount() {
-    OdfTable table = getTable();
-    Set<OdfTableCell> realCells = new HashSet<>();
-    List<CellCoverInfo> coverList =
-        table.getCellCoverInfos(0, 0, table.getColumnCount() - 1, table.getRowCount() - 1);
-    int rowIndex = getRowIndex();
-    for (int i = 0; i < table.getColumnCount(); i++) {
-      OdfTableCell cell = table.getOwnerCellByPosition(coverList, i, rowIndex);
-      realCells.add(cell);
+    // count cells by skipping covered-table-cell and taking into account number-columns-repeated attribute
+    int cellCount = 0;
+    for (Node node : new DomNodeList(maRowElement.getChildNodes())) {
+      if (node instanceof TableTableCellElement tableCell) {
+        if (tableCell.getTableNumberColumnsRepeatedAttribute() == null) {
+          cellCount++;
+        } else {
+          cellCount += tableCell.getTableNumberColumnsRepeatedAttribute();
+        }
+      }
     }
-    return realCells.size();
+    return cellCount;
   }
 
   /**

odfdom/src/test/java/org/odftoolkit/odfdom/doc/table/TableCellCountTest.java

@@ -0,0 +1,92 @@
+package org.odftoolkit.odfdom.doc.table;
+
+import static org.junit.Assert.assertEquals;
+import static org.odftoolkit.odfdom.utils.ResourceUtilities.getAbsoluteInputPath;
+
+import junit.framework.AssertionFailedError;
+import org.junit.Test;
+import org.odftoolkit.odfdom.doc.OdfSpreadsheetDocument;
+
+public class TableCellCountTest {
+
+  // The number of columns that Excel always uses.
+  // For example Excel puts <table:table-cell table:number-columns-repeated="16384"/> if no cell values or covered cells
+  private static final int EXCEL_COLUMN_COUNT = 16384;
+
+  @Test
+  public void verifyCellCountForLibreOfficeGeneratedSpreadsheet() {
+    // Spreadsheet created by LibreOffice on Mac version 25.8.4.2 (AARCH64)
+    try (OdfSpreadsheetDocument spreadsheet = loadSpreadsheetDocument("TestLibreOfficeSpreadsheetTableCellCount.ods")) {
+      OdfTable sheet = spreadsheet.getSpreadsheetTables().get(0);
+      assertEquals(3, sheet.getColumnCount());
+
+      // 3 cells merged so 2 covered cells
+      assertEquals(3 - 2 /* covered cells */, sheet.getRowByIndex(0).getCellCount());
+
+      // 2 cells merged so 1 covered cell
+      assertEquals(3 - 1 /* covered cell */, sheet.getRowByIndex(1).getCellCount());
+
+      // no merged cells
+      assertEquals(3, sheet.getRowByIndex(2).getCellCount());
+
+      // 2 cells merged over 2 rows (simplified XML):
+      //        <table:table-row>
+      //          <table:table-cell table:number-columns-spanned="2" table:number-rows-spanned="2">
+      //            <text:p>1</text:p>
+      //          </table:table-cell>
+      //          <table:covered-table-cell/>
+      //          <table:table-cell>
+      //            <text:p>2</text:p>
+      //          </table:table-cell>
+      //        </table:table-row>
+      //        <table:table-row>
+      //          <table:covered-table-cell table:number-columns-repeated="2"/>
+      //          <table:table-cell>
+      //            <text:p>2</text:p>
+      //          </table:table-cell>
+      //        </table:table-row>
+      // so 1 covered cell in first row
+      assertEquals(3 - 1 /* covered cell */, sheet.getRowByIndex(3).getCellCount());
+      // ... and 2 covered cells in second row
+      assertEquals(3 - 2 /* covered cells */, sheet.getRowByIndex(4).getCellCount());
+    }
+  }
+
+  @Test
+  public void verifyCellCountForExcelGeneratedSpreadsheet() {
+    // Spreadsheet created by Microsoft® Excel for Mac Version 16.106 (26020821)
+    try (OdfSpreadsheetDocument spreadsheet = loadSpreadsheetDocument("TestExcelSpreadsheetTableCellCount.ods")) {
+      OdfTable sheet = spreadsheet.getSpreadsheetTables().get(0);
+      assertEquals(EXCEL_COLUMN_COUNT, sheet.getColumnCount());
+
+      // 3 cells merged so 2 covered cells
+      assertEquals(EXCEL_COLUMN_COUNT - 2 /* covered cells */, sheet.getRowByIndex(0).getCellCount());
+
+      // 2 cells merged so 1 covered cell
+      assertEquals(EXCEL_COLUMN_COUNT - 1 /* covered cell */, sheet.getRowByIndex(1).getCellCount());
+
+      // no merged cells
+      assertEquals(EXCEL_COLUMN_COUNT, sheet.getRowByIndex(2).getCellCount());
+
+      // 2 cells merged over 2 rows so 1 covered cell in first row
+      assertEquals(EXCEL_COLUMN_COUNT - 1 /* covered cell */, sheet.getRowByIndex(3).getCellCount());
+      // ... and 2 covered cells in second row
+      assertEquals(EXCEL_COLUMN_COUNT - 2 /* covered cells */, sheet.getRowByIndex(4).getCellCount());
+
+      // Excel always adds
+      //         <table:table-row table:number-rows-repeated="1048573" table:style-name="ro1">
+      //          <table:table-cell table:number-columns-repeated="16384"/>
+      //        </table:table-row>
+      assertEquals(EXCEL_COLUMN_COUNT, sheet.getRowByIndex(5).getCellCount());
+    }
+  }
+
+  private OdfSpreadsheetDocument loadSpreadsheetDocument(String filename) {
+    try {
+      return OdfSpreadsheetDocument.loadDocument(
+        getAbsoluteInputPath(filename));
+    } catch (Exception ex) {
+      throw new AssertionFailedError(ex.getMessage());
+    }
+  }
+}