Support updating an existing spreadsheet

src/main/groovy/org/gsheets/ExcelFile.groovy

@@ -34,9 +34,8 @@ package org.gsheets
 
 import org.apache.poi.hssf.usermodel.HSSFRichTextString
 import org.apache.poi.hssf.usermodel.HSSFWorkbook
-import org.apache.poi.ss.util.CellRangeAddress
 import org.apache.poi.ss.usermodel.*
-import org.apache.poi.hssf.usermodel.HSSFCell
+import org.apache.poi.ss.util.CellRangeAddress
 
 /**
  * A Groovy builder that wraps Apache POI for generating binary Microsoft Excel sheets.
@@ -78,13 +77,49 @@ import org.apache.poi.hssf.usermodel.HSSFCell
  */
 class ExcelFile {
 
-    Workbook workbook = new HSSFWorkbook()
+    Workbook workbook
     private Sheet sheet
     private int rowsCounter
 
     private Map<String, CellStyle> cellStyles = [:]
     private Map<String, Font> fonts = [:]
 
+
+    /**
+     * Creates a builder for a new file
+     */
+    def ExcelFile() {
+        workbook = new HSSFWorkbook()
+    }
+
+    /**
+     * Creates a builder based on an existing file.
+     *
+     * All modifications will be on top of the given file, which
+     * serves as a base template.
+     *
+     * @param filename Path to the file
+     */
+    def ExcelFile(String filename) {
+        this(new File(filename))
+    }
+
+    /**
+     * Creates a builder based on an existing file.
+     *
+     * All modifications will be on top of the given file, which
+     * serves as a base template.
+     *
+     * @param file Existing file
+     */
+    def ExcelFile(File file) {
+        assert file.exists()
+
+        def input = new FileInputStream(file)
+        workbook = new HSSFWorkbook(input)
+        input.close()
+    }
+
     /**
      * Creates a new workbook.
      *
@@ -126,7 +161,7 @@ class ExcelFile {
         assert name
         assert closure
 
-        sheet = workbook.createSheet(name)
+        sheet = workbook.getSheet(name) ?: workbook.createSheet(name)
         rowsCounter = 0
 
         closure.delegate = sheet
@@ -273,6 +308,15 @@ class ExcelFile {
         sheet.createRow(rowsCounter++ as int)
     }
 
+    void onRow (int r, Closure closure) {
+        assert sheet
+        assert r >= 0
+
+        rowsCounter = r
+        closure.delegate = this
+        closure.call()
+    }
+
     void row(values) {
         assert sheet
         assert values

src/test/groovy/org/gsheets/ExcelFileTests.groovy

@@ -31,10 +31,9 @@
  */
 package org.gsheets
 
-import org.apache.poi.ss.usermodel.Workbook
 import org.apache.poi.ss.usermodel.CellStyle
 import org.apache.poi.ss.usermodel.Font
-import org.apache.poi.hssf.usermodel.HSSFCell
+import org.apache.poi.ss.usermodel.Workbook
 
 /**
  * @author [email protected]
@@ -83,4 +82,58 @@ class ExcelFileTests extends GroovyTestCase {
         workbook.write(excelOut)
         excelOut.close()
     }
+
+    void testOpenWorkbookAndEditExistingSheet() {
+        def file = createTestFile()
+        def workbook = new ExcelFile(file).workbook {
+            data {
+                sheet("SheetA") {
+                    onRow(4) {
+                        row (['new row', 'on existing', 'workbook'])
+                    }
+                }
+            }
+        }
+
+        def out = new FileOutputStream(file)
+        workbook.write(out)
+        out.close()
+    }
+
+    void testOpenWorkbookAndAddNewSheet() {
+        def file = createTestFile()
+        def workbook = new ExcelFile(file).workbook {
+            data {
+                sheet("SheetB") {
+                    header (["New", "Header"])
+                    row (["new", "row"])
+                }
+            }
+        }
+
+        def out = new FileOutputStream(file)
+        workbook.write(out)
+        out.close()
+    }
+
+    private File createTestFile() {
+        def file = File.createTempFile("gsheets", ".xls")
+        def workbook = new ExcelFile().workbook {
+            data {
+                sheet ("SheetA") {
+                    header (['HA', 'HB', 'HC'])
+
+                    3.times {
+                        row ([it, 2*it, 3*it])
+                    }
+                }
+            }
+        }
+
+        def out = new FileOutputStream(file)
+        workbook.write(out)
+        out.close()
+
+        file
+    }
 }