Get sheet names based on index

SheetID only seems to indicate the file name for the sheet.
Check the sheets list based on index instead. Reordering sheets
in Excel changes the order they appear in that list.

Fixes #457

sheet.go

@@ -317,15 +317,12 @@ func (f *File) SetSheetName(oldName, newName string) {
 // string.
 func (f *File) GetSheetName(index int) string {
 	wb := f.workbookReader()
-	if wb != nil {
-		for _, sheet := range wb.Sheets.Sheet {
-			if sheet.SheetID == index {
-				return sheet.Name
-			}
-		}
-	}
+	realIdx := index - 1 // sheets are 1 based index, but we're checking against an array
+	if wb == nil || realIdx < 0 || realIdx >= len(wb.Sheets.Sheet) {
 		return ""
 	}
+	return wb.Sheets.Sheet[realIdx].Name
+}
 
 // GetSheetIndex provides a function to get worksheet index of XLSX by given
 // sheet name. If given worksheet name is invalid, will return an integer type
@@ -357,8 +354,8 @@ func (f *File) GetSheetMap() map[int]string {
 	wb := f.workbookReader()
 	sheetMap := map[int]string{}
 	if wb != nil {
-		for _, sheet := range wb.Sheets.Sheet {
-			sheetMap[sheet.SheetID] = sheet.Name
+		for i, sheet := range wb.Sheets.Sheet {
+			sheetMap[i+1] = sheet.Name
 		}
 	}
 	return sheetMap

sheet_test.go

@@ -225,3 +225,24 @@ func TestUngroupSheets(t *testing.T) {
 	}
 	assert.NoError(t, f.UngroupSheets())
 }
+
+func TestGetSheetName(t *testing.T) {
+	f, _ := excelize.OpenFile(filepath.Join("test", "Book1.xlsx"))
+	assert.Equal(t, "Sheet1", f.GetSheetName(1))
+	assert.Equal(t, "Sheet2", f.GetSheetName(2))
+	assert.Equal(t, "", f.GetSheetName(0))
+	assert.Equal(t, "", f.GetSheetName(3))
+}
+
+func TestGetSheetMap(t *testing.T) {
+	expectedMap := map[int]string{
+		1: "Sheet1",
+		2: "Sheet2",
+	}
+	f, _ := excelize.OpenFile(filepath.Join("test", "Book1.xlsx"))
+	sheetMap := f.GetSheetMap()
+	for idx, name := range sheetMap {
+		assert.Equal(t, expectedMap[idx], name)
+	}
+	assert.Equal(t, len(sheetMap), 2)
+}