Merge pull request #2 from tespkg/fix/2-fix-export-slowness

#2; Improves export performance by avoiding create new style per time…

Author: abbasmahdian10

cell.go

@@ -144,7 +144,7 @@ func (f *File) SetCellValue(sheet, cell string, value interface{}) error {
 		if err != nil {
 			return err
 		}
-		err = f.setDefaultTimeStyle(sheet, cell, getDurationNumFmt(v))
+		err = f.setDefaultTimeStyle(sheet, cell, 21)
 	case time.Time:
 		err = f.setCellTimeFunc(sheet, cell, v)
 	case bool:
@@ -256,7 +256,7 @@ func (f *File) setCellTimeFunc(sheet, cell string, value time.Time) error {
 		return err
 	}
 	if isNum {
-		_ = f.setDefaultTimeStyle(sheet, cell, getTimeNumFmt(value))
+		_ = f.setDefaultTimeStyle(sheet, cell, 22)
 	}
 	return err
 }

excelize.go

@@ -242,18 +242,15 @@ func (f *File) xmlNewDecoder(rdr io.Reader) (ret *xml.Decoder) {
 // time.Time type cell value by given worksheet name, cell reference and
 // number format code.
 func (f *File) setDefaultTimeStyle(sheet, cell string, format int) error {
-	styleIdx, err := f.GetCellStyle(sheet, cell)
+	s, err := f.GetCellStyle(sheet, cell)
 	if err != nil {
 		return err
 	}
-	if styleIdx == 0 {
-		styleIdx, _ = f.NewStyle(&Style{NumFmt: format})
-	} else {
-		style, _ := f.GetStyle(styleIdx)
-		style.NumFmt = format
-		styleIdx, _ = f.NewStyle(style)
+	if s == 0 {
+		style, _ := f.NewStyle(&Style{NumFmt: format})
+		err = f.SetCellStyle(sheet, cell, cell, style)
 	}
-	return f.SetCellStyle(sheet, cell, cell, styleIdx)
+	return err
 }
 
 // workSheetReader provides a function to get the pointer to the structure