Add custom ZIP writer support for WriteToBuffer function

file.go

@@ -17,6 +17,7 @@ import (
 	"encoding/binary"
 	"encoding/xml"
 	"io"
+	"io/fs"
 	"math"
 	"os"
 	"path/filepath"
@@ -136,8 +137,22 @@ func (f *File) WriteTo(w io.Writer, opts ...Options) (int64, error) {
 // WriteToBuffer provides a function to get bytes.Buffer from the saved file,
 // and it allocates space in memory. Be careful when the file size is large.
 func (f *File) WriteToBuffer() (*bytes.Buffer, error) {
+	return f.WriteToBufferCustomZIP(func(w io.Writer) ZipWriter { return zip.NewWriter(w) })
+}
+
+type ZipWriter interface {
+	Create(name string) (io.Writer, error)
+	AddFS(fsys fs.FS) error
+	Close() error
+}
+
+// WriteToBufferCustomZIP provides a function to write to a buffer using a custom ZIP writer
+// factory. This allows for custom ZIP compression implementations to be used when saving
+// the spreadsheet to memory. The factory function should return a zipWriter interface
+// that will be used to create the final ZIP archive.
+func (f *File) WriteToBufferCustomZIP(factory func(io.Writer) ZipWriter) (*bytes.Buffer, error) {
 	buf := new(bytes.Buffer)
-	zw := zip.NewWriter(buf)
+	zw := factory(buf)
 
 	if err := f.writeToZip(zw); err != nil {
 		_ = zw.Close()
@@ -158,8 +173,8 @@ func (f *File) WriteToBuffer() (*bytes.Buffer, error) {
 	return buf, nil
 }
 
-// writeToZip provides a function to write to zip.Writer
-func (f *File) writeToZip(zw *zip.Writer) error {
+// writeToZip provides a function to write to zipWriter
+func (f *File) writeToZip(zw ZipWriter) error {
 	f.calcChainWriter()
 	f.commentsWriter()
 	f.contentTypesWriter()