feat: handle foreign encodings

pkg/message/parser/writer.go

@@ -19,19 +19,27 @@ func newWriter(root *Part) *Writer {
 	}
 }
 
+// WithCondition allows setting a condition when parts should be written.
+// Parts are passed to each condition set and if any condition returns false,
+// the part is not written.
+// This initially seemed like a good idea but is now kinda useless.
 func (w *Writer) WithCondition(cond Condition) *Writer {
 	w.cond = append(w.cond, cond)
 	return w
 }
 
-func (w *Writer) Write(ww io.Writer) (err error) {
-	msgWriter, err := message.CreateWriter(ww, w.root.Header)
-	if err != nil {
-		return
+func (w *Writer) Write(ww io.Writer) error {
+	if w.shouldFilter(w.root) {
+		w.root.Header.Add("Content-Transfer-Encoding", "base64")
 	}
 
-	if err = w.root.write(msgWriter, w); err != nil {
-		return
+	msgWriter, err := message.CreateWriter(ww, w.root.Header)
+	if err != nil {
+		return err
+	}
+
+	if err := w.write(msgWriter, w.root); err != nil {
+		return err
 	}
 
 	return msgWriter.Close()
@@ -46,3 +54,56 @@ func (w *Writer) shouldWrite(p *Part) bool {
 
 	return true
 }
+
+func (w *Writer) shouldFilter(p *Part) bool {
+	encoding := p.Header.Get("Content-Transfer-Encoding")
+
+	if encoding != "" && encoding == "quoted-printable" || encoding == "base64" {
+		return false
+	}
+
+	for _, b := range p.Body {
+		if uint8(b) > 1<<7 {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (w *Writer) write(writer *message.Writer, p *Part) error {
+	if len(p.children) > 0 {
+		for _, child := range p.children {
+			if err := w.writeAsChild(writer, child); err != nil {
+				return err
+			}
+		}
+	}
+
+	if _, err := writer.Write(p.Body); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (w *Writer) writeAsChild(writer *message.Writer, p *Part) error {
+	if !w.shouldWrite(p) {
+		return nil
+	}
+
+	if w.shouldFilter(p) {
+		p.Header.Add("Content-Transfer-Encoding", "base64")
+	}
+
+	childWriter, err := writer.CreatePart(p.Header)
+	if err != nil {
+		return err
+	}
+
+	if err := w.write(childWriter, p); err != nil {
+		return err
+	}
+
+	return childWriter.Close()
+}