feat: initial parser exposing walker/writer

pkg/message/parser/writer.go

@@ -0,0 +1,48 @@
+package parser
+
+import (
+	"io"
+
+	"github.com/emersion/go-message"
+)
+
+type Writer struct {
+	root *Part
+	cond []Condition
+}
+
+type Condition func(p *Part) bool
+
+func newWriter(root *Part) *Writer {
+	return &Writer{
+		root: root,
+	}
+}
+
+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
+	}
+
+	if err = w.root.write(msgWriter, w); err != nil {
+		return
+	}
+
+	return msgWriter.Close()
+}
+
+func (w *Writer) shouldWrite(p *Part) bool {
+	for _, cond := range w.cond {
+		if !cond(p) {
+			return false
+		}
+	}
+
+	return true
+}