feat: initial parser exposing walker/writer

pkg/message/parser/part.go

@@ -0,0 +1,96 @@
+package parser
+
+import (
+	"github.com/emersion/go-message"
+)
+
+type Part struct {
+	Header   message.Header
+	Body     []byte
+	children []*Part
+}
+
+func (p *Part) visit(w *Walker) (err error) {
+	if err = p.handle(w); err != nil {
+		return
+	}
+
+	for _, child := range p.children {
+		if err = child.visit(w); err != nil {
+			return
+		}
+	}
+
+	return
+}
+
+func (p *Part) getTypeHandler(w *Walker) (hdl PartHandler) {
+	t, _, err := p.Header.ContentType()
+	if err != nil {
+		return
+	}
+
+	return w.typeHandlers[t]
+}
+
+func (p *Part) getDispHandler(w *Walker) (hdl DispHandler) {
+	t, _, err := p.Header.ContentDisposition()
+	if err != nil {
+		return
+	}
+
+	return w.dispHandlers[t]
+}
+
+func (p *Part) handle(w *Walker) (err error) {
+	typeHandler := p.getTypeHandler(w)
+	dispHandler := p.getDispHandler(w)
+	defaultHandler := w.defaultHandler
+
+	switch {
+	case dispHandler != nil && typeHandler != nil:
+		return dispHandler(p, typeHandler)
+
+	case dispHandler != nil && typeHandler == nil:
+		return dispHandler(p, defaultHandler)
+
+	case dispHandler == nil && typeHandler != nil:
+		return typeHandler(p)
+
+	default:
+		return defaultHandler(p)
+	}
+}
+
+func (p *Part) write(writer *message.Writer, w *Writer) (err error) {
+	if len(p.children) > 0 {
+		for _, child := range p.children {
+			if err = child.writeAsChild(writer, w); err != nil {
+				return
+			}
+		}
+	}
+
+	if _, err = writer.Write(p.Body); err != nil {
+		return
+	}
+
+	return
+}
+
+func (p *Part) writeAsChild(writer *message.Writer, w *Writer) (err error) {
+	if !w.shouldWrite(p) {
+		return
+	}
+
+	childWriter, err := writer.CreatePart(p.Header)
+	if err != nil {
+		return
+	}
+
+	if err = p.write(childWriter, w); err != nil {
+		return
+	}
+
+	return childWriter.Close()
+}