mox!

message/headerwriter.go

@@ -0,0 +1,65 @@
+package message
+
+import (
+	"fmt"
+	"strings"
+)
+
+// HeaderWriter helps create headers, folding to the next line when it would
+// become too large. Useful for creating Received and DKIM-Signature headers.
+type HeaderWriter struct {
+	b        *strings.Builder
+	lineLen  int
+	nonfirst bool
+}
+
+// Addf formats the string and calls Add.
+func (w *HeaderWriter) Addf(separator string, format string, args ...any) {
+	w.Add(separator, fmt.Sprintf(format, args...))
+}
+
+// Add adds texts, each separated by separator. Individual elements in text are
+// not wrapped.
+func (w *HeaderWriter) Add(separator string, texts ...string) {
+	if w.b == nil {
+		w.b = &strings.Builder{}
+	}
+	for _, text := range texts {
+		n := len(text)
+		if w.nonfirst && w.lineLen > 1 && w.lineLen+len(separator)+n > 78 {
+			w.b.WriteString("\r\n\t")
+			w.lineLen = 1
+		} else if w.nonfirst && separator != "" {
+			w.b.WriteString(separator)
+			w.lineLen += len(separator)
+		}
+		w.b.WriteString(text)
+		w.lineLen += len(text)
+		w.nonfirst = true
+	}
+}
+
+// AddWrap adds data, folding anywhere in the buffer. E.g. for base64 data.
+func (w *HeaderWriter) AddWrap(buf []byte) {
+	for len(buf) > 0 {
+		line := buf
+		n := 78 - w.lineLen
+		if len(buf) > n {
+			line, buf = buf[:n], buf[n:]
+		} else {
+			buf = nil
+			n = len(buf)
+		}
+		w.b.Write(line)
+		w.lineLen += n
+		if len(buf) > 0 {
+			w.b.WriteString("\r\n\t")
+			w.lineLen = 1
+		}
+	}
+}
+
+// String returns the header in string form, ending with \r\n.
+func (w *HeaderWriter) String() string {
+	return w.b.String() + "\r\n"
+}