From 2f60d9c5df9a71844f06e19c429bf7ad46110fe9 Mon Sep 17 00:00:00 2001
From: Winni Neessen <winfried.neessen@cleverbridge.com>
Date: Mon, 5 Feb 2024 11:46:06 +0100
Subject: [PATCH] Add support for HTML alternative message body

The code has been updated to allow for an HTML alternative to the plain text body in messages. This includes adding a new function to add this alternative body along with additional unit tests to ensure the multipart messages are properly constructed. The tests also check the correct usage of different charsets.
---
 msgwriter_test.go | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/msgwriter_test.go b/msgwriter_test.go
index d19d334..4f61c6f 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -53,6 +53,7 @@ func TestMsgWriter_writeMsg(t *testing.T) {
 	m.SetDateWithValue(now)
 	m.SetMessageIDWithValue("message@id.com")
 	m.SetBodyString(TypeTextPlain, "This is the body")
+	m.AddAlternativeString(TypeTextHTML, "This is the alternative body")
 	buf := bytes.Buffer{}
 	mw := &msgWriter{w: &buf, c: CharsetUTF8, en: mime.QEncoding}
 	mw.writeMsg(m)
@@ -95,6 +96,26 @@ func TestMsgWriter_writeMsg(t *testing.T) {
 	if !strings.Contains(ms, "\r\n\r\nThis is the body") {
 		ea = append(ea, "Message body")
 	}
+
+	pl := m.GetParts()
+	if len(pl) <= 0 {
+		t.Errorf("expected multiple parts but got none")
+		return
+	}
+	if len(pl) == 2 {
+		ap := pl[1]
+		ap.SetCharset(CharsetISO88591)
+	}
+	buf.Reset()
+	mw.writeMsg(m)
+	ms = buf.String()
+	if !strings.Contains(ms, "\r\n\r\nThis is the alternative body") {
+		ea = append(ea, "Message alternative body")
+	}
+	if !strings.Contains(ms, `Content-Type: text/html; charset=ISO-8859-1`) {
+		ea = append(ea, "alternative body charset")
+	}
+
 	if len(ea) > 0 {
 		em := "writeMsg() failed. The following errors occurred:\n"
 		for e := range ea {