multipart mails

smtp/sender.go

@@ -1,16 +1,64 @@
 package smtp
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	stdhtml "html"
 	"log/slog"
+	"strings"
 
 	gomail "github.com/wneessen/go-mail"
 
 	"github.com/5000K/5000mails/config"
 	"github.com/5000K/5000mails/domain"
 )
 
+var blockElements = []string{
+	"address", "article", "aside", "blockquote", "br", "dd", "details",
+	"dialog", "div", "dl", "dt", "fieldset", "figcaption", "figure",
+	"footer", "form", "h1", "h2", "h3", "h4", "h5", "h6", "header",
+	"hgroup", "hr", "li", "main", "nav", "ol", "p", "pre", "section",
+	"summary", "table", "td", "th", "tr", "ul",
+}
+
+func htmlToPlainText(src []byte) []byte {
+	var buf bytes.Buffer
+	i := 0
+	for i < len(src) {
+		if src[i] != '<' {
+			buf.WriteByte(src[i])
+			i++
+			continue
+		}
+		end := bytes.IndexByte(src[i:], '>')
+		if end == -1 {
+			buf.Write(src[i:])
+			break
+		}
+		inner := src[i+1 : i+end]
+		if len(inner) > 0 && inner[0] == '/' {
+			inner = inner[1:]
+		}
+		tagName := strings.ToLower(string(inner))
+		if sp := strings.IndexByte(tagName, ' '); sp != -1 {
+			tagName = tagName[:sp]
+		}
+		for _, bt := range blockElements {
+			if tagName == bt {
+				buf.WriteByte('\n')
+				break
+			}
+		}
+		i += end + 1
+	}
+	text := stdhtml.UnescapeString(buf.String())
+	for strings.Contains(text, "\n\n\n") {
+		text = strings.ReplaceAll(text, "\n\n\n", "\n\n")
+	}
+	return []byte(strings.TrimSpace(text))
+}
+
 type Sender struct {
 	client      *gomail.Client
 	senderEmail string
@@ -61,6 +109,7 @@ func (s *Sender) SendMail(ctx context.Context, metadata domain.MailMetadata, bod
 
 	msg.Subject(metadata.Subject)
 	msg.SetBodyString(gomail.TypeTextHTML, body)
+	msg.AddAlternativeString(gomail.TypeTextPlain, string(htmlToPlainText([]byte(body))))
 
 	if err := s.client.DialAndSendWithContext(ctx, msg); err != nil {
 		s.logger.ErrorContext(ctx, "failed to send mail",

smtp/sender_test.go

@@ -0,0 +1,53 @@
+package smtp
+
+import (
+	"testing"
+)
+
+func TestHtmlToPlainText(t *testing.T) {
+	tests := []struct {
+		name string
+		src  string
+		want string
+	}{
+		{
+			name: "strips tags and keeps text",
+			src:  "<p>Hello, <strong>world</strong>!</p>",
+			want: "Hello, world!",
+		},
+		{
+			name: "inserts newline at block boundaries",
+			src:  "<p>First</p><p>Second</p>",
+			want: "First\n\nSecond",
+		},
+		{
+			name: "unescapes html entities",
+			src:  "<p>a &amp; b &lt;3</p>",
+			want: "a & b <3",
+		},
+		{
+			name: "collapses excessive blank lines",
+			src:  "<h1>Title</h1><br><br><p>Body</p>",
+			want: "Title\n\nBody",
+		},
+		{
+			name: "handles unclosed tag at end",
+			src:  "<p>Truncated <b",
+			want: "Truncated <b",
+		},
+		{
+			name: "empty input",
+			src:  "",
+			want: "",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := string(htmlToPlainText([]byte(tc.src)))
+			if got != tc.want {
+				t.Errorf("htmlToPlainText(%q)\n got  %q\n want %q", tc.src, got, tc.want)
+			}
+		})
+	}
+}