ultisuite-backend/internal/mail/sanitize/preheader.go

package sanitize

import (
	"bytes"
	"regexp"
	"strings"

	"golang.org/x/net/html"
)

// StripHiddenEmailHTML removes invisible preheader / preview blocks common in marketing mail.
// Must run before bluemonday, which strips display:none styles and would expose padding text.
func StripHiddenEmailHTML(raw string) string {
	if raw == "" {
		return raw
	}
	doc, err := html.Parse(strings.NewReader(raw))
	if err != nil {
		return stripHiddenEmailHTMLRegex(raw)
	}
	var remove []*html.Node
	var walk func(*html.Node)
	walk = func(n *html.Node) {
		if n.Type == html.ElementNode && shouldStripHiddenElement(n) {
			remove = append(remove, n)
			return
		}
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			walk(c)
		}
	}
	walk(doc)
	for _, n := range remove {
		if n.Parent != nil {
			n.Parent.RemoveChild(n)
		}
	}
	var buf bytes.Buffer
	if err := html.Render(&buf, doc); err != nil {
		return stripHiddenEmailHTMLRegex(raw)
	}
	return buf.String()
}

func shouldStripHiddenElement(n *html.Node) bool {
	if n.Type != html.ElementNode {
		return false
	}
	if attrVal(n, "hidden") != "" {
		return true
	}
	if strings.EqualFold(attrVal(n, "aria-hidden"), "true") {
		return true
	}
	class := strings.ToLower(attrVal(n, "class"))
	if strings.Contains(class, "mcnpreviewtext") ||
		strings.Contains(class, "preheader") ||
		strings.Contains(class, "preview-text") {
		return true
	}
	style := strings.ToLower(attrVal(n, "style"))
	if style == "" {
		return false
	}
	styleCompact := strings.ReplaceAll(style, " ", "")
	if strings.Contains(styleCompact, "display:none") ||
		strings.Contains(styleCompact, "mso-hide:all") ||
		strings.Contains(styleCompact, "max-height:0") ||
		strings.Contains(styleCompact, "opacity:0") ||
		strings.Contains(styleCompact, "visibility:hidden") ||
		strings.Contains(styleCompact, "overflow:hidden") && strings.Contains(styleCompact, "max-height:0") {
		return true
	}
	if strings.Contains(styleCompact, "font-size:0") && !hasSignificantChildElements(n) {
		return true
	}
	return false
}

// hasSignificantChildElements returns true when n contains child elements
// beyond trivial void elements (br, wbr, hr). Parents with font-size:0 that
// contain real child elements are layout wrappers (inline-block whitespace
// collapse), not hidden preheader text.
func hasSignificantChildElements(n *html.Node) bool {
	for c := n.FirstChild; c != nil; c = c.NextSibling {
		if c.Type != html.ElementNode {
			continue
		}
		tag := strings.ToLower(c.Data)
		if tag == "br" || tag == "wbr" || tag == "hr" {
			continue
		}
		return true
	}
	return false
}

func attrVal(n *html.Node, key string) string {
	for _, a := range n.Attr {
		if strings.EqualFold(a.Key, key) {
			return a.Val
		}
	}
	return ""
}

func stripHiddenEmailHTMLRegex(raw string) string {
	patterns := []*regexp.Regexp{
		regexp.MustCompile(`(?is)<span[^>]*class="[^"]*mcnPreviewText[^"]*"[^>]*>.*?</span>`),
		regexp.MustCompile(`(?is)<div[^>]*style="[^"]*display\s*:\s*none[^"]*"[^>]*>.*?</div>`),
	}
	out := raw
	for _, re := range patterns {
		out = re.ReplaceAllString(out, "")
	}
	return out
}

func isInvisiblePaddingRune(r rune) bool {
	switch r {
	case '\u034f', '\u200b', '\u200c', '\u200d', '\u200e', '\u200f', '\ufeff', '\u00a0', '\u2007':
		return true
	default:
		return false
	}
}

// StripInvisibleTextRuns removes repeated invisible Unicode padding from plain text.
// Line breaks are preserved so reply quotes stay splittable in the UI.
func StripInvisibleTextRuns(s string) string {
	if s == "" {
		return s
	}
	var b strings.Builder
	b.Grow(len(s))
	for _, r := range s {
		if isInvisiblePaddingRune(r) {
			continue
		}
		b.WriteRune(r)
	}
	lines := strings.Split(b.String(), "\n")
	for i, line := range lines {
		lines[i] = strings.Join(strings.Fields(line), " ")
	}
	return strings.TrimSpace(strings.Join(lines, "\n"))
}