added auto html to plain text mail generation

tools/mailer/html2text.go

@@ -0,0 +1,105 @@
+package mailer
+
+import (
+	"regexp"
+	"strings"
+
+	"github.com/pocketbase/pocketbase/tools/list"
+	"golang.org/x/net/html"
+)
+
+var whitespaceRegex = regexp.MustCompile(`\s+`)
+
+// Very rudimentary auto HTML to Text mail body converter.
+//
+// Caveats:
+// - This method doesn't check for correctness of the HTML document.
+// - Links will be converted to "[text](url)" format.
+// - List items (<li>) are prefixed with "- ".
+// - Indentation is stripped (both tabs and spaces).
+// - Trailing spaces are preserved.
+// - Multiple consequence newlines are collapsed as one unless multiple <br> tags are used.
+func html2Text(htmlDocument string) (string, error) {
+	var builder strings.Builder
+
+	doc, err := html.Parse(strings.NewReader(htmlDocument))
+	if err != nil {
+		return "", err
+	}
+
+	tagsToSkip := []string{
+		"style", "script", "iframe", "applet", "object", "svg", "img",
+		"button", "form", "textarea", "input", "select", "option", "template",
+	}
+
+	inlineTags := []string{
+		"a", "span", "small", "strike", "strong",
+		"sub", "sup", "em", "b", "u", "i",
+	}
+
+	var canAddNewLine bool
+
+	// see https://pkg.go.dev/golang.org/x/net/html#Parse
+	var f func(*html.Node)
+	f = func(n *html.Node) {
+		// start link wrapping for producing "[text](link)" formatted string
+		isLink := n.Type == html.ElementNode && n.Data == "a"
+		if isLink {
+			builder.WriteString("[")
+		}
+
+		switch n.Type {
+		case html.TextNode:
+			txt := whitespaceRegex.ReplaceAllString(n.Data, " ")
+
+			// the prev node has new line so it is safe to trim the indentation
+			if !canAddNewLine {
+				txt = strings.TrimLeft(txt, " ")
+			}
+
+			if txt != "" {
+				builder.WriteString(txt)
+				canAddNewLine = true
+			}
+		case html.ElementNode:
+			if n.Data == "br" {
+				// always write new lines when <br> tag is used
+				builder.WriteString("\r\n")
+				canAddNewLine = false
+			} else if canAddNewLine && !list.ExistInSlice(n.Data, inlineTags) {
+				builder.WriteString("\r\n")
+				canAddNewLine = false
+			}
+
+			// prefix list items with dash
+			if n.Data == "li" {
+				builder.WriteString("- ")
+			}
+		}
+
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
+			if c.Type != html.ElementNode || !list.ExistInSlice(c.Data, tagsToSkip) {
+				f(c)
+			}
+		}
+
+		// end link wrapping
+		if isLink {
+			builder.WriteString("]")
+			for _, a := range n.Attr {
+				if a.Key == "href" {
+					if a.Val != "" {
+						builder.WriteString("(")
+						builder.WriteString(a.Val)
+						builder.WriteString(")")
+					}
+					break
+				}
+			}
+		}
+	}
+
+	f(doc)
+
+	return strings.TrimSpace(builder.String()), nil
+}