app/email/engine.go

package email

import (
	"bytes"
	_ "embed"
	"html/template"
	"strings"

	"github.com/vanng822/go-premailer/premailer"
	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/extension"
)

//go:embed base.gohtml
var mailTmpl string

var engine *template.Template
var app App

var mdParser = goldmark.New(
	goldmark.WithExtensions(extension.GFM),
)

func init() {
	engine = template.Must(template.New("email").Funcs(template.FuncMap{
		"ToHTML": ToHTML,
		"add": func(a, b int) int {
			return a + b
		},
		"mul": func(a, b int) int {
			return a * b
		},
		"replace": func(from, to, input string) string {
			return strings.Replace(input, from, to, -1)
		},
	}).Parse(mailTmpl))
}

func ToHTML(mdtext string) (template.HTML, error) {
	var buf bytes.Buffer
	if err := mdParser.Convert([]byte(mdtext), &buf); err != nil {
		return "", err
	}
	return template.HTML(buf.String()), nil
}

func Parse(b *Body) (string, error) {
	merge(&b.App)
	buf := &bytes.Buffer{}
	err := engine.Execute(buf, b)
	if err != nil {
		return "", err
	}
	pre, err := premailer.NewPremailerFromString(buf.String(), premailer.NewOptions())
	if err != nil {
		return "", err
	}
	html, err := pre.Transform()
	return html, err
}

func merge(base *App) {
	if base.Name == "" {
		base.Name = app.Name
	}
	if base.Link == "" {
		base.Link = app.Link
	}
	if base.Logo == "" {
		base.Logo = app.Logo
	}
	if base.Copyright == "" {
		base.Copyright = app.Copyright
	}
	if base.TroubleText == "" {
		base.TroubleText = app.TroubleText
	}
}