package mail

import (
	"bytes"
	"fmt"
	htmltmpl "html/template"
	"strings"

	"github.com/yuin/goldmark"
)

// Announcement is one product email to one person: a feature launch, a guide,
// an offer. vantage-admin's announce package builds it per recipient, because
// the unsubscribe links carry that person's token.
type Announcement struct {
	CategoryLabel  string // "New features"
	Subject        string
	BodyMarkdown   string
	UnsubscribeURL string // RFC 8058 one-click target for this category
	PreferencesURL string // the no-login preferences page
	PostalAddress  string // optional footer line
}

type announcementData struct {
	Announcement
	BodyHTML htmltmpl.HTML
}

// md renders with goldmark's defaults, which omit raw HTML: a pasted <script>
// never reaches an inbox.
var md = goldmark.New()

func announcementMessage(a Announcement, publicURL string) (message, error) {
	var buf bytes.Buffer
	if err := md.Convert([]byte(a.BodyMarkdown), &buf); err != nil {
		return message{}, fmt.Errorf("mail: markdown: %w", err)
	}
	return render("announcement", announcementData{Announcement: a, BodyHTML: htmltmpl.HTML(buf.String())}, publicURL)
}

// RenderAnnouncement is the staff preview: exactly what a recipient gets.
func RenderAnnouncement(a Announcement, publicURL string) (subject, html, text string, err error) {
	m, err := announcementMessage(a, publicURL)
	if err != nil {
		return "", "", "", err
	}
	return m.Subject, m.HTML, m.Text, nil
}

// SendAnnouncement sends one announcement to one address, with the
// List-Unsubscribe pair Gmail and Yahoo require of bulk senders.
func (s Sender) SendAnnouncement(to string, a Announcement) error {
	if strings.Contains(to, ",") {
		return fmt.Errorf("mail: an announcement goes to exactly one address")
	}
	if a.UnsubscribeURL == "" {
		return fmt.Errorf("mail: announcement without an unsubscribe URL")
	}
	m, err := announcementMessage(a, s.PublicURL)
	if err != nil {
		return err
	}
	m.To = to
	m.Headers = map[string]string{
		"List-Unsubscribe":      "<" + a.UnsubscribeURL + ">",
		"List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
	}
	return s.send(m)
}