guestguard/internal/notification/templates.go

package notification

import (
	"bytes"
	"embed"
	"fmt"
	htmltemplate "html/template"
	"io/fs"
	"path"
	"strings"
	texttemplate "text/template"
)

//go:embed templates
var templatesFS embed.FS

// TemplateName names one of the email templates. There must be a
// matching `<name>.html` and `<name>.txt` under templates/.
type TemplateName string

const (
	TmplVerification  TemplateName = "verification"
	TmplPasswordReset TemplateName = "reset"
	TmplInvitation    TemplateName = "invitation"
	TmplConfirmation  TemplateName = "confirmation"
	TmplReminder      TemplateName = "reminder"
)

// Templates renders branded transactional emails for both HTML and
// plaintext bodies. Loaded once at construction; thread-safe afterwards.
//
// Each page-level HTML template gets its own *html/template.Template
// holding a copy of the `base.html` partial. This avoids html/template's
// per-template contextual-escape pass interfering between pages that all
// define a sibling named "body".
type Templates struct {
	html map[string]*htmltemplate.Template // page-name (no ext) -> root
	text *texttemplate.Template
}

func NewTemplates() (*Templates, error) {
	root, err := fs.Sub(templatesFS, "templates")
	if err != nil {
		return nil, fmt.Errorf("templates fs: %w", err)
	}

	baseHTML, err := fs.ReadFile(root, "base.html")
	if err != nil {
		return nil, fmt.Errorf("read base.html: %w", err)
	}

	out := &Templates{
		html: make(map[string]*htmltemplate.Template),
		text: texttemplate.New("__root__"),
	}

	walk := func(p string, d fs.DirEntry, _ error) error {
		if d == nil || d.IsDir() {
			return nil
		}
		base := path.Base(p)
		if base == "base.html" {
			return nil // partial — folded into each page template below
		}
		b, err := fs.ReadFile(root, p)
		if err != nil {
			return err
		}
		switch {
		case strings.HasSuffix(p, ".html"):
			name := strings.TrimSuffix(base, ".html")
			t := htmltemplate.New(name)
			if _, err := t.Parse(string(baseHTML)); err != nil {
				return fmt.Errorf("parse _base for %s: %w", p, err)
			}
			if _, err := t.Parse(string(b)); err != nil {
				return fmt.Errorf("parse %s: %w", p, err)
			}
			out.html[name] = t
		case strings.HasSuffix(p, ".txt"):
			if _, err := out.text.New(base).Parse(string(b)); err != nil {
				return fmt.Errorf("parse %s: %w", p, err)
			}
		}
		return nil
	}
	if err := fs.WalkDir(root, ".", walk); err != nil {
		return nil, err
	}
	return out, nil
}

// Render returns (htmlBody, textBody) for the named template using data.
func (t *Templates) Render(name TemplateName, data map[string]any) (htmlBody, textBody string, err error) {
	if data == nil {
		data = map[string]any{}
	}
	if _, ok := data["Subject"]; !ok {
		data["Subject"] = "GuestGuard"
	}

	htmlTpl, ok := t.html[string(name)]
	if !ok {
		return "", "", fmt.Errorf("unknown html template %q", name)
	}

	var hBuf, tBuf bytes.Buffer
	// Each page-root template's entry point is "base" (defined by base.html).
	if err := htmlTpl.ExecuteTemplate(&hBuf, "base", data); err != nil {
		return "", "", fmt.Errorf("render html %s: %w", name, err)
	}
	if err := t.text.ExecuteTemplate(&tBuf, string(name)+".txt", data); err != nil {
		return "", "", fmt.Errorf("render text %s: %w", name, err)
	}
	return hBuf.String(), tBuf.String(), nil
}