package classes

import (
	"strings"
)

// Template placeholders (D-03): the content's title, its own description
// or excerpt, and the site name.
const (
	PlaceholderTitle       = "%title%"
	PlaceholderDescription = "%description%"
	PlaceholderSite        = "%site%"
)

// TemplateVars are the values a template's placeholders take.
type TemplateVars struct {
	Title       string
	Description string
	Site        string
}

// templateTrim are the separator characters left at either end when a
// placeholder renders empty ("About | " without a site name).
const templateTrim = " \t|-–—·:,"

// RenderTemplate fills a title or description template. Runs of
// whitespace collapse to one space and separators left at either end by an
// empty placeholder are trimmed. An empty template renders "".
func RenderTemplate(tpl string, vars TemplateVars) string {
	if strings.TrimSpace(tpl) == "" {
		return ""
	}
	out := strings.NewReplacer(
		PlaceholderTitle, vars.Title,
		PlaceholderDescription, vars.Description,
		PlaceholderSite, vars.Site,
	).Replace(tpl)
	out = strings.Join(strings.Fields(out), " ")
	return strings.Trim(out, templateTrim)
}