separate template logic into own file

Signed-off-by: Naman Sood <mail@nsood.in>

template.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/aymerick/raymond"
+)
+
+func loadTemplate(file string) (*raymond.Template, error) {
+	tpl, err := raymond.ParseFile("templates/" + file + ".html")
+	if err != nil {
+		return nil, fmt.Errorf("Could not parse %s template: %w", file, err)
+	}
+	tpl.RegisterHelper("datetime", func(timeStr string) string {
+		timestamp, err := strconv.ParseInt(timeStr, 10, 64)
+		if err != nil {
+			log.Printf("Could not parse timestamp '%v', falling back to current time", timeStr)
+			timestamp = time.Now().Unix()
+		}
+		return time.Unix(timestamp, 0).Format("Jan 2 2006, 3:04 PM")
+	})
+	log.Printf("Loaded template: %s", file)
+	return tpl, nil
+}
+
+// loadTemplates, for each f in files, loads `templates/$f.html`
+// as a handlebars HTML template. If any single template fails to
+// load, only an error is returned. Conversely, if there is no error,
+// every template name passed is guaranteed to have loaded successfully.
+func loadTemplates(files []string) (map[string]*raymond.Template, error) {
+	templates := make(map[string]*raymond.Template)
+	for _, f := range files {
+		tpl, err := loadTemplate(f)
+		if err != nil {
+			return nil, err
+		}
+		templates[f] = tpl
+	}
+	log.Printf("Loaded templates: %s", files)
+	return templates, nil
+}
+
+func newTemplateListener(update func(func(map[string]*raymond.Template))) *listener {
+	return &listener{
+		folder: "templates/",
+		update: func(file string) error {
+			tplName := strings.TrimSuffix(file, ".html")
+			newTpl, err := loadTemplate(tplName)
+			if err != nil {
+				return err
+			}
+			update(func(oldMap map[string]*raymond.Template) {
+				oldMap[tplName] = newTpl
+			})
+			return nil
+		},
+		clean: func(file string) error {
+			tplName := strings.TrimSuffix(file, ".html")
+			update(func(oldMap map[string]*raymond.Template) {
+				delete(oldMap, tplName)
+			})
+			log.Printf("Unloaded template: %s", tplName)
+			return nil
+		},
+	}
+}