Add persitent URLs for year/month/day in the diary

diary.go

@@ -21,6 +21,67 @@ func init() {
 
 type diaryHandler struct{}
 
+// redirect resolves the persistent date links (today, this-month, this-year)
+// inside any diary root to a dated URL. Returns ok=false otherwise.
+func (d *diaryHandler) redirect(root, fsPath, urlPath string) (string, bool) {
+	base := path.Base(strings.TrimSuffix(urlPath, "/"))
+	switch base {
+	case "today", "this-month", "this-year":
+	default:
+		return "", false
+	}
+
+	parentFS := filepath.Dir(fsPath)
+	parentURLPath := parentURL(urlPath)
+	_, diaryRootFS, diaryRootURL, ok := findDiaryContext(root, parentFS, parentURLPath)
+	if !ok {
+		return "", false
+	}
+
+	now := time.Now()
+	year := fmt.Sprintf("%d", now.Year())
+	month := fmt.Sprintf("%02d", int(now.Month()))
+	day := fmt.Sprintf("%02d", now.Day())
+
+	switch base {
+	case "today":
+		target := path.Join(diaryRootURL, year, month, day) + "/"
+		dayFS := filepath.Join(diaryRootFS, year, month, day)
+		if _, err := os.Stat(dayFS); err != nil {
+			target += "?edit"
+		}
+		return target, true
+	case "this-month":
+		return path.Join(diaryRootURL, year, month) + "/", true
+	case "this-year":
+		return path.Join(diaryRootURL, year) + "/", true
+	}
+	return "", false
+}
+
+// defaultHeading returns the German long-form date as the editor pre-fill
+// heading for a diary day folder (depth 3 inside a diary root).
+func (d *diaryHandler) defaultHeading(root, fsPath, urlPath string) (string, bool) {
+	depth, _, _, ok := findDiaryContext(root, fsPath, urlPath)
+	if !ok || depth != 3 {
+		return "", false
+	}
+	day, err := strconv.Atoi(filepath.Base(fsPath))
+	if err != nil {
+		return "", false
+	}
+	monthFS := filepath.Dir(fsPath)
+	month, err := strconv.Atoi(filepath.Base(monthFS))
+	if err != nil || month < 1 || month > 12 {
+		return "", false
+	}
+	year, err := strconv.Atoi(filepath.Base(filepath.Dir(monthFS)))
+	if err != nil {
+		return "", false
+	}
+	return formatGermanDate(time.Date(year, time.Month(month), day, 0, 0, 0, 0, time.UTC)), true
+}
+
 func (d *diaryHandler) handle(root, fsPath, urlPath string) *specialPage {
 	depth, diaryRootFS, diaryRootURL, ok := findDiaryContext(root, fsPath, urlPath)
 	if !ok {