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+;;; test-calendar-sync--parse-byday-entry.el --- Tests for calendar-sync--parse-byday-entry  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for parsing a single RRULE BYDAY entry ("2WE", "-1TU", "SU") into
+;; an (ordinal . weekday-number) cons.  Ordinal is nil for a bare weekday.
+
+;;; Code:
+
+(require 'ert)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--parse-byday-entry-normal-positive-ordinal ()
+  "Normal: \"2WE\" parses to ordinal 2, Wednesday (3)."
+  (should (equal (calendar-sync--parse-byday-entry "2WE") '(2 . 3))))
+
+(ert-deftest test-calendar-sync--parse-byday-entry-normal-negative-ordinal ()
+  "Normal: \"-1TU\" parses to ordinal -1, Tuesday (2)."
+  (should (equal (calendar-sync--parse-byday-entry "-1TU") '(-1 . 2))))
+
+(ert-deftest test-calendar-sync--parse-byday-entry-normal-bare-weekday ()
+  "Normal: \"SU\" parses to nil ordinal, Sunday (7)."
+  (should (equal (calendar-sync--parse-byday-entry "SU") '(nil . 7))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--parse-byday-entry-boundary-double-digit-ordinal ()
+  "Boundary: \"53MO\" (yearly-scale ordinal) parses without truncation."
+  (should (equal (calendar-sync--parse-byday-entry "53MO") '(53 . 1))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--parse-byday-entry-error-garbage-nil ()
+  "Error: an unrecognizable entry returns nil."
+  (should (null (calendar-sync--parse-byday-entry "XX")))
+  (should (null (calendar-sync--parse-byday-entry "")))
+  (should (null (calendar-sync--parse-byday-entry nil))))
+
+(provide 'test-calendar-sync--parse-byday-entry)
+;;; test-calendar-sync--parse-byday-entry.el ends here