From 8be7e98557df8ba708b3f7e285a29f279b609e46 Mon Sep 17 00:00:00 2001
From: Philipp Stephani <p.stephani2@gmail.com>
Date: Tue, 25 Mar 2025 00:12:20 +0100
Subject: [PATCH] Add an ERT explainer for 'time-equal-p'.

* lisp/emacs-lisp/ert.el (ert--explain-time-equal-p): New explainer
function.

* test/lisp/emacs-lisp/ert-tests.el (ert-test-explain-time-equal-p): New
test.
---
 lisp/emacs-lisp/ert.el            | 13 +++++++++++++
 test/lisp/emacs-lisp/ert-tests.el |  8 ++++++++
 2 files changed, 21 insertions(+)

diff --git a/lisp/emacs-lisp/ert.el b/lisp/emacs-lisp/ert.el
index c8cee72025f..ef00dc73f91 100644
--- a/lisp/emacs-lisp/ert.el
+++ b/lisp/emacs-lisp/ert.el
@@ -669,6 +669,19 @@ Return nil if they are."
 (put 'equal-including-properties 'ert-explainer
      'ert--explain-equal-including-properties)
 
+(defun ert--explain-time-equal-p (a b)
+  "Explainer function for `time-equal-p'.
+A and B are the time values to compare."
+  (declare (ftype (function (t t) list))
+           (side-effect-free t))
+  (unless (time-equal-p a b)
+    `(different-time-values
+      ,(format-time-string "%F %T.%N %Z" a t)
+      ,(format-time-string "%F %T.%N %Z" b t)
+      difference
+      ,(format-time-string "%s.%N" (time-subtract a b) t))))
+(function-put #'time-equal-p 'ert-explainer #'ert--explain-time-equal-p)
+
 ;;; Implementation of `ert-info'.
 
 ;; TODO(ohler): The name `info' clashes with
diff --git a/test/lisp/emacs-lisp/ert-tests.el b/test/lisp/emacs-lisp/ert-tests.el
index aec2c92ba81..7a08cb47d82 100644
--- a/test/lisp/emacs-lisp/ert-tests.el
+++ b/test/lisp/emacs-lisp/ert-tests.el
@@ -792,6 +792,14 @@ This macro is used to test if macroexpansion in `should' works."
                  '(char 1 "o" (different-properties-for-key a (different-atoms b foo))
                         context-before "f" context-after "o"))))
 
+(ert-deftest ert-test-explain-time-equal-p ()
+  (should-not (ert--explain-time-equal-p 123 '(0 123 0 0)))
+  (should (equal (ert--explain-time-equal-p 123 '(0 120 0 0))
+                 '(different-time-values
+                   "1970-01-01 00:02:03.000000000 UTC"
+                   "1970-01-01 00:02:00.000000000 UTC"
+                   difference "3.000000000"))))
+
 (ert-deftest ert-test-stats-set-test-and-result ()
   (let* ((test-1 (make-ert-test :name 'test-1
                                 :body (lambda () nil)))