Fix timer.el minor rounding error

* lisp/emacs-lisp/timer.el (timer-next-integral-multiple-of-time):
Fix rounding error by using integers rather than floats.
* test/lisp/emacs-lisp/timer-tests.el (timer-test-multiple-of-time):
New test.

2 files changed, 10 insertions, 5 deletions

diff --git a/lisp/emacs-lisp/timer.el b/lisp/emacs-lisp/timer.el
index 795554fec58..74d37b0eaed 100644
--- a/lisp/emacs-lisp/timer.el
+++ b/lisp/emacs-lisp/timer.el
@@ -102,14 +102,14 @@ fire each time Emacs is idle for that many seconds."
   "Yield the next value after TIME that is an integral multiple of SECS.
 More precisely, the next value, after TIME, that is an integral multiple
 of SECS seconds since the epoch.  SECS may be a fraction."
-  (let* ((trillion 1e12)
+  (let* ((trillion 1000000000000)
          (time-sec (+ (nth 1 time)
-                      (* 65536.0 (nth 0 time))))
+                      (* 65536 (nth 0 time))))
          (delta-sec (mod (- time-sec) secs))
-         (next-sec (+ time-sec (ffloor delta-sec)))
-         (next-sec-psec (ffloor (* trillion (mod delta-sec 1))))
+         (next-sec (+ time-sec (floor delta-sec)))
+         (next-sec-psec (floor (* trillion (mod delta-sec 1))))
          (sub-time-psec (+ (or (nth 3 time) 0)
-                           (* 1e6 (nth 2 time))))
+                           (* 1000000 (nth 2 time))))
          (psec-diff (- sub-time-psec next-sec-psec)))
     (if (and (<= next-sec time-sec) (< 0 psec-diff))
         (setq next-sec-psec (+ sub-time-psec
diff --git a/test/lisp/emacs-lisp/timer-tests.el b/test/lisp/emacs-lisp/timer-tests.el
index 65e5dc9bde9..fa92c1b64aa 100644
--- a/test/lisp/emacs-lisp/timer-tests.el
+++ b/test/lisp/emacs-lisp/timer-tests.el
@@ -39,4 +39,9 @@
   (if (fboundp 'debug-timer-check)
       (should (debug-timer-check)) t))
 
+(ert-deftest timer-test-multiple-of-time ()
+  (should (equal
+           (timer-next-integral-multiple-of-time '(0 0 0 1) (1+ (ash 1 53)))
+           (list (ash 1 (- 53 16)) 1 0 0))))
+
 ;;; timer-tests.el ends here