time-equal, and time values of infinity and NaN

* doc/lispref/os.texi (Time Calculations):
Document time-equal, and the behavior on NaNs and infinities of
time-less-p, time-add, time-subtract.
* etc/NEWS: Mention the change.
* src/editfns.c (time_arith): Change last arg from function
to bool.  All callers changed.  Do the right thing with
infinities and NaNs.
(time_cmp): New function, which handlesx infinities and NaNs.
(Ftime_less_p): Use it.
(Ftime_equal): New function.
* test/lisp/emacs-lisp/timer-tests.el (timer-test-multiple-of-time):
Use it.

doc/lispref/os.texi

@@ -1736,17 +1736,26 @@ integer number stands for the number of seconds since the epoch.
 @defun time-less-p t1 t2
 This returns @code{t} if time value @var{t1} is less than time value
 @var{t2}.
+The result is @code{nil} if either argument is a NaN.
+@end defun
+
+@defun time-equal t1 t2
+This returns @code{t} if @var{t1} and @var{t2} are equal time values.
+The result is @code{nil} if either argument is a NaN.
 @end defun
 
 @defun time-subtract t1 t2
 This returns the time difference @var{t1} @minus{} @var{t2} between
-two time values, as a time value.  If you need the difference in units
+two time values, as a time value.  However, the result is a float
+if either argument is a float infinity or NaN@.
+If you need the difference in units
 of elapsed seconds, use @code{float-time} (@pxref{Time of Day,
 float-time}) to convert the result into seconds.
 @end defun
 
 @defun time-add t1 t2
 This returns the sum of two time values, as a time value.
+However, the result is a float if either argument is a float infinity or NaN@.
 One argument should represent a time difference rather than a point in time,
 either as a list or as a single number of elapsed seconds.
 Here is how to add a number of seconds to a time value:

etc/NEWS

@@ -951,6 +951,14 @@ like file-attributes that compute file sizes and other attributes,
 functions like process-id that compute process IDs, and functions like
 user-uid and group-gid that compute user and group IDs.
 
++++
+** 'time-add', 'time-subtract', and 'time-less-p' now accept
+infinities and NaNs too, and propagate them or return nil like
+floating-point operators do.
+
++++
+** New function 'time-equal' compares time values for equality.
+
 ** define-minor-mode automatically documents the meaning of ARG.
 
 +++

src/editfns.c

@@ -1589,13 +1589,21 @@ time_subtract (struct lisp_time ta, struct lisp_time tb)
 }
 
 static Lisp_Object
-time_arith (Lisp_Object a, Lisp_Object b,
-	    struct lisp_time (*op) (struct lisp_time, struct lisp_time))
+time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
 {
+  if (FLOATP (a) && !isfinite (XFLOAT_DATA (a)))
+    {
+      double da = XFLOAT_DATA (a);
+      double db = XFLOAT_DATA (Ffloat_time (b));
+      return make_float (subtract ? da - db : da + db);
+    }
+  if (FLOATP (b) && !isfinite (XFLOAT_DATA (b)))
+    return subtract ? make_float (-XFLOAT_DATA (b)) : b;
+
   int alen, blen;
   struct lisp_time ta = lisp_time_struct (a, &alen);
   struct lisp_time tb = lisp_time_struct (b, &blen);
-  struct lisp_time t = op (ta, tb);
+  struct lisp_time t = (subtract ? time_subtract : time_add) (ta, tb);
   if (FIXNUM_OVERFLOW_P (t.hi))
     time_overflow ();
   Lisp_Object val = Qnil;
@@ -1623,7 +1631,7 @@ A nil value for either argument stands for the current time.
 See `current-time-string' for the various forms of a time value.  */)
   (Lisp_Object a, Lisp_Object b)
 {
-  return time_arith (a, b, time_add);
+  return time_arith (a, b, false);
 }
 
 DEFUN ("time-subtract", Ftime_subtract, Stime_subtract, 2, 2, 0,
@@ -1633,7 +1641,30 @@ A nil value for either argument stands for the current time.
 See `current-time-string' for the various forms of a time value.  */)
   (Lisp_Object a, Lisp_Object b)
 {
-  return time_arith (a, b, time_subtract);
+  return time_arith (a, b, true);
+}
+
+/* Return negative, 0, positive if a < b, a == b, a > b respectively.
+   Return positive if either a or b is a NaN; this is good enough
+   for the current callers.  */
+static int
+time_cmp (Lisp_Object a, Lisp_Object b)
+{
+  if ((FLOATP (a) && !isfinite (XFLOAT_DATA (a)))
+      || (FLOATP (b) && !isfinite (XFLOAT_DATA (b))))
+    {
+      double da = FLOATP (a) ? XFLOAT_DATA (a) : 0;
+      double db = FLOATP (b) ? XFLOAT_DATA (b) : 0;
+      return da < db ? -1 : da != db;
+    }
+
+  int alen, blen;
+  struct lisp_time ta = lisp_time_struct (a, &alen);
+  struct lisp_time tb = lisp_time_struct (b, &blen);
+  return (ta.hi != tb.hi ? (ta.hi < tb.hi ? -1 : 1)
+	  : ta.lo != tb.lo ? (ta.lo < tb.lo ? -1 : 1)
+	  : ta.us != tb.us ? (ta.us < tb.us ? -1 : 1)
+	  : ta.ps < tb.ps ? -1 : ta.ps != tb.ps);
 }
 
 DEFUN ("time-less-p", Ftime_less_p, Stime_less_p, 2, 2, 0,
@@ -1642,14 +1673,14 @@ A nil value for either argument stands for the current time.
 See `current-time-string' for the various forms of a time value.  */)
   (Lisp_Object t1, Lisp_Object t2)
 {
-  int t1len, t2len;
-  struct lisp_time a = lisp_time_struct (t1, &t1len);
-  struct lisp_time b = lisp_time_struct (t2, &t2len);
-  return ((a.hi != b.hi ? a.hi < b.hi
-	   : a.lo != b.lo ? a.lo < b.lo
-	   : a.us != b.us ? a.us < b.us
-	   : a.ps < b.ps)
-	  ? Qt : Qnil);
+  return time_cmp (t1, t2) < 0 ? Qt : Qnil;
+}
+
+DEFUN ("time-equal", Ftime_equal, Stime_equal, 2, 2, 0,
+       doc: /* Return non-nil if T1 and T2 are equal time values.  */)
+  (Lisp_Object t1, Lisp_Object t2)
+{
+  return time_cmp (t1, t2) == 0 ? Qt : Qnil;
 }
 
 
@@ -5734,6 +5765,7 @@ it to be non-nil.  */);
   defsubr (&Scurrent_time);
   defsubr (&Stime_add);
   defsubr (&Stime_subtract);
+  defsubr (&Stime_equal);
   defsubr (&Stime_less_p);
   defsubr (&Sget_internal_run_time);
   defsubr (&Sformat_time_string);

test/lisp/emacs-lisp/timer-tests.el

@@ -40,10 +40,8 @@
       (should (debug-timer-check)) t))
 
 (ert-deftest timer-test-multiple-of-time ()
-  (should (zerop
-	   (float-time
-	    (time-subtract
-	     (timer-next-integral-multiple-of-time '(0 0 0 1) (1+ (ash 1 53)))
-	     (list (ash 1 (- 53 16)) 1))))))
+  (should (time-equal
+	   (timer-next-integral-multiple-of-time '(0 0 0 1) (1+ (ash 1 53)))
+	   (list (ash 1 (- 53 16)) 1))))
 
 ;;; timer-tests.el ends here