Fix time-add rounding bug

Without this fix, time arithmetic yielded results that were not
mathematically accurate, even though the exact results were
representable; for example, (time-add 0 1e-13) yielded a timestamp
equal to 0 instead of to 1e-13.
* lisp/timezone.el (timezone-time-from-absolute):
Let time-add do its thing rather than using floating point
internally, which has rounding errors.  We now have bignums and so
don’t need floating point to avoid overflow issues.
* src/timefns.c (timeform_sub_ps_p): New function.
(time_arith): If either argument is a float, represent the
result exactly instead of discarding sub-ps info.
* test/lisp/timezone-tests.el (timezone-tests-time-from-absolute):
Don’t assume (HI LO US PS) timestamp format.
* test/src/emacs-module-tests.el (mod-test-add-nanosecond/valid):
Don’t assume that time-add discards sub-ns info.
* test/src/timefns-tests.el (time-rounding-tests):
Add regression test to detect time-add rounding bug.

diff --git a/lisp/timezone.el b/lisp/timezone.el
index ff0b266245fa8a792ab1d3e7237d67bf3f3d38b0..ce881a8c951d524245f0af40416b3958acd503a6 100644 (file)
--- a/lisp/timezone.el
+++ b/lisp/timezone.el
@@ -284,14 +284,14 @@ or an integer of the form +-HHMM, or a time zone name."
 
 (defun timezone-time-from-absolute (date seconds)
   "Compute the UTC time equivalent to DATE at time SECONDS after midnight.
-Return a list suitable as an argument to `current-time-zone',
+Return a Lisp timestamp suitable as an argument to `current-time-zone',
 or nil if the date cannot be thus represented.
 DATE is the number of days elapsed since the (imaginary)
 Gregorian date Sunday, December 31, 1 BC."
   (let* ((current-time-origin 719163)
            ;; (timezone-absolute-from-gregorian 1 1 1970)
         (days (- date current-time-origin))
-        (seconds-per-day (float 86400))
+        (seconds-per-day 86400)
         (day-seconds (* days seconds-per-day)))
     (condition-case nil (time-add day-seconds seconds)
       (range-error))))

diff --git a/src/timefns.c b/src/timefns.c
index e9d1a9bf64b0d79e7fa1045b7a202f094f04fdd9..a4c1c4cb2842b0b0431c3311e2a47eb7f986972e 100644 (file)
--- a/src/timefns.c
+++ b/src/timefns.c
@@ -661,10 +661,18 @@ enum timeform
    TIMEFORM_HI_LO_US, /* seconds plus microseconds (HI LO US) */
    TIMEFORM_NIL, /* current time in nanoseconds */
    TIMEFORM_HI_LO_US_PS, /* seconds plus micro and picoseconds (HI LO US PS) */
+   /* These two should be last; see timeform_sub_ps_p.  */
    TIMEFORM_FLOAT, /* time as a float */
    TIMEFORM_TICKS_HZ /* fractional time: HI is ticks, LO is ticks per second */
   };
 
+/* True if Lisp times of form FORM can express sub-picosecond timestamps.  */
+static bool
+timeform_sub_ps_p (enum timeform form)
+{
+  return TIMEFORM_FLOAT <= form;
+}
+
 /* From the valid form FORM and the time components HIGH, LOW, USEC
    and PSEC, generate the corresponding time value.  If LOW is
    floating point, the other components should be zero and FORM should
@@ -1016,8 +1024,8 @@ lispint_arith (Lisp_Object a, Lisp_Object b, bool subtract)
 
 /* Given Lisp operands A and B, add their values, and return the
    result as a Lisp timestamp that is in (TICKS . HZ) form if either A
-   or B are in that form, (HI LO US PS) form otherwise.  Subtract
-   instead of adding if SUBTRACT.  */
+   or B are in that form or are floats, (HI LO US PS) form otherwise.
+   Subtract instead of adding if SUBTRACT.  */
 static Lisp_Object
 time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
 {
@@ -1077,7 +1085,7 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
      otherwise the (HI LO US PS) form for backward compatibility.  */
   return (EQ (hz, make_fixnum (1))
          ? ticks
-         : aform == TIMEFORM_TICKS_HZ || bform == TIMEFORM_TICKS_HZ
+         : timeform_sub_ps_p (aform) || timeform_sub_ps_p (bform)
          ? Fcons (ticks, hz)
          : ticks_hz_list4 (ticks, hz));
 }

diff --git a/test/lisp/timezone-tests.el b/test/lisp/timezone-tests.el
index 4b5f5617ecd423eabcc552b7a9376796d930cf6e..c374042fa5d4bd2c79a3b100402dc20e963bcc29 100644 (file)
--- a/test/lisp/timezone-tests.el
+++ b/test/lisp/timezone-tests.el
@@ -135,9 +135,10 @@
   (should (equal (timezone-zone-to-minute "*invalid*") 0)))
 
 (ert-deftest timezone-tests-time-from-absolute ()
-  (should (equal (timezone-time-from-absolute (* 2020 365)  ; Jan 1 2020
-                                      (* 12 60 60)) ; 12:00
-                 '(23911 48704 0 0))))
+  (should (time-equal-p
+          (timezone-time-from-absolute (* 2020 365)  ; Jan 1 2020
+                                       (* 12 60 60)) ; 12:00
+          '(23911 48704 0 0))))
 
 ;; TODO: Write tests for timezone-tests-time-zone-from-absolute, which is a
 ;;       bit tricky since the results depend on `current-time-zone'.

diff --git a/test/src/emacs-module-tests.el b/test/src/emacs-module-tests.el
index c44c386d30b1c8bf858de68e193b1368a488d918..c5107847318c23d39eb163cddb5c222c0492d282 100644 (file)
--- a/test/src/emacs-module-tests.el
+++ b/test/src/emacs-module-tests.el
@@ -335,12 +335,15 @@ Interactively, you can try hitting \\[keyboard-quit] to quit."
                   ;; New (TICKS . HZ) format.
                   '(123456789 . 1000000000)))
     (ert-info ((format "input: %s" input))
-      (let ((result (mod-test-add-nanosecond input)))
+      (let ((result (mod-test-add-nanosecond input))
+           (desired-result
+            (let ((hz 1000000000))
+              (time-add (time-convert input hz) (cons 1 hz)))))
         (should (consp result))
         (should (integerp (car result)))
         (should (integerp (cdr result)))
         (should (cl-plusp (cdr result)))
-        (should (time-equal-p result (time-add input '(0 0 0 1000))))))))
+        (should (time-equal-p result desired-result))))))
 
 (ert-deftest mod-test-add-nanosecond/nil ()
   (should (<= (float-time (mod-test-add-nanosecond nil))

diff --git a/test/src/timefns-tests.el b/test/src/timefns-tests.el
index 1b1032deaa10280c34f045a012095cae042c0c57..362e7655a91dd07b8fcdbcd5871f50521ee8f941 100644 (file)
--- a/test/src/timefns-tests.el
+++ b/test/src/timefns-tests.el
@@ -145,6 +145,9 @@
                      (< 0.99 (/ (- (float-time a)) (float-time b))
                         1.01))))))))
 
+(ert-deftest time-rounding-tests ()
+  (should (time-equal-p 1e-13 (time-add 0 1e-13))))
+
 (ert-deftest encode-time-dst-numeric-zone ()
     "Check for Bug#35502."
     (should (time-equal-p