Commit f9fd12a3 authored by Paul Eggert's avatar Paul Eggert

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.
parent e82923c8
Pipeline #2866 passed with stage
in 55 minutes and 2 seconds
......@@ -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))))
......
......@@ -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));
}
......
......@@ -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'.
......
......@@ -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))
......
......@@ -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
......
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