Improvements on (TICKS . HZ)

This patch is in response to Eli's review (Bug#32902#10).
* src/systime.c: Doc strings of affected functions now refer
to format-time-string instead of to Lisp manual, and
format-time-string's doc string covers time values.
* test/src/systime-tests.el (format-time-string-with-zone):
Check decode-time too.
(decode-then-encode-time, time-arith-tests): New tests.

4 files changed, 140 insertions, 71 deletions

diff --git a/doc/lispref/buffers.texi b/doc/lispref/buffers.texi
index 8789a8d56f6..b2a4b0eab1a 100644
--- a/doc/lispref/buffers.texi
+++ b/doc/lispref/buffers.texi
@@ -654,7 +654,7 @@ If the buffer has no recorded last modification time, this function
 returns zero.  This case occurs, for instance, if the buffer is not
 visiting a file or if the time has been explicitly cleared by
 @code{clear-visited-file-modtime}.  Note, however, that
-@code{visited-file-modtime} returns a list for some non-file buffers
+@code{visited-file-modtime} returns a timestamp for some non-file buffers
 too.  For instance, in a Dired buffer listing a directory, it returns
 the last modification time of that directory, as recorded by Dired.
 
diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index ea6915350e8..64c327c3809 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1457,14 +1457,14 @@ seconds east of Greenwich.
 @var{dow} and @var{utcoff}.
 @end defun
 
-@defun encode-time time &optional form
+@defun encode-time &optional time form &rest obsolescent-arguments
 This function converts @var{time} to a Lisp timestamp.
 It can act as the inverse of @code{decode-time}.
 
-The first argument can be a Lisp time value such as @code{nil} for the
-current time, a number of seconds, a pair @code{(@var{ticks}
-. @var{hz})}, or a list @code{(@var{high} @var{low} @var{micro}
-@var{pico})} (@pxref{Time of Day}).  It can also be a list
+The first argument can be a time value such as a number of seconds, a
+pair @code{(@var{ticks} . @var{hz})}, a list @code{(@var{high}
+@var{low} @var{micro} @var{pico})}, or @code{nil} (the default) for
+the current time (@pxref{Time of Day}).  It can also be a list
 @code{(@var{second} @var{minute} @var{hour} @var{day} @var{month}
 @var{year} @var{ignored} @var{dst} @var{zone})} that specifies a
 decoded time in the style of @code{decode-time}, so that
@@ -1494,10 +1494,10 @@ or more arguments.  The first six arguments @var{second},
 specify most of the components of a decoded time.  If there are more
 than six arguments the @emph{last} argument is used as @var{zone} and
 any other extra arguments are ignored, so that @code{(apply
-'encode-time (decode-time ...))} works; otherwise @var{zone} defaults
+#\\='encode-time (decode-time ...))} works; otherwise @var{zone} defaults
 to the current time zone rule (@pxref{Time Zone Rules}).  The decoded
 time's @var{dst} component is treated as if it was @minus{}1, and
-@var{form} so it takes its default value.
+@var{form} takes its default value.
 
 Year numbers less than 100 are not treated specially.  If you want them
 to stand for years above 1900, or years above 2000, you must alter them
diff --git a/src/timefns.c b/src/timefns.c
index 72cb54d3a0c..7bce3b1e500 100644
--- a/src/timefns.c
+++ b/src/timefns.c
@@ -1035,8 +1035,8 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
 }
 
 DEFUN ("time-add", Ftime_add, Stime_add, 2, 2, 0,
-       doc: /* Return the sum of two time values A and B, as a timestamp.
-See Info node `(elisp)Time of Day' for time value formats.
+       doc: /* Return the sum of two time values A and B, as a time value.
+See `format-time-string' for the various forms of a time value.
 For example, nil stands for the current time.  */)
   (Lisp_Object a, Lisp_Object b)
 {
@@ -1044,9 +1044,9 @@ For example, nil stands for the current time.  */)
 }
 
 DEFUN ("time-subtract", Ftime_subtract, Stime_subtract, 2, 2, 0,
-       doc: /* Return the difference between two time values A and B, as a timestamp.
+       doc: /* Return the difference between two time values A and B, as a time value.
 You can use `float-time' to convert the difference into elapsed seconds.
-See Info node `(elisp)Time of Day' for time value formats.
+See `format-time-string' for the various forms of a time value.
 For example, nil stands for the current time.  */)
   (Lisp_Object a, Lisp_Object b)
 {
@@ -1092,7 +1092,7 @@ time_cmp (Lisp_Object a, Lisp_Object b)
 
 DEFUN ("time-less-p", Ftime_less_p, Stime_less_p, 2, 2, 0,
        doc: /* Return non-nil if time value A is less than time value B.
-See Info node `(elisp)Time of Day' for time value formats.
+See `format-time-string' for the various forms of a time value.
 For example, nil stands for the current time.  */)
   (Lisp_Object a, Lisp_Object b)
 {
@@ -1101,7 +1101,7 @@ For example, nil stands for the current time.  */)
 
 DEFUN ("time-equal-p", Ftime_equal_p, Stime_equal_p, 2, 2, 0,
        doc: /* Return non-nil if A and B are equal time values.
-See Info node `(elisp)Time of Day' for time value formats.  */)
+See `format-time-string' for the various forms of a time value.  */)
   (Lisp_Object a, Lisp_Object b)
 {
   return time_cmp (a, b) == 0 ? Qt : Qnil;
@@ -1110,12 +1110,12 @@ See Info node `(elisp)Time of Day' for time value formats.  */)
 
 DEFUN ("float-time", Ffloat_time, Sfloat_time, 0, 1, 0,
        doc: /* Return the current time, as a float number of seconds since the epoch.
-If SPECIFIED-TIME is given, it is a Lisp time value to convert to
-float instead of the current time.  See Info node `(elisp)Time of Day'
-for time value formats.
+If SPECIFIED-TIME is given, it is a time value to convert to float
+instead of the current time.  See `format-time-string' for the various
+forms of a time value.
 
 WARNING: Since the result is floating point, it may not be exact.
-If precise time stamps are required, use either `current-time',
+If precise time stamps are required, use either `encode-time',
 or (if you need time as a string) `format-time-string'.  */)
   (Lisp_Object specified_time)
 {
@@ -1226,8 +1226,12 @@ format_time_string (char const *format, ptrdiff_t formatlen,
 }
 
 DEFUN ("format-time-string", Fformat_time_string, Sformat_time_string, 1, 3, 0,
-       doc: /* Use FORMAT-STRING to format the time TIME, or now if omitted or nil.
-TIME is a Lisp time value; see Info node `(elisp)Time of Day'.
+       doc: /* Use FORMAT-STRING to format the time value TIME.
+A time value that is omitted or nil stands for the current time,
+a number stands for that many seconds, an integer pair (TICKS . HZ)
+stands for TICKS/HZ seconds, and an integer list (HI LO US PS) stands
+for HI*2**16 + LO + US/10**6 + PS/10**12 seconds.  This function
+treats seconds as time since the epoch of 1970-01-01 00:00:00 UTC.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
@@ -1300,8 +1304,8 @@ usage: (format-time-string FORMAT-STRING &optional TIME ZONE)  */)
 
 DEFUN ("decode-time", Fdecode_time, Sdecode_time, 0, 2, 0,
        doc: /* Decode a time value as (SEC MINUTE HOUR DAY MONTH YEAR DOW DST UTCOFF).
-The optional TIME is the Lisp time value to convert.  See Info node
-`(elisp)Time of Day' for time value formats.
+The optional TIME is the time value to convert.  See
+`format-time-string' for the various forms of a time value.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
@@ -1381,22 +1385,23 @@ check_tm_member (Lisp_Object obj, int offset)
 }
 
 DEFUN ("encode-time", Fencode_time, Sencode_time, 1, MANY, 0,
-       doc: /* Convert TIME to a timestamp.
+       doc: /* Convert optional TIME to a timestamp.
 Optional FORM specifies how the returned value should be encoded.
 This can act as the reverse operation of `decode-time', which see.
 
 If TIME is a list (SECOND MINUTE HOUR DAY MONTH YEAR IGNORED DST ZONE)
-it a decoded time in the style of `decode-time', so that (encode-time
-(decode-time ...)) works.  TIME can also be a Lisp time value; see
-Info node `(elisp)Time of Day'.
+it is a decoded time in the style of `decode-time', so that (encode-time
+(decode-time ...)) works.  TIME can also be a time value.
+See `format-time-string' for the various forms of a time value.
+For example, an omitted TIME stands for the current time.
 
 If FORM is a positive integer, the time is returned as a pair of
 integers (TICKS . FORM), where TICKS is the number of clock ticks and FORM
 is the clock frequency in ticks per second.  (Currently the positive
 integer should be at least 65536 if the returned value is expected to
 be given to standard functions expecting Lisp timestamps.)  If FORM is
-t, the time is returned as (TICKS . PHZ), where PHZ is a
-platform-dependent clock frequency.  If FORM is `integer', the time is
+t, the time is returned as (TICKS . PHZ), where PHZ is a platform dependent
+clock frequency in ticks per second.  If FORM is `integer', the time is
 returned as an integer count of seconds.  If FORM is `list', the time is
 returned as an integer list (HIGH LOW USEC PSEC), where HIGH has the
 most significant bits of the seconds, LOW has the least significant 16
@@ -1405,11 +1410,12 @@ Returned values are rounded toward minus infinity.  Although an
 omitted or nil FORM currently acts like `list', this is planned to
 change, so callers requiring list timestamps should specify `list'.
 
-As an obsolescent calling convention, the first 6 arguments SECOND,
-MINUTE, HOUR, DAY, MONTH, and YEAR specify the components of a decoded
-time, where DST assumed to be -1 and FORM is omitted.  If there are more
+As an obsolescent calling convention, if this function is called with
+6 or more arguments, the first 6 arguments are SECOND, MINUTE, HOUR,
+DAY, MONTH, and YEAR, and specify the components of a decoded time,
+where DST assumed to be -1 and FORM is omitted.  If there are more
 than 6 arguments the *last* argument is used as ZONE and any other
-extra arguments are ignored, so that (apply \\='encode-time
+extra arguments are ignored, so that (apply #\\='encode-time
 (decode-time ...)) works; otherwise ZONE is assumed to be nil.
 
 If the input is a decoded time, ZONE is nil for Emacs local time, t
@@ -1430,7 +1436,7 @@ If you want them to stand for years in this century, you must do that yourself.
 Years before 1970 are not guaranteed to work.  On some systems,
 year values as low as 1901 do work.
 
-usage: (encode-time TIME &optional FORM)  */)
+usage: (encode-time &optional TIME FORM &rest OBSOLESCENT-ARGUMENTS)  */)
   (ptrdiff_t nargs, Lisp_Object *args)
 {
   time_t value;
@@ -1490,13 +1496,13 @@ usage: (encode-time TIME &optional FORM)  */)
 }
 
 DEFUN ("current-time", Fcurrent_time, Scurrent_time, 0, 0, 0,
-       doc: /* Return the current time, counting the number of seconds since the epoch.
-
-See Info node `(elisp)Time of Day' for the format of the returned
-timestamp.  Although this is currently list format, it may change in
-future versions of Emacs.  Use `encode-time' if you need a particular
-form; for example, (encode-time nil \\='list) returns the current time
-in list form.  */)
+       doc: /* Return the current time, as the number of seconds since 1970-01-01 00:00:00.
+The time is returned as a list of integers (HIGH LOW USEC PSEC).
+HIGH has the most significant bits of the seconds, while LOW has the
+least significant 16 bits.  USEC and PSEC are the microsecond and
+picosecond counts.  Use `encode-time' if you need a particular
+timestamp form; for example, (encode-time nil \\='integer) returns the
+current time in seconds.  */)
   (void)
 {
   return make_lisp_time (current_timespec ());
@@ -1512,9 +1518,9 @@ The format is `Sun Sep 16 01:03:52 1973'.
 However, see also the functions `decode-time' and `format-time-string'
 which provide a much more powerful and general facility.
 
-If SPECIFIED-TIME is given, it is the Lisp time value to format
-instead of the current time.  See Info node `(elisp)Time of Day' for
-time value formats.
+If SPECIFIED-TIME is given, it is the time value to format instead of
+the current time.  See `format-time-string' for the various forms of a
+time value.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
@@ -1559,7 +1565,8 @@ OFFSET is an integer number of seconds ahead of UTC (east of Greenwich).
 NAME is a string giving the name of the time zone.
 If SPECIFIED-TIME is given, the time zone offset is determined from it
 instead of using the current time.  The argument should be a Lisp
-time value; see Info node `(elisp)Time of Day'.
+time value; see `format-time-string' for the various forms of a time
+value.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
diff --git a/test/src/timefns-tests.el b/test/src/timefns-tests.el
index 435dcf7db70..ebeb43de163 100644
--- a/test/src/timefns-tests.el
+++ b/test/src/timefns-tests.el
@@ -19,7 +19,7 @@
 
 (require 'ert)
 
-;;; Check format-time-string with various TZ settings.
+;;; Check format-time-string and decode-time with various TZ settings.
 ;;; Use only POSIX-compatible TZ values, since the tests should work
 ;;; even if tzdb is not in use.
 (ert-deftest format-time-string-with-zone ()
@@ -35,32 +35,61 @@
   ;; Similarly, stick to the limited set of time zones that are
   ;; supported by both POSIX and MS-Windows: exactly 3 ASCII letters
   ;; in the abbreviation, and no DST.
-  (let ((look '(1202 22527 999999 999999))
-        (format "%Y-%m-%d %H:%M:%S.%3N %z (%Z)"))
-    ;; UTC.
-    (should (string-equal
-             (format-time-string "%Y-%m-%d %H:%M:%S.%3N %z" look t)
-             "1972-06-30 23:59:59.999 +0000"))
-    ;; "UTC0".
-    (should (string-equal
-             (format-time-string format look "UTC0")
-             "1972-06-30 23:59:59.999 +0000 (UTC)"))
-    ;; Negative UTC offset, as a Lisp list.
-    (should (string-equal
-             (format-time-string format look '(-28800 "PST"))
-             "1972-06-30 15:59:59.999 -0800 (PST)"))
-    ;; Negative UTC offset, as a Lisp integer.
-    (should (string-equal
-             (format-time-string format look -28800)
-             ;; MS-Windows build replaces unrecognizable TZ values,
-             ;; such as "-08", with "ZZZ".
-             (if (eq system-type 'windows-nt)
-                 "1972-06-30 15:59:59.999 -0800 (ZZZ)"
-               "1972-06-30 15:59:59.999 -0800 (-08)")))
-    ;; Positive UTC offset that is not an hour multiple, as a string.
-    (should (string-equal
-             (format-time-string format look "IST-5:30")
-             "1972-07-01 05:29:59.999 +0530 (IST)"))))
+  (let ((format "%Y-%m-%d %H:%M:%S.%3N %z (%Z)"))
+    (dolist (look '((1202 22527 999999 999999)
+                    (7879679999900 . 100000)
+                    (78796799999999999999 . 1000000000000)))
+      ;; UTC.
+      (should (string-equal
+               (format-time-string "%Y-%m-%d %H:%M:%S.%3N %z" look t)
+               "1972-06-30 23:59:59.999 +0000"))
+      (should (equal (decode-time look t)
+                     '(59 59 23 30 6 1972 5 nil 0)))
+      ;; "UTC0".
+      (should (string-equal
+               (format-time-string format look "UTC0")
+               "1972-06-30 23:59:59.999 +0000 (UTC)"))
+      (should (equal (decode-time look "UTC0")
+                     '(59 59 23 30 6 1972 5 nil 0)))
+      ;; Negative UTC offset, as a Lisp list.
+      (should (string-equal
+               (format-time-string format look '(-28800 "PST"))
+               "1972-06-30 15:59:59.999 -0800 (PST)"))
+      (should (equal (decode-time look '(-28800 "PST"))
+                     '(59 59 15 30 6 1972 5 nil -28800)))
+      ;; Negative UTC offset, as a Lisp integer.
+      (should (string-equal
+               (format-time-string format look -28800)
+               ;; MS-Windows build replaces unrecognizable TZ values,
+               ;; such as "-08", with "ZZZ".
+               (if (eq system-type 'windows-nt)
+                   "1972-06-30 15:59:59.999 -0800 (ZZZ)"
+                 "1972-06-30 15:59:59.999 -0800 (-08)")))
+      (should (equal (decode-time look -28800)
+                     '(59 59 15 30 6 1972 5 nil -28800)))
+      ;; Positive UTC offset that is not an hour multiple, as a string.
+      (should (string-equal
+               (format-time-string format look "IST-5:30")
+               "1972-07-01 05:29:59.999 +0530 (IST)"))
+      (should (equal (decode-time look "IST-5:30")
+                     '(59 29 5 1 7 1972 6 nil 19800))))))
+
+(ert-deftest decode-then-encode-time ()
+  (let ((time-values (list 0 -2 1 0.0 -0.0 -2.0 1.0
+                           most-negative-fixnum most-positive-fixnum
+                           (1- most-negative-fixnum)
+                           (1+ most-positive-fixnum)
+                           1e+INF -1e+INF 1e+NaN -1e+NaN
+                           '(0 1 0 0) '(1 0 0 0) '(-1 0 0 0)
+                           '(123456789000000 . 1000000)
+                           (cons (1+ most-positive-fixnum) 1000000000000)
+                           (cons 1000000000000 (1+ most-positive-fixnum)))))
+    (dolist (a time-values)
+      (let* ((d (ignore-errors (decode-time a t)))
+             (e (encode-time d))
+             (diff (float-time (time-subtract a e))))
+        (should (or (not d)
+                    (and (<= 0 diff) (< diff 1))))))))
 
 ;;; This should not dump core.
 (ert-deftest format-time-string-with-outlandish-zone ()
@@ -80,3 +109,36 @@
 
 (ert-deftest time-equal-p-nil-nil ()
   (should (time-equal-p nil nil)))
+
+(ert-deftest time-arith-tests ()
+  (let ((time-values (list 0 -1 1 0.0 -0.0 -1.0 1.0
+                           most-negative-fixnum most-positive-fixnum
+                           (1- most-negative-fixnum)
+                           (1+ most-positive-fixnum)
+                           1e+INF -1e+INF 1e+NaN -1e+NaN
+                           '(0 0 0 1) '(0 0 1 0) '(0 1 0 0) '(1 0 0 0)
+                           '(-1 0 0 0) '(1 2 3 4) '(-1 2 3 4)
+                           '(-123456789 . 100000) '(123456789 . 1000000)
+                           (cons (1+ most-positive-fixnum) 1000000000000)
+                           (cons 1000000000000 (1+ most-positive-fixnum)))))
+    (dolist (a time-values)
+      (dolist (b time-values)
+        (let ((aa (time-subtract (time-add a b) b)))
+          (should (or (time-equal-p a aa) (and (floatp aa) (isnan aa)))))
+        (should (= 1 (+ (if (time-less-p a b) 1 0)
+                        (if (time-equal-p a b) 1 0)
+                        (if (time-less-p b a) 1 0)
+                        (if (or (and (floatp a) (isnan a))
+                                (and (floatp b) (isnan b)))
+                            1 0))))
+        (should (or (not (time-less-p 0 b))
+                    (time-less-p a (time-add a b))
+                    (time-equal-p a (time-add a b))
+                    (and (floatp (time-add a b)) (isnan (time-add a b)))))
+        (let ((x (float-time (time-add a b)))
+              (y (+ (float-time a) (float-time b))))
+          (should (or (and (isnan x) (isnan y))
+                      (= x y)
+                      (< 0.99 (/ x y) 1.01)
+                      (< 0.99 (/ (- (float-time a)) (float-time b))
+                         1.01))))))))