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   1  ;;; cal-dst.el --- calendar functions for daylight savings rules.
   2  
   3  ;; Copyright (C) 1993, 1994 Free Software Foundation, Inc.
   4  
   5  ;; Author: Paul Eggert <eggert@twinsun.com>
   6  ;;    Edward M. Reingold <reingold@cs.uiuc.edu>
   7  ;; Keywords: calendar
   8  ;; Human-Keywords: daylight savings time, calendar, diary, holidays
   9  
  10  ;; This file is part of XEmacs.
  11  
  12  ;; XEmacs is free software; you can redistribute it and/or modify it
  13  ;; under the terms of the GNU General Public License as published by
  14  ;; the Free Software Foundation; either version 2, or (at your option)
  15  ;; any later version.
  16  
  17  ;; XEmacs is distributed in the hope that it will be useful, but
  18  ;; WITHOUT ANY WARRANTY; without even the implied warranty of
  19  ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  20  ;; General Public License for more details.
  21  
  22  ;; You should have received a copy of the GNU General Public License
  23  ;; along with XEmacs; see the file COPYING.  If not, write to the Free
  24  ;; Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
  25  
  26  ;;; Commentary:
  27  
  28  ;; This collection of functions implements the features of calendar.el and
  29  ;; holiday.el that deal with daylight savings time.
  30  
  31  ;; Comments, corrections, and improvements should be sent to
  32  ;;  Edward M. Reingold               Department of Computer Science
  33  ;;  (217) 333-6733                   University of Illinois at Urbana-Champaign
  34  ;;  reingold@cs.uiuc.edu             1304 West Springfield Avenue
  35  ;;                                   Urbana, Illinois 61801
  36  
  37  ;;; Code:
  38  
  39  ;(require 'calendar)
  40  
  41  (defvar calendar-current-time-zone-cache nil
  42    "Cache for result of calendar-current-time-zone.")
  43  
  44  (defvar calendar-system-time-basis
  45    (calendar-absolute-from-gregorian '(1 1 1900))
  46    "Absolute date of starting date of system clock.")
  47  
  48  (defun calendar-absolute-from-time (x utc-diff)
  49    "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
  50  
  51  X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
  52  high and low 16 bits, respectively, of the number of seconds since
  53  1970-01-01 00:00:00 UTC, ignoring leap seconds.
  54  
  55  Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
  56  absolute date ABS-DATE is the equivalent moment to X."
  57    (let* ((h (car x))
  58       (xtail (cdr x))
  59           (l (+ utc-diff (if (numberp xtail) xtail (car xtail))))
  60           (u (+ (* 512 (mod h 675)) (floor l 128))))
  61      ;; Overflow is a terrible thing!
  62      (cons (+ calendar-system-time-basis
  63           ;; floor((2^16 h +l) / (60*60*24))
  64           (* 512 (floor h 675)) (floor u 675))
  65        ;; (2^16 h +l) mod (60*60*24)
  66        (+ (* (mod u 675) 128) (mod l 128)))))
  67  
  68  (defun calendar-time-from-absolute (abs-date s)
  69    "Time of absolute date ABS-DATE, S seconds after midnight.
  70  
  71  Returns the pair (HIGH . LOW) where HIGH and LOW are the high and low
  72  16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
  73  ignoring leap seconds, that is the equivalent moment to S seconds after
  74  midnight UTC on absolute date ABS-DATE."
  75    (let* ((a (- abs-date calendar-system-time-basis))
  76           (u (+ (* 163 (mod a 512)) (floor s 128))))
  77      ;; Overflow is a terrible thing!
  78      (cons
  79       ;; floor((60*60*24*a + s) / 2^16)
  80       (+ a (* 163 (floor a 512)) (floor u 512))
  81       ;; (60*60*24*a + s) mod 2^16
  82       (+ (* 128 (mod u 512)) (mod s 128)))))
  83  
  84  (defun calendar-next-time-zone-transition (time)
  85    "Return the time of the next time zone transition after TIME.
  86  Both TIME and the result are acceptable arguments to current-time-zone.
  87  Return nil if no such transition can be found."
  88    (let* ((base 65536);; 2^16 = base of current-time output
  89       (quarter-multiple 120);; approx = (seconds per quarter year) / base
  90       (time-zone (current-time-zone time))
  91       (time-utc-diff (car time-zone))
  92           hi
  93       hi-zone
  94           (hi-utc-diff time-utc-diff)
  95           (quarters '(2 1 3)))
  96      ;; Heuristic: probe the time zone offset in the next three calendar
  97      ;; quarters, looking for a time zone offset different from TIME.
  98      (while (and quarters (eq time-utc-diff hi-utc-diff))
  99        (setq hi (cons (+ (car time) (* (car quarters) quarter-multiple)) 0))
 100        (setq hi-zone (current-time-zone hi))
 101        (setq hi-utc-diff (car hi-zone))
 102        (setq quarters (cdr quarters)))
 103      (and
 104       time-utc-diff
 105       hi-utc-diff
 106       (not (eq time-utc-diff hi-utc-diff))
 107       ;; Now HI is after the next time zone transition.
 108       ;; Set LO to TIME, and then binary search to increase LO and decrease HI
 109       ;; until LO is just before and HI is just after the time zone transition.
 110       (let* ((tail (cdr time))
 111          (lo (cons (car time) (if (numberp tail) tail (car tail))))
 112          probe)
 113         (while
 114         ;; Set PROBE to halfway between LO and HI, rounding down.
 115         ;; If PROBE equals LO, we are done.
 116         (let* ((lsum (+ (cdr lo) (cdr hi)))
 117            (hsum (+ (car lo) (car hi) (/ lsum base)))
 118            (hsumodd (logand 1 hsum)))
 119           (setq probe (cons (/ (- hsum hsumodd) 2)
 120                     (/ (+ (* hsumodd base) (% lsum base)) 2)))
 121           (not (equal lo probe)))
 122       ;; Set either LO or HI to PROBE, depending on probe results.
 123       (if (eq (car (current-time-zone probe)) hi-utc-diff)
 124           (setq hi probe)
 125         (setq lo probe)))
 126         hi))))
 127  
 128  (defun calendar-time-zone-daylight-rules (abs-date utc-diff)
 129    "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
 130  ABS-DIFF must specify a day that contains a daylight savings transition.
 131  The result has the proper form for calendar-daylight-savings-starts'."
 132    (let* ((date (calendar-gregorian-from-absolute abs-date))
 133       (weekday (% abs-date 7))
 134       (m (extract-calendar-month date))
 135       (d (extract-calendar-day date))
 136       (y (extract-calendar-year date))
 137           (last (calendar-last-day-of-month m y))
 138       (candidate-rules
 139        (append
 140         ;; Day D of month M.
 141         (list (list 'list m d 'year))
 142         ;; The first WEEKDAY of month M.
 143             (if (< d 8)
 144                 (list (list 'calendar-nth-named-day 1 weekday m 'year)))
 145         ;; The last WEEKDAY of month M.
 146             (if (> d (- last 7))
 147                 (list (list 'calendar-nth-named-day -1 weekday m 'year)))
 148         ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
 149             (let (l)
 150               (calendar-for-loop j from (max 2 (- d 6)) to (min d (- last 8)) do
 151          (setq l
 152                (cons
 153                 (list 'calendar-nth-named-day 1 weekday m 'year j)
 154                 l)))
 155           l)))
 156       (prevday-sec (- -1 utc-diff)) ;; last sec of previous local day
 157       (year (1+ y)))
 158      ;; Scan through the next few years until only one rule remains.
 159      (while
 160      (let ((rules candidate-rules)
 161            new-rules)
 162        (while
 163            (let*
 164            ((rule (car rules))
 165             (date
 166              ;; The following is much faster than
 167              ;; (calendar-absolute-from-gregorian (eval rule)).
 168              (cond ((eq (car rule) 'calendar-nth-named-day)
 169                 (eval (cons 'calendar-nth-named-absday (cdr rule))))
 170                ((eq (car rule) 'calendar-gregorian-from-absolute)
 171                 (eval (car (cdr rule))))
 172                (t (let ((g (eval rule)))
 173                     (calendar-absolute-from-gregorian g))))))
 174          (or (equal
 175               (current-time-zone
 176                (calendar-time-from-absolute date prevday-sec))
 177               (current-time-zone
 178                (calendar-time-from-absolute (1+ date) prevday-sec)))
 179              (setq new-rules (cons rule new-rules)))
 180          (setq rules (cdr rules))))
 181        ;; If no rules remain, just use the first candidate rule;
 182        ;; it's wrong in general, but it's right for at least one year.
 183        (setq candidate-rules (if new-rules (nreverse new-rules)
 184                    (list (car candidate-rules))))
 185        (setq year (1+ year))
 186        (cdr candidate-rules)))
 187      (car candidate-rules)))
 188  
 189  (defun calendar-current-time-zone ()
 190    "Return UTC difference, dst offset, names and rules for current time zone.
 191  
 192  Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
 193  DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
 194  system knows:
 195  
 196  UTC-DIFF is an integer specifying the number of minutes difference between
 197      standard time in the current time zone and Coordinated Universal Time
 198      (Greenwich Mean Time).  A negative value means west of Greenwich.
 199  DST-OFFSET is an integer giving the daylight savings time offset in minutes.
 200  STD-ZONE is a string giving the name of the time zone when no seasonal time
 201      adjustment is in effect.
 202  DST-ZONE is a string giving the name of the time zone when there is a seasonal
 203      time adjustment in effect.
 204  DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
 205      savings time start and end rules, in the form expected by
 206      `calendar-daylight-savings-starts'.
 207  DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
 208      after midnight that daylight savings time starts and ends.
 209  
 210  If the local area does not use a seasonal time adjustment, STD-ZONE and
 211  DST-ZONE are equal, and all the DST-* integer variables are 0.
 212  
 213  Some operating systems cannot provide all this information to Emacs; in this
 214  case, `calendar-current-time-zone' returns a list containing nil for the data
 215  it can't find."
 216    (or
 217     calendar-current-time-zone-cache
 218     (setq
 219      calendar-current-time-zone-cache
 220      (let* ((t0 (get-decoded-time))
 221        ; (t0-zone (current-time-zone t0))
 222         (t0-zone (list nil))
 223         (t0-utc-diff (car t0-zone))
 224         (t0-name (car (cdr t0-zone))))
 225        (if (not t0-utc-diff)
 226        ;; Little or no time zone information is available.
 227        (list nil nil t0-name t0-name nil nil nil nil)
 228      (let* ((t1 (calendar-next-time-zone-transition t0))
 229             (t2 (and t1 (calendar-next-time-zone-transition t1))))
 230        (if (not t2)
 231            ;; This locale does not have daylight savings time.
 232            (list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
 233          ;; Use heuristics to find daylight savings parameters.
 234          (let* ((t1-zone (current-time-zone t1))
 235             (t1-utc-diff (car t1-zone))
 236             (t1-name (car (cdr t1-zone)))
 237             (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
 238             (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
 239             (t1-rules (calendar-time-zone-daylight-rules
 240                    (car t1-date-sec) t0-utc-diff))
 241             (t2-rules (calendar-time-zone-daylight-rules
 242                    (car t2-date-sec) t1-utc-diff))
 243             (t1-time (/ (cdr t1-date-sec) 60))
 244             (t2-time (/ (cdr t2-date-sec) 60)))
 245            (cons
 246             (/ (min t0-utc-diff t1-utc-diff) 60)
 247             (cons
 248          (/ (abs (- t0-utc-diff t1-utc-diff)) 60)
 249          (if (< t0-utc-diff t1-utc-diff)
 250              (list t0-name t1-name t1-rules t2-rules t1-time t2-time)
 251              (list t1-name t0-name t2-rules t1-rules t2-time t1-time)
 252              )))))))))))
 253  
 254  ;;; The following six defvars relating to daylight savings time should NOT be
 255  ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
 256  ;;; dumped.  These variables' appropriate values depend on the conditions under
 257  ;;; which the code is INVOKED; so it's inappropriate to initialize them when
 258  ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
 259  ;;; They default to US Eastern time if time zone info is not available.
 260  
 261  (calendar-current-time-zone)
 262  
 263  (defvar calendar-time-zone (or (car calendar-current-time-zone-cache) -300)
 264    "*Number of minutes difference between local standard time at
 265  `calendar-location-name' and Coordinated Universal (Greenwich) Time.  For
 266  example, -300 for New York City, -480 for Los Angeles.")
 267  
 268  (defvar calendar-daylight-time-offset
 269    (or (car (cdr calendar-current-time-zone-cache)) 60)
 270    "*Number of minutes difference between daylight savings and standard time.
 271    
 272  If the locale never uses daylight savings time, set this to 0.")
 273  
 274  (defvar calendar-standard-time-zone-name
 275    (or (car (nthcdr 2 calendar-current-time-zone-cache)) "EST")
 276    "*Abbreviated name of standard time zone at `calendar-location-name'.
 277  For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
 278  
 279  (defvar calendar-daylight-time-zone-name
 280    (or (car (nthcdr 3 calendar-current-time-zone-cache)) "EDT")
 281    "*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
 282  For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
 283    
 284  ;;;###autoload
 285  ;(put 'calendar-daylight-savings-starts 'risky-local-variable t)
 286  (defvar calendar-daylight-savings-starts
 287    (or (car (nthcdr 4 calendar-current-time-zone-cache))
 288        (and (not (zerop calendar-daylight-time-offset))
 289         '(calendar-nth-named-day 1 0 4 year)))
 290    "*Sexp giving the date on which daylight savings time starts.
 291  This is an expression in the variable `year' whose value gives the Gregorian
 292  date in the form (month day year) on which daylight savings time starts.  It is
 293  used to determine the starting date of daylight savings time for the holiday
 294  list and for correcting times of day in the solar and lunar calculations.
 295  
 296  For example, if daylight savings time is mandated to start on October 1,
 297  you would set `calendar-daylight-savings-starts' to
 298  
 299        '(10 1 year)
 300  
 301  If it starts on the first Sunday in April, you would set it to
 302  
 303        '(calendar-nth-named-day 1 0 4 year)
 304  
 305  If the locale never uses daylight savings time, set this to nil.")
 306  
 307  ;;;###autoload
 308  ;(put 'calendar-daylight-savings-ends 'risky-local-variable t)
 309  (defvar calendar-daylight-savings-ends
 310    (or (car (nthcdr 5 calendar-current-time-zone-cache))
 311        (and (not (zerop calendar-daylight-time-offset))
 312         '(calendar-nth-named-day -1 0 10 year)))
 313    "*Sexp giving the date on which daylight savings time ends.
 314  This is an expression in the variable `year' whose value gives the Gregorian
 315  date in the form (month day year) on which daylight savings time ends.  It is
 316  used to determine the starting date of daylight savings time for the holiday
 317  list and for correcting times of day in the solar and lunar calculations.
 318  
 319  For example, if daylight savings time ends on the last Sunday in October:
 320  
 321        '(calendar-nth-named-day -1 0 10 year)
 322  
 323  If the locale never uses daylight savings time, set this to nil.")
 324    
 325  (defvar calendar-daylight-savings-starts-time
 326    (or (car (nthcdr 6 calendar-current-time-zone-cache)) 120)
 327    "*Number of minutes after midnight that daylight savings time starts.")
 328    
 329  (defvar calendar-daylight-savings-ends-time
 330    (or (car (nthcdr 7 calendar-current-time-zone-cache))
 331        calendar-daylight-savings-starts-time)
 332    "*Number of minutes after midnight that daylight savings time ends.")
 333  
 334  ;(provide 'cal-dst)
 335  
 336  ;;; cal-dst.el ends here