code.delx.au - gnu-emacs/blob - lisp/calendar/cal-julian.el

   1 ;;; cal-julian.el --- calendar functions for the Julian calendar
   2
   3 ;; Copyright (C) 1995, 1997 Free Software Foundation, Inc.
   4
   5 ;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
   6 ;; Keywords: calendar
   7 ;; Human-Keywords: Julian calendar, Julian day number, calendar, diary
   8
   9 ;; This file is part of GNU Emacs.
  10
  11 ;; GNU Emacs is free software; you can redistribute it and/or modify
  12 ;; it under the terms of the GNU General Public License as published by
  13 ;; the Free Software Foundation; either version 2, or (at your option)
  14 ;; any later version.
  15
  16 ;; GNU Emacs is distributed in the hope that it will be useful,
  17 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 ;; GNU General Public License for more details.
  20
  21 ;; You should have received a copy of the GNU General Public License
  22 ;; along with GNU Emacs; see the file COPYING.  If not, write to the
  23 ;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  24 ;; Boston, MA 02110-1301, USA.
  25
  26 ;;; Commentary:
  27
  28 ;; This collection of functions implements the features of calendar.el and
  29 ;; diary.el that deal with the Julian calendar.
  30
  31 ;; Technical details of all the calendrical calculations can be found in
  32 ;; ``Calendrical Calculations: The Millennium Edition'' by Edward M. Reingold
  33 ;; and Nachum Dershowitz, Cambridge University Press (2001).
  34
  35 ;; Comments, corrections, and improvements should be sent to
  36 ;;  Edward M. Reingold               Department of Computer Science
  37 ;;  (217) 333-6733                   University of Illinois at Urbana-Champaign
  38 ;;  reingold@cs.uiuc.edu             1304 West Springfield Avenue
  39 ;;                                   Urbana, Illinois 61801
  40
  41 ;;; Code:
  42
  43 (defvar date)
  44 (defvar displayed-month)
  45 (defvar displayed-year)
  46
  47 (require 'calendar)
  48
  49 (defun calendar-julian-from-absolute (date)
  50   "Compute the Julian (month day year) corresponding to the absolute DATE.
  51 The absolute date is the number of days elapsed since the (imaginary)
  52 Gregorian date Sunday, December 31, 1 BC."
  53   (let* ((approx (/ (+ date 2) 366));; Approximation from below.
  54          (year        ;; Search forward from the approximation.
  55           (+ approx
  56              (calendar-sum y approx
  57                 (>= date (calendar-absolute-from-julian (list 1 1 (1+ y))))
  58                 1)))
  59          (month       ;; Search forward from January.
  60           (1+ (calendar-sum m 1
  61                  (> date
  62                     (calendar-absolute-from-julian
  63                      (list m
  64                            (if (and (= m 2) (= (% year 4) 0))
  65                                29
  66                              (aref [31 28 31 30 31 30 31 31 30 31 30 31]
  67                                    (1- m)))
  68                            year)))
  69                  1)))
  70          (day         ;; Calculate the day by subtraction.
  71           (- date (1- (calendar-absolute-from-julian (list month 1 year))))))
  72     (list month day year)))
  73
  74 (defun calendar-absolute-from-julian (date)
  75   "The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
  76 The Gregorian date Sunday, December 31, 1 BC is imaginary."
  77   (let ((month (extract-calendar-month date))
  78         (day (extract-calendar-day date))
  79         (year (extract-calendar-year date)))
  80     (+ (calendar-day-number date)
  81        (if (and (= (% year 100) 0)
  82                 (/= (% year 400) 0)
  83                 (> month 2))
  84            1 0);; Correct for Julian but not Gregorian leap year.
  85        (* 365 (1- year))
  86        (/ (1- year) 4)
  87        -2)))
  88
  89 (defun calendar-julian-date-string (&optional date)
  90   "String of Julian date of Gregorian DATE.
  91 Defaults to today's date if DATE is not given.
  92 Driven by the variable `calendar-date-display-form'."
  93   (calendar-date-string
  94    (calendar-julian-from-absolute
  95     (calendar-absolute-from-gregorian
  96      (or date (calendar-current-date))))
  97    nil t))
  98
  99 (defun calendar-print-julian-date ()
 100   "Show the Julian calendar equivalent of the date under the cursor."
 101   (interactive)
 102   (message "Julian date: %s"
 103            (calendar-julian-date-string (calendar-cursor-to-date t))))
 104
 105 (defun calendar-goto-julian-date (date &optional noecho)
 106   "Move cursor to Julian DATE; echo Julian date unless NOECHO is t."
 107   (interactive
 108    (let* ((today (calendar-current-date))
 109           (year (calendar-read
 110                  "Julian calendar year (>0): "
 111                  '(lambda (x) (> x 0))
 112                  (int-to-string
 113                   (extract-calendar-year
 114                    (calendar-julian-from-absolute
 115                     (calendar-absolute-from-gregorian
 116                      today))))))
 117           (month-array calendar-month-name-array)
 118           (completion-ignore-case t)
 119           (month (cdr (assoc-string
 120                         (completing-read
 121                          "Julian calendar month name: "
 122                          (mapcar 'list (append month-array nil))
 123                          nil t)
 124                        (calendar-make-alist month-array 1) t)))
 125           (last
 126            (if (and (zerop (% year 4)) (= month 2))
 127                29
 128              (aref [31 28 31 30 31 30 31 31 30 31 30 31] (1- month))))
 129           (day (calendar-read
 130                 (format "Julian calendar day (%d-%d): "
 131                         (if (and (= year 1) (= month 1)) 3 1) last)
 132                 '(lambda (x)
 133                    (and (< (if (and (= year 1) (= month 1)) 2 0) x)
 134                         (<= x last))))))
 135      (list (list month day year))))
 136   (calendar-goto-date (calendar-gregorian-from-absolute
 137                        (calendar-absolute-from-julian date)))
 138   (or noecho (calendar-print-julian-date)))
 139
 140 (defun holiday-julian (month day string)
 141   "Holiday on MONTH, DAY  (Julian) called STRING.
 142 If MONTH, DAY (Julian) is visible, the value returned is corresponding
 143 Gregorian date in the form of the list (((month day year) STRING)).  Returns
 144 nil if it is not visible in the current calendar window."
 145   (let ((m1 displayed-month)
 146         (y1 displayed-year)
 147         (m2 displayed-month)
 148         (y2 displayed-year)
 149         (year))
 150     (increment-calendar-month m1 y1 -1)
 151     (increment-calendar-month m2 y2 1)
 152     (let* ((start-date (calendar-absolute-from-gregorian
 153                         (list m1 1 y1)))
 154            (end-date (calendar-absolute-from-gregorian
 155                       (list m2 (calendar-last-day-of-month m2 y2) y2)))
 156            (julian-start (calendar-julian-from-absolute start-date))
 157            (julian-end (calendar-julian-from-absolute end-date))
 158            (julian-y1 (extract-calendar-year julian-start))
 159            (julian-y2 (extract-calendar-year julian-end)))
 160       (setq year (if (< 10 month) julian-y1 julian-y2))
 161       (let ((date (calendar-gregorian-from-absolute
 162                    (calendar-absolute-from-julian
 163                     (list month day year)))))
 164         (if (calendar-date-is-visible-p date)
 165             (list (list date string)))))))
 166
 167 (defun diary-julian-date ()
 168   "Julian calendar equivalent of date diary entry."
 169   (format "Julian date: %s" (calendar-julian-date-string date)))
 170
 171 (defun calendar-absolute-from-astro (d)
 172   "Absolute date of astronomical (Julian) day number D."
 173   (- d 1721424.5))
 174
 175 (defun calendar-astro-from-absolute (d)
 176   "Astronomical (Julian) day number of absolute date D."
 177   (+ d 1721424.5))
 178
 179 (defun calendar-astro-date-string (&optional date)
 180   "String of astronomical (Julian) day number after noon UTC of Gregorian DATE.
 181 Defaults to today's date if DATE is not given."
 182   (int-to-string
 183    (ceiling
 184     (calendar-astro-from-absolute
 185      (calendar-absolute-from-gregorian
 186       (or date (calendar-current-date)))))))
 187
 188 (defun calendar-print-astro-day-number ()
 189   "Show astronomical (Julian) day number after noon UTC on date shown by cursor."
 190   (interactive)
 191   (message
 192    "Astronomical (Julian) day number (at noon UTC): %s.0"
 193    (calendar-astro-date-string (calendar-cursor-to-date t))))
 194
 195 (defun calendar-goto-astro-day-number (daynumber &optional noecho)
 196   "Move cursor to astronomical (Julian) DAYNUMBER.
 197 Echo astronomical (Julian) day number unless NOECHO is t."
 198   (interactive (list (calendar-read
 199                       "Astronomical (Julian) day number (>1721425): "
 200                       '(lambda (x) (> x 1721425)))))
 201   (calendar-goto-date
 202    (calendar-gregorian-from-absolute
 203     (floor
 204      (calendar-absolute-from-astro daynumber))))
 205   (or noecho (calendar-print-astro-day-number)))
 206
 207 (defun diary-astro-day-number ()
 208   "Astronomical (Julian) day number diary entry."
 209   (format "Astronomical (Julian) day number at noon UTC: %s.0"
 210           (calendar-astro-date-string date)))
 211
 212 (provide 'cal-julian)
 213
 214 ;;; arch-tag: 0520acdd-1c60-4188-9aa8-9b8c24d856ae
 215 ;;; cal-julian.el ends here