cal-julian.el 8.36 KB
Newer Older
1
;;; cal-julian.el --- calendar functions for the Julian calendar
Edward M. Reingold's avatar
Edward M. Reingold committed
2

3
;; Copyright (C) 1995, 1997 Free Software Foundation, Inc.
Edward M. Reingold's avatar
Edward M. Reingold committed
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
;; Keywords: calendar
;; Human-Keywords: Julian calendar, Julian day number, calendar, diary

;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
Erik Naggum's avatar
Erik Naggum committed
22 23 24
;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.
Edward M. Reingold's avatar
Edward M. Reingold committed
25 26 27 28 29 30

;;; Commentary:

;; This collection of functions implements the features of calendar.el and
;; diary.el that deal with the Julian calendar.

31 32 33 34
;; Technical details of all the calendrical calculations can be found in
;; ``Calendrical Calculations'' by Nachum Dershowitz and Edward M. Reingold,
;; Cambridge University Press (1997).

Edward M. Reingold's avatar
Edward M. Reingold committed
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114
;; Comments, corrections, and improvements should be sent to
;;  Edward M. Reingold               Department of Computer Science
;;  (217) 333-6733                   University of Illinois at Urbana-Champaign
;;  reingold@cs.uiuc.edu             1304 West Springfield Avenue
;;                                   Urbana, Illinois 61801

;;; Code:

(require 'calendar)

(defun calendar-julian-from-absolute (date)
  "Compute the Julian (month day year) corresponding to the absolute DATE.
The absolute date is the number of days elapsed since the (imaginary)
Gregorian date Sunday, December 31, 1 BC."
  (let* ((approx (/ (+ date 2) 366));; Approximation from below.
         (year        ;; Search forward from the approximation.
          (+ approx
             (calendar-sum y approx
                (>= date (calendar-absolute-from-julian (list 1 1 (1+ y))))
                1)))
         (month       ;; Search forward from January.
          (1+ (calendar-sum m 1
                 (> date
                    (calendar-absolute-from-julian
                     (list m
                           (if (and (= m 2) (= (% year 4) 0))
                               29
                             (aref [31 28 31 30 31 30 31 31 30 31 30 31]
                                   (1- m)))
                           year)))
                 1)))
         (day         ;; Calculate the day by subtraction.
          (- date (1- (calendar-absolute-from-julian (list month 1 year))))))
    (list month day year)))

(defun calendar-absolute-from-julian (date)
  "The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
The Gregorian date Sunday, December 31, 1 BC is imaginary."
  (let ((month (extract-calendar-month date))
        (day (extract-calendar-day date))
        (year (extract-calendar-year date)))
    (+ (calendar-day-number date)
       (if (and (= (% year 100) 0)
                (/= (% year 400) 0)
                (> month 2))
           1 0);; Correct for Julian but not Gregorian leap year.
       (* 365 (1- year))
       (/ (1- year) 4)
       -2)))

(defun calendar-julian-date-string (&optional date)
  "String of Julian date of Gregorian DATE.
Defaults to today's date if DATE is not given.
Driven by the variable `calendar-date-display-form'."
  (calendar-date-string
   (calendar-julian-from-absolute
    (calendar-absolute-from-gregorian
     (or date (calendar-current-date))))
   nil t))

(defun calendar-print-julian-date ()
  "Show the Julian calendar equivalent of the date under the cursor."
  (interactive)
  (message "Julian date: %s"
           (calendar-julian-date-string (calendar-cursor-to-date t))))

(defun calendar-goto-julian-date (date &optional noecho)
  "Move cursor to Julian DATE; echo Julian date unless NOECHO is t."
  (interactive
   (let* ((today (calendar-current-date))
          (year (calendar-read
                 "Julian calendar year (>0): "
                 '(lambda (x) (> x 0))
                 (int-to-string
                  (extract-calendar-year
                   (calendar-julian-from-absolute
                    (calendar-absolute-from-gregorian
                     today))))))
          (month-array calendar-month-name-array)
          (completion-ignore-case t)
115
          (month (cdr (assoc-ignore-case
Edward M. Reingold's avatar
Edward M. Reingold committed
116 117 118
                        (completing-read
                         "Julian calendar month name: "
                         (mapcar 'list (append month-array nil))
119 120
                         nil t)
                       (calendar-make-alist month-array 1))))
Edward M. Reingold's avatar
Edward M. Reingold committed
121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167
          (last 
           (if (and (zerop (% year 4)) (= month 2))
               29
             (aref [31 28 31 30 31 30 31 31 30 31 30 31] (1- month))))
          (day (calendar-read
                (format "Julian calendar day (%d-%d): "
                        (if (and (= year 1) (= month 1)) 3 1) last)
                '(lambda (x) 
                   (and (< (if (and (= year 1) (= month 1)) 2 0) x)
                        (<= x last))))))
     (list (list month day year))))
  (calendar-goto-date (calendar-gregorian-from-absolute
                       (calendar-absolute-from-julian date)))
  (or noecho (calendar-print-julian-date)))

(defun holiday-julian (month day string)
  "Holiday on MONTH, DAY  (Julian) called STRING.
If MONTH, DAY (Julian) is visible, the value returned is corresponding
Gregorian date in the form of the list (((month day year) STRING)).  Returns
nil if it is not visible in the current calendar window."
  (let ((m1 displayed-month)
        (y1 displayed-year)
        (m2 displayed-month)
        (y2 displayed-year)
        (year))
    (increment-calendar-month m1 y1 -1)
    (increment-calendar-month m2 y2 1)
    (let* ((start-date (calendar-absolute-from-gregorian
                        (list m1 1 y1)))
           (end-date (calendar-absolute-from-gregorian
                      (list m2 (calendar-last-day-of-month m2 y2) y2)))
           (julian-start (calendar-julian-from-absolute start-date))
           (julian-end (calendar-julian-from-absolute end-date))
           (julian-y1 (extract-calendar-year julian-start))
           (julian-y2 (extract-calendar-year julian-end)))
      (setq year (if (< 10 month) julian-y1 julian-y2))
      (let ((date (calendar-gregorian-from-absolute
                   (calendar-absolute-from-julian
                    (list month day year)))))
        (if (calendar-date-is-visible-p date)
            (list (list date string)))))))

(defun diary-julian-date ()
  "Julian calendar equivalent of date diary entry."
  (format "Julian date: %s" (calendar-julian-date-string date)))

(defun calendar-absolute-from-astro (d)
168
  "Absolute date of astronomical (Julian) day number D."
Edward M. Reingold's avatar
Edward M. Reingold committed
169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187
  (- d 1721424.5))

(defun calendar-astro-from-absolute (d)
  "Astronomical (Julian) day number of absolute date D."
  (+ d 1721424.5))

(defun calendar-astro-date-string (&optional date)
  "String of astronomical (Julian) day number after noon UTC of Gregorian DATE.
Defaults to today's date if DATE is not given."
  (int-to-string
   (ceiling
    (calendar-astro-from-absolute
     (calendar-absolute-from-gregorian
      (or date (calendar-current-date)))))))

(defun calendar-print-astro-day-number ()
  "Show astronomical (Julian) day number after noon UTC on date shown by cursor."
  (interactive)
  (message
Edward M. Reingold's avatar
Edward M. Reingold committed
188
   "Astronomical (Julian) day number (at noon UTC): %s.0"
Edward M. Reingold's avatar
Edward M. Reingold committed
189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204
   (calendar-astro-date-string (calendar-cursor-to-date t))))

(defun calendar-goto-astro-day-number (daynumber &optional noecho)
  "Move cursor to astronomical (Julian) DAYNUMBER.
Echo astronomical (Julian) day number unless NOECHO is t."
  (interactive (list (calendar-read
                      "Astronomical (Julian) day number (>1721425): "
                      '(lambda (x) (> x 1721425)))))
  (calendar-goto-date
   (calendar-gregorian-from-absolute
    (floor
     (calendar-absolute-from-astro daynumber))))
  (or noecho (calendar-print-astro-day-number)))

(defun diary-astro-day-number ()
  "Astronomical (Julian) day number diary entry."
205
  (format "Astronomical (Julian) day number at noon UTC: %s.0"
Edward M. Reingold's avatar
Edward M. Reingold committed
206 207 208 209 210
          (calendar-astro-date-string date)))

(provide 'cal-julian)

;;; cal-julian.el ends here