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., 59 Temple Place - Suite 330,
  24 ;; Boston, MA 02111-1307, 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'' by Nachum Dershowitz and Edward M. Reingold,
  33 ;; Cambridge University Press (1997).
  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 (require 'calendar)
  44
  45 (defun calendar-julian-from-absolute (date)
  46   "Compute the Julian (month day year) corresponding to the absolute DATE.
  47 The absolute date is the number of days elapsed since the (imaginary)
  48 Gregorian date Sunday, December 31, 1 BC."
  49   (let* ((approx (/ (+ date 2) 366));; Approximation from below.
  50          (year        ;; Search forward from the approximation.
  51           (+ approx
  52              (calendar-sum y approx
  53                 (>= date (calendar-absolute-from-julian (list 1 1 (1+ y))))
  54                 1)))
  55          (month       ;; Search forward from January.
  56           (1+ (calendar-sum m 1
  57                  (> date
  58                     (calendar-absolute-from-julian
  59                      (list m
  60                            (if (and (= m 2) (= (% year 4) 0))
  61                                29
  62                              (aref [31 28 31 30 31 30 31 31 30 31 30 31]
  63                                    (1- m)))
  64                            year)))
  65                  1)))
  66          (day         ;; Calculate the day by subtraction.
  67           (- date (1- (calendar-absolute-from-julian (list month 1 year))))))
  68     (list month day year)))
  69
  70 (defun calendar-absolute-from-julian (date)
  71   "The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
  72 The Gregorian date Sunday, December 31, 1 BC is imaginary."
  73   (let ((month (extract-calendar-month date))
  74         (day (extract-calendar-day date))
  75         (year (extract-calendar-year date)))
  76     (+ (calendar-day-number date)
  77        (if (and (= (% year 100) 0)
  78                 (/= (% year 400) 0)
  79                 (> month 2))
  80            1 0);; Correct for Julian but not Gregorian leap year.
  81        (* 365 (1- year))
  82        (/ (1- year) 4)
  83        -2)))
  84
  85 (defun calendar-julian-date-string (&optional date)
  86   "String of Julian date of Gregorian DATE.
  87 Defaults to today's date if DATE is not given.
  88 Driven by the variable `calendar-date-display-form'."
  89   (calendar-date-string
  90    (calendar-julian-from-absolute
  91     (calendar-absolute-from-gregorian
  92      (or date (calendar-current-date))))
  93    nil t))
  94
  95 (defun calendar-print-julian-date ()
  96   "Show the Julian calendar equivalent of the date under the cursor."
  97   (interactive)
  98   (message "Julian date: %s"
  99            (calendar-julian-date-string (calendar-cursor-to-date t))))
 100
 101 (defun calendar-goto-julian-date (date &optional noecho)
 102   "Move cursor to Julian DATE; echo Julian date unless NOECHO is t."
 103   (interactive
 104    (let* ((today (calendar-current-date))
 105           (year (calendar-read
 106                  "Julian calendar year (>0): "
 107                  '(lambda (x) (> x 0))
 108                  (int-to-string
 109                   (extract-calendar-year
 110                    (calendar-julian-from-absolute
 111                     (calendar-absolute-from-gregorian
 112                      today))))))
 113           (month-array calendar-month-name-array)
 114           (completion-ignore-case t)
 115           (month (cdr (assoc-ignore-case
 116                         (completing-read
 117                          "Julian calendar month name: "
 118                          (mapcar 'list (append month-array nil))
 119                          nil t)
 120                        (calendar-make-alist month-array 1))))
 121           (last
 122            (if (and (zerop (% year 4)) (= month 2))
 123                29
 124              (aref [31 28 31 30 31 30 31 31 30 31 30 31] (1- month))))
 125           (day (calendar-read
 126                 (format "Julian calendar day (%d-%d): "
 127                         (if (and (= year 1) (= month 1)) 3 1) last)
 128                 '(lambda (x)
 129                    (and (< (if (and (= year 1) (= month 1)) 2 0) x)
 130                         (<= x last))))))
 131      (list (list month day year))))
 132   (calendar-goto-date (calendar-gregorian-from-absolute
 133                        (calendar-absolute-from-julian date)))
 134   (or noecho (calendar-print-julian-date)))
 135
 136 (defun holiday-julian (month day string)
 137   "Holiday on MONTH, DAY  (Julian) called STRING.
 138 If MONTH, DAY (Julian) is visible, the value returned is corresponding
 139 Gregorian date in the form of the list (((month day year) STRING)).  Returns
 140 nil if it is not visible in the current calendar window."
 141   (let ((m1 displayed-month)
 142         (y1 displayed-year)
 143         (m2 displayed-month)
 144         (y2 displayed-year)
 145         (year))
 146     (increment-calendar-month m1 y1 -1)
 147     (increment-calendar-month m2 y2 1)
 148     (let* ((start-date (calendar-absolute-from-gregorian
 149                         (list m1 1 y1)))
 150            (end-date (calendar-absolute-from-gregorian
 151                       (list m2 (calendar-last-day-of-month m2 y2) y2)))
 152            (julian-start (calendar-julian-from-absolute start-date))
 153            (julian-end (calendar-julian-from-absolute end-date))
 154            (julian-y1 (extract-calendar-year julian-start))
 155            (julian-y2 (extract-calendar-year julian-end)))
 156       (setq year (if (< 10 month) julian-y1 julian-y2))
 157       (let ((date (calendar-gregorian-from-absolute
 158                    (calendar-absolute-from-julian
 159                     (list month day year)))))
 160         (if (calendar-date-is-visible-p date)
 161             (list (list date string)))))))
 162
 163 (defun diary-julian-date ()
 164   "Julian calendar equivalent of date diary entry."
 165   (format "Julian date: %s" (calendar-julian-date-string date)))
 166
 167 (defun calendar-absolute-from-astro (d)
 168   "Absolute date of astronomical (Julian) day number D."
 169   (- d 1721424.5))
 170
 171 (defun calendar-astro-from-absolute (d)
 172   "Astronomical (Julian) day number of absolute date D."
 173   (+ d 1721424.5))
 174
 175 (defun calendar-astro-date-string (&optional date)
 176   "String of astronomical (Julian) day number after noon UTC of Gregorian DATE.
 177 Defaults to today's date if DATE is not given."
 178   (int-to-string
 179    (ceiling
 180     (calendar-astro-from-absolute
 181      (calendar-absolute-from-gregorian
 182       (or date (calendar-current-date)))))))
 183
 184 (defun calendar-print-astro-day-number ()
 185   "Show astronomical (Julian) day number after noon UTC on date shown by cursor."
 186   (interactive)
 187   (message
 188    "Astronomical (Julian) day number (at noon UTC): %s.0"
 189    (calendar-astro-date-string (calendar-cursor-to-date t))))
 190
 191 (defun calendar-goto-astro-day-number (daynumber &optional noecho)
 192   "Move cursor to astronomical (Julian) DAYNUMBER.
 193 Echo astronomical (Julian) day number unless NOECHO is t."
 194   (interactive (list (calendar-read
 195                       "Astronomical (Julian) day number (>1721425): "
 196                       '(lambda (x) (> x 1721425)))))
 197   (calendar-goto-date
 198    (calendar-gregorian-from-absolute
 199     (floor
 200      (calendar-absolute-from-astro daynumber))))
 201   (or noecho (calendar-print-astro-day-number)))
 202
 203 (defun diary-astro-day-number ()
 204   "Astronomical (Julian) day number diary entry."
 205   (format "Astronomical (Julian) day number at noon UTC: %s.0"
 206           (calendar-astro-date-string date)))
 207
 208 (provide 'cal-julian)
 209
 210 ;;; cal-julian.el ends here