1;;; cal-julian.el --- calendar functions for the Julian calendar
2
3;; Copyright (C) 1995, 1997, 2001, 2002, 2003, 2004, 2005, 2006, 2007
4;;   Free Software Foundation, Inc.
5
6;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
7;; Maintainer: Glenn Morris <rgm@gnu.org>
8;; Keywords: calendar
9;; Human-Keywords: Julian calendar, Julian day number, calendar, diary
10
11;; This file is part of GNU Emacs.
12
13;; GNU Emacs is free software; you can redistribute it and/or modify
14;; it under the terms of the GNU General Public License as published by
15;; the Free Software Foundation; either version 2, or (at your option)
16;; any later version.
17
18;; GNU Emacs is distributed in the hope that it will be useful,
19;; but WITHOUT ANY WARRANTY; without even the implied warranty of
20;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
21;; GNU General Public License for more details.
22
23;; You should have received a copy of the GNU General Public License
24;; along with GNU Emacs; see the file COPYING.  If not, write to the
25;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
26;; Boston, MA 02110-1301, USA.
27
28;;; Commentary:
29
30;; This collection of functions implements the features of calendar.el and
31;; diary.el that deal with the Julian calendar.
32
33;; Technical details of all the calendrical calculations can be found in
34;; ``Calendrical Calculations: The Millennium Edition'' by Edward M. Reingold
35;; and Nachum Dershowitz, Cambridge University Press (2001).
36
37;;; Code:
38
39(defvar date)
40(defvar displayed-month)
41(defvar displayed-year)
42
43(require 'calendar)
44
45(defun calendar-julian-from-absolute (date)
46  "Compute the Julian (month day year) corresponding to the absolute DATE.
47The absolute date is the number of days elapsed since the (imaginary)
48Gregorian 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.
72The 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.
87Defaults to today's date if DATE is not given.
88Driven 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-string
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) t)))
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.
138If MONTH, DAY (Julian) is visible, the value returned is corresponding
139Gregorian date in the form of the list (((month day year) STRING)).  Returns
140nil 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.
177Defaults 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.
193Echo 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;;; arch-tag: 0520acdd-1c60-4188-9aa8-9b8c24d856ae
211;;; cal-julian.el ends here
212