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+;;; cal-julian.el --- calendar functions for the Julian calendar.
+
+;; Copyright (C) 1995 Free Software Foundation, Inc.
+
+;; 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
+;; along with GNU Emacs; see the file COPYING.  If not, write to
+;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+
+;;; Commentary:
+
+;; This collection of functions implements the features of calendar.el and
+;; diary.el that deal with the Julian calendar.
+
+;; 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)
+          (month (cdr (assoc
+                       (capitalize
+                        (completing-read
+                         "Julian calendar month name: "
+                         (mapcar 'list (append month-array nil))
+                         nil t))
+                       (calendar-make-alist month-array 1 'capitalize))))
+          (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)
+  "Absolute date of astronical (Julian) day number D."
+  (- 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
+   "Astronomical (Julian) day number (after noon UTC): %s"
+   (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."
+  (format "Astronomical (Julian) day number %s"
+          (calendar-astro-date-string date)))
+
+(provide 'cal-julian)
+
+;;; cal-julian.el ends here