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;;; cal-french.el --- calendar functions for the French Revolutionary calendar.

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;; Copyright (C) 1988, 89, 92, 94, 95, 1997 Free Software Foundation, Inc.
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;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
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;; Keywords: calendar
;; Human-Keywords: French Revolutionary calendar, calendar, diary
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;; This file is part of GNU Emacs.

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;; 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.

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;; GNU Emacs is distributed in the hope that it will be useful,
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;; 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
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;; 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.
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;;; Commentary:

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

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;; Technical details of the French Revolutionary calendar can be found in
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;; ``Calendrical Calculations, Part II: Three Historical Calendars'' by
;; E. M. Reingold, N. Dershowitz, and S. M. Clamen, Software--Practice and
;; Experience, Volume 23, Number 4 (April, 1993), pages 383-404.
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;; 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)

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(defun french-calendar-accents ()
  "True if diacritical marks are available."
  (and (or window-system
	   (terminal-coding-system))
       (or enable-multibyte-characters
	   (and (char-table-p standard-display-table)
		(equal (aref standard-display-table 161) [161])))))
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(defconst french-calendar-epoch (calendar-absolute-from-gregorian '(9 22 1792))
  "Absolute date of start of French Revolutionary calendar = September 22, 1792.")

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(defconst french-calendar-month-name-array
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  ["Vende'miaire" "Brumaire" "Frimaire" "Nivo^se" "Pluvio^se" "Vento^se"
   "Germinal" "Flore'al" "Prairial" "Messidor" "Thermidor" "Fructidor"])

(defconst french-calendar-multibyte-month-name-array
  ["Vendmiaire" "Brumaire" "Frimaire" "Nivse" "Pluvise" "Ventse"
   "Germinal" "Floral" "Prairial" "Messidor" "Thermidor" "Fructidor"])
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(defun french-calendar-month-name-array ()
  (if (french-calendar-accents)
      french-calendar-multibyte-month-name-array
    french-calendar-month-name-array))

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(defconst french-calendar-day-name-array
  ["Primidi" "Duodi" "Tridi" "Quartidi" "Quintidi" "Sextidi" "Septidi"
   "Octidi" "Nonidi" "Decadi"])

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(defconst french-calendar-multibyte-special-days-array
  ["de la Vertu" "du Gnie" "du Labour" "de la Raison"
   "de la Rcompense" "de la Rvolution"])

(defun french-calendar-day-name-array ()
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  french-calendar-day-name-array)
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(defconst french-calendar-special-days-array
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  ["de la Vertu" "du Ge'nie" "du Labour" "de la Raison" "de la Re'compense"
   "de la Re'volution"])
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(defun french-calendar-special-days-array ()
  (if (french-calendar-accents)
      french-calendar-multibyte-special-days-array
    french-calendar-special-days-array))

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(defun french-calendar-leap-year-p (year)
  "True if YEAR is a leap year on the French Revolutionary calendar.
For Gregorian years 1793 to 1805, the years of actual operation of the
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calendar, follows historical practice based on equinoxes (years 3, 7,
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and 11 were leap years; 15 and 20 would have been leap years).  For later
years uses the proposed rule of Romme (never adopted)--leap years fall every
four years except century years not divisible 400 and century years that are
multiples of 4000."
  (or (memq year '(3 7 11));; Actual practice--based on equinoxes
      (memq year '(15 20)) ;; Anticipated practice--based on equinoxes
      (and (> year 20)     ;; Romme's proposal--never adopted
           (zerop (% year 4))
           (not (memq (% year 400) '(100 200 300)))
           (not (zerop (% year 4000))))))

(defun french-calendar-last-day-of-month (month year)
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  "Return last day of MONTH, YEAR on the French Revolutionary calendar.
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The 13th month is not really a month, but the 5 (6 in leap years) day period of
`sansculottides' at the end of the year."
  (if (< month 13)
      30
    (if (french-calendar-leap-year-p year)
        6
      5)))

(defun calendar-absolute-from-french (date)
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  "Compute absolute date from French Revolutionary date DATE.
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The absolute date is the number of days elapsed since the (imaginary)
Gregorian date Sunday, December 31, 1 BC."
  (let ((month (extract-calendar-month date))
        (day (extract-calendar-day date))
        (year (extract-calendar-year date)))
    (+ (* 365 (1- year));; Days in prior years
       ;; Leap days in prior years
       (if (< year 20)
           (/ year 4);; Actual and anticipated practice (years 3, 7, 11, 15)
         ;; Romme's proposed rule (using the Principle of Inclusion/Exclusion)
         (+ (/ (1- year) 4);; Luckily, there were 4 leap years before year 20
            (- (/ (1- year) 100))
            (/ (1- year) 400)
            (- (/ (1- year) 4000))))
       (* 30 (1- month));; Days in prior months this year
       day;; Days so far this month
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       (1- french-calendar-epoch))));; Days before start of calendar
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(defun calendar-french-from-absolute (date)
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  "Compute the French Revolutionary equivalent for absolute date DATE.
The result is a list of the form (MONTH DAY YEAR).
The absolute date is the number of days elapsed since the
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\(imaginary) Gregorian date Sunday, December 31, 1 BC."
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  (if (< date french-calendar-epoch)
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      (list 0 0 0);; pre-French Revolutionary date
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    (let* ((approx              ;; Approximation from below.
            (/ (- date french-calendar-epoch) 366))
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           (year                ;; Search forward from the approximation.
            (+ approx
               (calendar-sum y approx
                 (>= date (calendar-absolute-from-french (list 1 1 (1+ y))))
                 1)))
           (month               ;; Search forward from Vendemiaire.
            (1+ (calendar-sum m 1
                  (> date
                     (calendar-absolute-from-french
                      (list m
                            (french-calendar-last-day-of-month m year)
                            year)))
                  1)))
           (day                   ;; Calculate the day by subtraction.
            (- date
               (1- (calendar-absolute-from-french (list month 1 year))))))
    (list month day year))))

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(defun calendar-french-date-string (&optional date)
  "String of French Revolutionary date of Gregorian DATE.
Returns the empty string if DATE is pre-French Revolutionary.
Defaults to today's date if DATE is not given."
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  (let* ((french-date (calendar-french-from-absolute
                       (calendar-absolute-from-gregorian
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                        (or date (calendar-current-date)))))
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         (y (extract-calendar-year french-date))
         (m (extract-calendar-month french-date))
         (d (extract-calendar-day french-date)))
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    (cond
     ((< y 1) "")
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     ((= m 13) (format (if (french-calendar-accents)
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                           "Jour %s de l'Anne %d de la Rvolution"
                         "Jour %s de l'Anne'e %d de la Re'volution")
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                       (aref (french-calendar-special-days-array) (1- d))
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                       y))
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     (t (format
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         (if (french-calendar-accents)
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             "Dcade %s, %s de %s de l'Anne %d de la Rvolution"
           "De'cade %s, %s de %s de l'Anne'e %d de la Re'volution")
         (make-string (1+ (/ (1- d) 10)) ?I)
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         (aref (french-calendar-day-name-array) (% (1- d) 10))
         (aref (french-calendar-month-name-array) (1- m))
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         y)))))
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(defun calendar-print-french-date ()
  "Show the French Revolutionary calendar equivalent of the selected date."
  (interactive)
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  (let ((f (calendar-french-date-string (calendar-cursor-to-date t)))
	(enable-multibyte-characters t))
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    (if (string-equal f "")
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        (message "Date is pre-French Revolution")
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      (message "French Revolutionary date: %s" f))))
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(defun calendar-goto-french-date (date &optional noecho)
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  "Move cursor to French Revolutionary date DATE.
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Echo French Revolutionary date unless NOECHO is t."
  (interactive
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   (let* ((oldval enable-multibyte-characters)
	  (year (unwind-protect
		    (progn
		      (setq-default enable-multibyte-characters t)
		      (calendar-read
		       (if (french-calendar-accents)
			   "Anne de la Rvolution (>0): "
			 "Anne'e de la Re'volution (>0): ")
		       '(lambda (x) (> x 0))
		       (int-to-string
			(extract-calendar-year
			 (calendar-french-from-absolute
			  (calendar-absolute-from-gregorian
			   (calendar-current-date)))))))
		  (setq-default enable-multibyte-characters oldval)))
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          (month-list
           (mapcar 'list
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                   (append (french-calendar-month-name-array)
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                           (if (french-calendar-leap-year-p year)
                               (mapcar
                                '(lambda (x) (concat "Jour " x))
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                                (french-calendar-special-days-array))
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                             (reverse
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                              (cdr;; we don't want rev. day in a non-leap yr.
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                               (reverse
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                                (mapcar
                                 '(lambda (x) (concat "Jour " x))
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                                 (french-calendar-special-days-array)))))))))
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          (completion-ignore-case t)
          (month (cdr (assoc
                       (capitalize
                        (completing-read
                         "Mois ou Sansculottide: "
                         month-list
                         nil t))
                       (calendar-make-alist
                        month-list
                        1
                        '(lambda (x) (capitalize (car x)))))))
          (decade (if (> month 12)
                      1
                    (calendar-read
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                     (if (french-calendar-accents)
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                         "Dcade (1-3): "
                       "De'cade (1-3): ")
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                     '(lambda (x) (memq x '(1 2 3))))))
          (day (if (> month 12)
                   (- month 12)
                 (calendar-read
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                  "Jour (1-10): "
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                  '(lambda (x) (and (<= 1 x) (<= x 10))))))
          (month (if (> month 12) 13 month))
          (day (+ day (* 10 (1- decade)))))
     (list (list month day year))))
  (calendar-goto-date (calendar-gregorian-from-absolute
                       (calendar-absolute-from-french date)))
  (or noecho (calendar-print-french-date)))

(defun diary-french-date ()
  "French calendar equivalent of date diary entry."
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  (let ((f (calendar-french-date-string date)))
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    (if (string-equal f "")
        "Date is pre-French Revolution"
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      (format "French Revolutionary date: %s" f))))
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(provide 'cal-french)

;;; cal-french.el ends here