emacs/lisp/calendar/cal-french.el

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

;; Copyright (C) 1988, 1989, 1992, 1994, 1995, 1997, 2001, 2002, 2003,
;;   2004, 2005, 2006, 2007, 2008  Free Software Foundation, Inc.

;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
;; Maintainer: Glenn Morris <rgm@gnu.org>
;; Keywords: calendar
;; Human-Keywords: French Revolutionary calendar, 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 3, 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, Inc., 51 Franklin Street, Fifth Floor,
;; Boston, MA 02110-1301, USA.

;;; Commentary:

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

;; Technical details of the French Revolutionary calendar can be found in
;; ``Calendrical Calculations: The Millennium Edition'' by Edward M. Reingold
;; and Nachum Dershowitz, Cambridge University Press (2001), and in
;; ``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.

;;; Code:

(require 'calendar)

(defconst french-calendar-epoch (calendar-absolute-from-gregorian '(9 22 1792))
  "Absolute date of start of French Revolutionary calendar = Sept 22, 1792.")

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

(defconst french-calendar-multibyte-month-name-array
  ["Vend<EFBFBD>miaire" "Brumaire" "Frimaire" "Niv<EFBFBD>se" "Pluvi<EFBFBD>se" "Vent<EFBFBD>se"
   "Germinal" "Flor<EFBFBD>al" "Prairial" "Messidor" "Thermidor" "Fructidor"]
  "Array of multibyte month names in the French calendar.")

(defconst french-calendar-day-name-array
  ["Primidi" "Duodi" "Tridi" "Quartidi" "Quintidi" "Sextidi" "Septidi"
   "Octidi" "Nonidi" "Decadi"]
  "Array of day names in the French calendar.")

(defconst french-calendar-special-days-array
  ["de la Vertu" "du Ge'nie" "du Travail" "de la Raison" "des Re'compenses"
   "de la Re'volution"]
  "Array of special day names in the French calendar.")

(defconst french-calendar-multibyte-special-days-array
  ["de la Vertu" "du G<>nie" "du Travail" "de la Raison" "des R<>compenses"
   "de la R<>volution"]
  "Array of multibyte special day names in the French calendar.")

(defun french-calendar-accents ()
  "Return non-nil 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])))))

(defun french-calendar-month-name-array ()
  "Return the array of month names, depending on whether accents are available."
  (if (french-calendar-accents)
      french-calendar-multibyte-month-name-array
    french-calendar-month-name-array))

(defun french-calendar-day-name-array ()
  "Return the array of day names."
  french-calendar-day-name-array)

(defun french-calendar-special-days-array ()
  "Return the special day names, depending on whether accents are available."
  (if (french-calendar-accents)
      french-calendar-multibyte-special-days-array
    french-calendar-special-days-array))

(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
calendar, follows historical practice based on equinoxes (years 3, 7,
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)
  "Return last day of MONTH, YEAR on the French Revolutionary calendar.
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)
  "Compute absolute date from French Revolutionary date DATE.
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
       (1- french-calendar-epoch))))  ; days before start of calendar

(defun calendar-french-from-absolute (date)
  "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
\(imaginary) Gregorian date Sunday, December 31, 1 BC."
  (if (< date french-calendar-epoch)
      (list 0 0 0)                     ; pre-French Revolutionary date
    (let* ((approx                     ; approximation from below
            (/ (- date french-calendar-epoch) 366))
           (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))))

;;;###cal-autoload
(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."
  (let* ((french-date (calendar-french-from-absolute
                       (calendar-absolute-from-gregorian
                        (or date (calendar-current-date)))))
         (y (extract-calendar-year french-date))
         (m (extract-calendar-month french-date))
         (d (extract-calendar-day french-date)))
    (cond
     ((< y 1) "")
     ((= m 13) (format (if (french-calendar-accents)
                           "Jour %s de l'Ann<6E>e %d de la R<>volution"
                         "Jour %s de l'Anne'e %d de la Re'volution")
                       (aref (french-calendar-special-days-array) (1- d))
                       y))
     (t (format
         (if (french-calendar-accents)
             "%d %s an %d de la R<>volution"
           "%d %s an %d de la Re'volution")
         d
         (aref (french-calendar-month-name-array) (1- m))
         y)))))

;;;###cal-autoload
(defun calendar-print-french-date ()
  "Show the French Revolutionary calendar equivalent of the selected date."
  (interactive)
  (let ((f (calendar-french-date-string (calendar-cursor-to-date t))))
    (if (string-equal f "")
        (message "Date is pre-French Revolution")
      (message "French Revolutionary date: %s" f))))

;;;###cal-autoload
(defun calendar-goto-french-date (date &optional noecho)
  "Move cursor to French Revolutionary date DATE.
Echo French Revolutionary date unless NOECHO is non-nil."
  (interactive
   (let ((accents (french-calendar-accents))
         (months (french-calendar-month-name-array))
         (special-days (french-calendar-special-days-array)))
     (let* ((year
             (progn
               (calendar-read
                (if accents
                    "Ann<EFBFBD>e de la R<>volution (>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))))))))
            (month-list
             (mapcar 'list
                     (append months
                             (if (french-calendar-leap-year-p year)
                                 (mapcar
                                  (lambda (x) (concat "Jour " x))
                                  french-calendar-special-days-array)
                               (reverse
                                (cdr ; we don't want rev. day in a non-leap yr
                                 (reverse
                                  (mapcar
                                   (lambda (x)
                                     (concat "Jour " x))
                                   special-days))))))))
            (completion-ignore-case t)
            (month (cdr (assoc-string
                         (completing-read
                          "Mois ou Sansculottide: "
                          month-list
                          nil t)
                         (calendar-make-alist month-list 1 'car) t)))
            (day (if (> month 12)
                     (- month 12)
                   (calendar-read
                    "Jour (1-30): "
                    (lambda (x) (and (<= 1 x) (<= x 30))))))
            (month (if (> month 12) 13 month)))
       (list (list month day year)))))
  (calendar-goto-date (calendar-gregorian-from-absolute
                       (calendar-absolute-from-french date)))
  (or noecho (calendar-print-french-date)))

(defvar date)

;; To be called from list-sexp-diary-entries, where DATE is bound.
;;;###diary-autoload
(defun diary-french-date ()
  "French calendar equivalent of date diary entry."
  (let ((f (calendar-french-date-string date)))
    (if (string-equal f "")
        "Date is pre-French Revolution"
      (format "French Revolutionary date: %s" f))))

(provide 'cal-french)

;; arch-tag: 7e8045a3-8609-46b5-9cde-cf40ce541cf9
;;; cal-french.el ends here