lisp/calendar/cal-persia.el

   1 ;;; cal-persia.el --- calendar functions for the Persian calendar.
   2
   3 ;; Copyright (C) 1996, 1997 Free Software Foundation, Inc.
   4
   5 ;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
   6 ;; Keywords: calendar
   7 ;; Human-Keywords: Persian calendar, 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 Persian 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 'cal-julian)
  44
  45 (defvar persian-calendar-month-name-array
  46   ["Farvardin" "Ordibehest" "Xordad" "Tir" "Mordad" "Sahrivar" "Mehr" "Aban"
  47    "Azar" "Dey" "Bahman" "Esfand"])
  48
  49 (defvar persian-calendar-epoch (calendar-absolute-from-julian '(3 19 622))
  50   "Absolute date of start of Persian calendar = March 19, 622 A.D. (Julian).")
  51
  52 (defun persian-calendar-leap-year-p (year)
  53   "True if YEAR is a leap year on the Persian calendar."
  54   (< (mod (* (mod (mod (if (<= 0 year)
  55                            ; No year zero
  56                            (+ year 2346)
  57                          (+ year 2347))
  58                        2820)
  59                   768)
  60               683)
  61            2820)
  62       683))
  63
  64 (defun persian-calendar-last-day-of-month (month year)
  65   "Return last day of MONTH, YEAR on the Persian calendar."
  66   (cond
  67    ((< month 7) 31)
  68    ((or (< month 12) (persian-calendar-leap-year-p year)) 30)
  69    (t 29)))
  70
  71 (defun calendar-absolute-from-persian (date)
  72   "Compute absolute date from Persian date DATE.
  73 The absolute date is the number of days elapsed since the (imaginary)
  74 Gregorian date Sunday, December 31, 1 BC."
  75   (let ((month (extract-calendar-month date))
  76         (day (extract-calendar-day date))
  77         (year (extract-calendar-year date)))
  78     (if (< year 0)
  79         (+ (calendar-absolute-from-persian
  80             (list month day (1+ (mod year 2820))))
  81            (* 1029983 (floor year 2820)))
  82       (+ (1- persian-calendar-epoch); Days before epoch
  83          (* 365 (1- year)) ; Days in prior years.
  84          (* 683        ; Leap days in prior 2820-year cycles
  85             (floor (+ year 2345) 2820))
  86          (* 186        ; Leap days in prior 768 year cycles
  87             (floor (mod (+ year 2345) 2820) 768))
  88          (floor; Leap years in current 768 or 516 year cycle
  89           (* 683 (mod (mod (+ year 2345) 2820) 768))
  90           2820)
  91          -568          ; Leap years in Persian years -2345...-1
  92          (calendar-sum ; Days in prior months this year.
  93           m 1 (< m month)
  94           (persian-calendar-last-day-of-month m year))
  95          day))))        ; Days so far this month.
  96
  97 (defun calendar-persian-year-from-absolute (date)
  98   "Persian year corresponding to the absolute DATE."
  99   (let* ((d0        ; Prior days since start of 2820 cycles
 100           (- date (calendar-absolute-from-persian (list 1 1 -2345))))
 101          (n2820     ; Completed 2820-year cycles
 102           (floor d0 1029983))
 103          (d1        ; Prior days not in n2820
 104           (mod d0 1029983))
 105          (n768      ; 768-year cycles not in n2820
 106           (floor d1 280506))
 107          (d2        ; Prior days not in n2820 or n768
 108           (mod d1 280506))
 109          (n1        ; Years not in n2820 or n768
 110           ; we want is
 111           ; (floor (+ (* 2820 d2) (* 2820 366)) 1029983))
 112           ; but that causes overflow, so we use
 113           (let ((a (floor d2 366)); we use 366 as the divisor because
 114                                   ; (2820*366 mod 1029983) is small
 115                 (b (mod d2 366)))
 116             (+ 1 a (floor (+ (* 2137 a) (* 2820 b) 2137) 1029983))))
 117          (year (+ (* 2820 n2820); Complete 2820 year cycles
 118                   (* 768 n768)  ; Complete 768 year cycles
 119                   (if           ; Remaining years
 120                       ; Last day of 2820 year cycle
 121                       (= d1 1029617)
 122                       (1- n1)
 123                     n1)
 124                   -2345)))      ; Years before year 1
 125     (if (< year 1)
 126         (1- year); No year zero
 127       year)))
 128
 129 (defun calendar-persian-from-absolute (date)
 130   "Compute the Persian equivalent for absolute date DATE.
 131 The result is a list of the form (MONTH DAY YEAR).
 132 The absolute date is the number of days elapsed since the imaginary
 133 Gregorian date Sunday, December 31, 1 BC."
 134   (let* ((year (calendar-persian-year-from-absolute date))
 135          (month         ; Search forward from Farvardin
 136           (1+ (calendar-sum m 1
 137                             (> date
 138                                (calendar-absolute-from-persian
 139                                 (list
 140                                  m
 141                                  (persian-calendar-last-day-of-month m year)
 142                                  year)))
 143                             1)))
 144          (day           ; Calculate the day by subtraction
 145           (- date (1- (calendar-absolute-from-persian
 146                        (list month 1 year))))))
 147     (list month day year)))
 148
 149 (defun calendar-persian-date-string (&optional date)
 150   "String of Persian date of Gregorian DATE.
 151 Defaults to today's date if DATE is not given."
 152   (let* ((persian-date (calendar-persian-from-absolute
 153                        (calendar-absolute-from-gregorian
 154                         (or date (calendar-current-date)))))
 155          (y (extract-calendar-year persian-date))
 156          (m (extract-calendar-month persian-date)))
 157     (let ((monthname (aref persian-calendar-month-name-array (1- m)))
 158           (day (int-to-string (extract-calendar-day persian-date)))
 159           (dayname nil)
 160           (month (int-to-string m))
 161           (year (int-to-string y)))
 162       (mapconcat 'eval calendar-date-display-form ""))))
 163
 164 (defun calendar-print-persian-date ()
 165   "Show the Persian calendar equivalent of the selected date."
 166   (interactive)
 167   (message "Persian date: %s"
 168            (calendar-persian-date-string (calendar-cursor-to-date t))))
 169
 170 (defun calendar-goto-persian-date (date &optional noecho)
 171   "Move cursor to Persian date DATE.
 172 Echo Persian date unless NOECHO is t."
 173   (interactive (persian-prompt-for-date))
 174   (calendar-goto-date (calendar-gregorian-from-absolute
 175                        (calendar-absolute-from-persian date)))
 176   (or noecho (calendar-print-persian-date)))
 177
 178 (defun persian-prompt-for-date ()
 179   "Ask for a Persian date."
 180   (let* ((today (calendar-current-date))
 181          (year (calendar-read
 182                 "Persian calendar year (not 0): "
 183                 '(lambda (x) (/= x 0))
 184                 (int-to-string
 185                  (extract-calendar-year
 186                   (calendar-persian-from-absolute
 187                    (calendar-absolute-from-gregorian today))))))
 188          (completion-ignore-case t)
 189          (month (cdr (assoc
 190                        (completing-read
 191                         "Persian calendar month name: "
 192                         (mapcar 'list
 193                                 (append persian-calendar-month-name-array nil))
 194                         nil t)
 195                       (calendar-make-alist persian-calendar-month-name-array
 196                                            1))))
 197          (last (persian-calendar-last-day-of-month month year))
 198          (day (calendar-read
 199                (format "Persian calendar day (1-%d): " last)
 200                '(lambda (x) (and (< 0 x) (<= x last))))))
 201     (list (list month day year))))
 202
 203 (defun diary-persian-date ()
 204   "Persian calendar equivalent of date diary entry."
 205   (format "Persian date: %s" (calendar-persian-date-string date)))
 206
 207 (provide 'cal-persia)
 208
 209 ;;; cal-persia.el ends here