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Edward M. Reingold 1996-03-29 17:34:02 +00:00
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;;; cal-persian.el --- calendar functions for the Persian calendar.
;; Copyright (C) 1996 Free Software Foundation, Inc.
;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
;; Keywords: calendar
;; Human-Keywords: Persian 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 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 Persian 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 'cal-julian)
(defvar persian-calendar-month-name-array
["Farvardin" "Ordibehest" "Xordad" "Tir" "Mordad" "Sahrivar" "Mehr" "Aban"
"Azar" "Dey" "Bahman" "Esfand"])
(defvar persian-calendar-epoch (calendar-absolute-from-julian '(3 19 622))
"Absolute date of start of Persian calendar = March 19, 622 A.D. (Julian).")
(defun persian-calendar-leap-year-p (year)
"True if YEAR is a leap year on the Persian calendar."
(< (mod (* (mod (mod (if (<= 0 year)
; No year zero
(+ year 2346)
(+ year 2347))
2820)
768)
683)
2820)
683))
(defun persian-calendar-last-day-of-month (month year)
"Return last day of MONTH, YEAR on the Persian calendar."
(cond
((< month 7) 31)
((or (< month 12) (persian-calendar-leap-year-p year)) 30)
(t 29)))
(defun calendar-absolute-from-persian (date)
"Compute absolute date from Persian 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)))
(if (< year 0)
(+ (calendar-absolute-from-persian
(list month day (1+ (mod year 2820))))
(* 1029983 (floor year 2820)))
(+ (1- persian-calendar-epoch); Days before epoch
(* 365 (1- year)) ; Days in prior years.
(* 683 ; Leap days in prior 2820-year cycles
(floor (+ year 2345) 2820))
(* 186 ; Leap days in prior 768 year cycles
(floor (mod (+ year 2345) 2820) 768))
(floor; Leap years in current 768 or 516 year cycle
(* 683 (mod (mod (+ year 2345) 2820) 768))
2820)
-568 ; Leap years in Persian years -2345...-1
(calendar-sum ; Days in prior months this year.
m 1 (< m month)
(persian-calendar-last-day-of-month m year))
day)))) ; Days so far this month.
(defun calendar-persian-year-from-absolute (date)
"Persian year corresponding to the absolute DATE."
(let* ((d0 ; Prior days since start of 2820 cycles
(- date (calendar-absolute-from-persian (list 1 1 -2345))))
(n2820 ; Completed 2820-year cycles
(floor d0 1029983))
(d1 ; Prior days not in n2820
(mod d0 1029983))
(n768 ; 768-year cycles not in n2820
(floor d1 280506))
(d2 ; Prior days not in n2820 or n768
(mod d1 280506))
(n1 ; Years not in n2820 or n768
; we want is
; (floor (+ (* 2820 d2) (* 2820 366)) 1029983))
; but that causes overflow, so we use
(let ((a (floor d2 366)); we use 366 as the divisor because
; (2820*366 mod 1029983) is small
(b (mod d2 366)))
(+ 1 a (floor (+ (* 2137 a) (* 2820 b) 2137) 1029983))))
(year (+ (* 2820 n2820); Complete 2820 year cycles
(* 768 n768) ; Complete 768 year cycles
(if ; Remaining years
; Last day of 2820 year cycle
(= d1 1029617)
(1- n1)
n1)
-2345))) ; Years before year 1
(if (< year 1)
(1- year); No year zero
year)))
(defun calendar-persian-from-absolute (date)
"Compute the Persian 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."
(let* ((year (calendar-persian-year-from-absolute date))
(month ; Search forward from Farvardin
(1+ (calendar-sum m 1
(> date
(calendar-absolute-from-persian
(list
m
(persian-calendar-last-day-of-month m year)
year)))
1)))
(day ; Calculate the day by subtraction
(- date (1- (calendar-absolute-from-persian
(list month 1 year))))))
(list month day year)))
(defun calendar-persian-date-string (&optional date)
"String of Persian date of Gregorian DATE.
Defaults to today's date if DATE is not given."
(let* ((persian-date (calendar-persian-from-absolute
(calendar-absolute-from-gregorian
(or date (calendar-current-date)))))
(y (extract-calendar-year persian-date))
(m (extract-calendar-month persian-date)))
(let ((monthname (aref persian-calendar-month-name-array (1- m)))
(day (int-to-string (extract-calendar-day persian-date)))
(dayname nil)
(month (int-to-string m))
(year (int-to-string y)))
(mapconcat 'eval calendar-date-display-form ""))))
(defun calendar-print-persian-date ()
"Show the Persian calendar equivalent of the selected date."
(interactive)
(message "Persian date: %s"
(calendar-persian-date-string (calendar-cursor-to-date t))))
(defun calendar-goto-persian-date (date &optional noecho)
"Move cursor to Persian date DATE.
Echo Persian date unless NOECHO is t."
(interactive (persian-prompt-for-date))
(calendar-goto-date (calendar-gregorian-from-absolute
(calendar-absolute-from-persian date)))
(or noecho (calendar-print-persian-date)))
(defun persian-prompt-for-date ()
"Ask for a Persian date."
(let* ((today (calendar-current-date))
(year (calendar-read
"Persian calendar year (not 0): "
'(lambda (x) (/= x 0))
(int-to-string
(extract-calendar-year
(calendar-persian-from-absolute
(calendar-absolute-from-gregorian today))))))
(completion-ignore-case t)
(month (cdr (assoc
(capitalize
(completing-read
"Persian calendar month name: "
(mapcar 'list
(append persian-calendar-month-name-array nil))
nil t))
(calendar-make-alist persian-calendar-month-name-array
1 'capitalize))))
(last (persian-calendar-last-day-of-month month year))
(day (calendar-read
(format "Persian calendar day (1-%d): " last)
'(lambda (x) (and (< 0 x) (<= x last))))))
(list (list month day year))))
(defun diary-persian-date ()
"Persian calendar equivalent of date diary entry."
(calendar-persian-date-string (calendar-cursor-to-date t)))
(provide 'cal-persian)
;;; cal-persian.el ends here