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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