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+;;; cal-coptic.el --- calendar functions for the Coptic/Ethiopic calendars.
+
+;; Copyright (C) 1995 Free Software Foundation, Inc.
+
+;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
+;; Keywords: calendar
+;; Human-Keywords: Coptic calendar, Ethiopic 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 Coptic and Ethiopic calendars.
+
+;; 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 coptic-calendar-month-name-array
+  ["Tot" "Babe" "Hatur" "Kihak" "Tube" "Amshir" "Baramhat" "Barmuda"
+   "Bashnas" "Bauna" "Abib" "Misra" "Nisi"])
+
+(defvar coptic-calendar-epoch (calendar-absolute-from-julian '(8 29 284))
+  "Absolute date of start of Coptic calendar = August 29, 284 A.D. (Julian).")
+
+(defconst coptic-name "Coptic")
+
+(defun coptic-calendar-leap-year-p (year)
+  "True if YEAR is a leap year on the Coptic calendar."
+  (zerop (mod (1+ year) 4)))
+
+(defun coptic-calendar-last-day-of-month (month year)
+  "Return last day of MONTH, YEAR on the Coptic calendar.
+The 13th month is not really a month, but the 5 (6 in leap years) day period of
+Nisi (Kebus)  at the end of the year."
+  (if (< month 13)
+      30
+    (if (coptic-calendar-leap-year-p year)
+        6
+      5)))
+
+(defun calendar-absolute-from-coptic (date)
+  "Compute absolute date from Coptic 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)))
+    (+ (1- coptic-calendar-epoch);; Days before start of calendar
+       (* 365 (1- year))         ;; Days in prior years
+       (/ year 4)                ;; Leap days in prior years
+       (* 30 (1- month))         ;; Days in prior months this year
+       day)))                    ;; Days so far this month
+       
+
+(defun calendar-coptic-from-absolute (date)
+  "Compute the Coptic 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 coptic-calendar-epoch)
+      (list 0 0 0);; pre-Coptic date
+    (let* ((approx (/ (- date coptic-calendar-epoch)
+                      366))   ;; Approximation from below.
+           (year              ;; Search forward from the approximation.
+            (+ approx
+               (calendar-sum y approx
+                 (>= date (calendar-absolute-from-coptic (list 1 1 (1+ y))))
+                 1)))
+           (month             ;; Search forward from Tot.
+            (1+ (calendar-sum m 1
+                  (> date
+                     (calendar-absolute-from-coptic
+                      (list m
+                            (coptic-calendar-last-day-of-month m year)
+                            year)))
+                  1)))
+           (day                ;; Calculate the day by subtraction.
+            (- date
+               (1- (calendar-absolute-from-coptic (list month 1 year))))))
+    (list month day year))))
+
+(defun calendar-coptic-date-string (&optional date)
+  "String of Coptic date of Gregorian DATE.
+Returns the empty string if DATE is pre-Coptic calendar.
+Defaults to today's date if DATE is not given."
+  (let* ((coptic-date (calendar-coptic-from-absolute
+                       (calendar-absolute-from-gregorian
+                        (or date (calendar-current-date)))))
+         (y (extract-calendar-year coptic-date))
+         (m (extract-calendar-month coptic-date)))
+    (if (< y 1)
+        ""
+      (let ((monthname (aref coptic-calendar-month-name-array (1- m)))
+            (day (int-to-string (extract-calendar-day coptic-date)))
+            (dayname nil)
+            (month (int-to-string m))
+            (year (int-to-string y)))
+        (mapconcat 'eval calendar-date-display-form "")))))
+
+(defun calendar-print-coptic-date ()
+  "Show the Coptic calendar equivalent of the selected date."
+  (interactive)
+  (let ((f (calendar-coptic-date-string (calendar-cursor-to-date t))))
+    (if (string-equal f "")
+        (message "Date is pre-%s calendar" coptic-name)
+      (message f))))
+
+(defun calendar-goto-coptic-date (date &optional noecho)
+  "Move cursor to Coptic date DATE.
+Echo Coptic date unless NOECHO is t."
+  (interactive (coptic-prompt-for-date))
+  (calendar-goto-date (calendar-gregorian-from-absolute
+                       (calendar-absolute-from-coptic date)))
+  (or noecho (calendar-print-coptic-date)))
+
+(defun coptic-prompt-for-date ()
+  "Ask for a Coptic date."
+  (let* ((today (calendar-current-date))
+         (year (calendar-read
+                (format "%s calendar year (>0): " coptic-name)
+                '(lambda (x) (> x 0))
+                (int-to-string
+                 (extract-calendar-year
+                  (calendar-coptic-from-absolute
+                   (calendar-absolute-from-gregorian today))))))
+         (completion-ignore-case t)
+         (month (cdr (assoc
+                      (capitalize
+                       (completing-read
+                        (format "%s calendar month name: " coptic-name)
+                        (mapcar 'list
+                                (append coptic-calendar-month-name-array nil))
+                        nil t))
+                      (calendar-make-alist coptic-calendar-month-name-array
+                                           1 'capitalize))))
+         (last (coptic-calendar-last-day-of-month month year))
+         (day (calendar-read
+               (format "%s calendar day (1-%d): " coptic-name last)
+               '(lambda (x) (and (< 0 x) (<= x last))))))
+    (list (list month day year))))
+
+(defun diary-coptic-date ()
+  "Coptic calendar equivalent of date diary entry."
+  (let ((f (calendar-coptic-date-string (calendar-cursor-to-date t))))
+    (if (string-equal f "")
+        (format "Date is pre-%s calendar" coptic-name)
+      f)))
+
+(defconst ethiopic-calendar-month-name-array
+  ["Maskarram" "Tekemt" "Hadar" "Tahsas" "Tarr" "Yekatit" "Magawit" "Miaziah"
+   "Genbot" "Sanni" "Hamle" "Nas'hi" "Pagnem"])
+
+(defconst ethiopic-calendar-epoch (calendar-absolute-from-julian '(8 29 7))
+  "Absolute date of start of Ethiopic calendar = August 29, 7 A.D. (Julian).")
+
+(defconst ethiopic-name "Ethiopic")
+
+(defun calendar-absolute-from-ethiopic (date)
+  "Compute absolute date from Ethiopic date DATE.
+The absolute date is the number of days elapsed since the (imaginary)
+Gregorian date Sunday, December 31, 1 BC."
+  (let ((coptic-calendar-epoch ethiopic-calendar-epoch))
+    (calendar-absolute-from-coptic date)))
+
+(defun calendar-ethiopic-from-absolute (date)
+  "Compute the Ethiopic 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 ((coptic-calendar-epoch ethiopic-calendar-epoch))
+    (calendar-coptic-from-absolute date)))
+
+(defun calendar-ethiopic-date-string (&optional date)
+  "String of Ethiopic date of Gregorian DATE.
+Returns the empty string if DATE is pre-Ethiopic calendar.
+Defaults to today's date if DATE is not given."
+  (let ((coptic-calendar-epoch ethiopic-calendar-epoch)
+        (coptic-name ethiopic-name)
+        (coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
+    (calendar-coptic-date-string date)))
+
+(defun calendar-print-ethiopic-date ()
+  "Show the Ethiopic calendar equivalent of the selected date."
+  (interactive)
+  (let ((coptic-calendar-epoch ethiopic-calendar-epoch)
+        (coptic-name ethiopic-name)
+        (coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
+    (call-interactively 'calendar-print-coptic-date)))
+
+(defun calendar-goto-ethiopic-date (date &optional noecho)
+  "Move cursor to Ethiopic date DATE.
+Echo Ethiopic date unless NOECHO is t."
+  (interactive
+   (let ((coptic-calendar-epoch ethiopic-calendar-epoch)
+         (coptic-name ethiopic-name)
+         (coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
+     (coptic-prompt-for-date)))
+  (calendar-goto-date (calendar-gregorian-from-absolute
+                       (calendar-absolute-from-ethiopic date)))
+  (or noecho (calendar-print-ethiopic-date)))
+
+(defun diary-ethiopic-date ()
+  "Ethiopic calendar equivalent of date diary entry."
+  (let ((coptic-calendar-epoch ethiopic-calendar-epoch)
+        (coptic-name ethiopic-name)
+        (coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
+    (diary-coptic-date)))
+
+(provide 'cal-coptic)
+
+;;; cal-coptic.el ends here