1;;; cal-iso.el --- calendar functions for the ISO calendar
2
3;; Copyright (C) 1995, 1997, 2001, 2002, 2003, 2004, 2005, 2006, 2007
4;;   Free Software Foundation, Inc.
5
6;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
7;; Maintainer: Glenn Morris <rgm@gnu.org>
8;; Keywords: calendar
9;; Human-Keywords: ISO calendar, calendar, diary
10
11;; This file is part of GNU Emacs.
12
13;; GNU Emacs is free software; you can redistribute it and/or modify
14;; it under the terms of the GNU General Public License as published by
15;; the Free Software Foundation; either version 2, or (at your option)
16;; any later version.
17
18;; GNU Emacs is distributed in the hope that it will be useful,
19;; but WITHOUT ANY WARRANTY; without even the implied warranty of
20;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
21;; GNU General Public License for more details.
22
23;; You should have received a copy of the GNU General Public License
24;; along with GNU Emacs; see the file COPYING.  If not, write to the
25;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
26;; Boston, MA 02110-1301, USA.
27
28;;; Commentary:
29
30;; This collection of functions implements the features of calendar.el and
31;; diary.el that deal with the ISO calendar.
32
33;; Technical details of all the calendrical calculations can be found in
34;; ``Calendrical Calculations: The Millennium Edition'' by Edward M. Reingold
35;; and Nachum Dershowitz, Cambridge University Press (2001).
36
37;;; Code:
38
39(defvar date)
40
41(require 'calendar)
42
43(defun calendar-absolute-from-iso (date)
44  "The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
45The `ISO year' corresponds approximately to the Gregorian year, but
46weeks start on Monday and end on Sunday.  The first week of the ISO year is
47the first such week in which at least 4 days are in a year.  The ISO
48commercial DATE has the form (week day year) in which week is in the range
491..52 and day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 =
50Sunday).  The Gregorian date Sunday, December 31, 1 BC is imaginary."
51  (let* ((week (extract-calendar-month date))
52         (day (extract-calendar-day date))
53         (year (extract-calendar-year date)))
54    (+ (calendar-dayname-on-or-before
55        1 (+ 3 (calendar-absolute-from-gregorian (list 1 1 year))))
56       (* 7 (1- week))
57       (if (= day 0) 6 (1- day)))))
58
59(defun calendar-iso-from-absolute (date)
60  "Compute the `ISO commercial date' corresponding to the absolute DATE.
61The ISO year corresponds approximately to the Gregorian year, but weeks
62start on Monday and end on Sunday.  The first week of the ISO year is the
63first such week in which at least 4 days are in a year.  The ISO commercial
64date has the form (week day year) in which week is in the range 1..52 and
65day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 = Sunday).  The
66absolute date is the number of days elapsed since the (imaginary) Gregorian
67date Sunday, December 31, 1 BC."
68  (let* ((approx (extract-calendar-year
69                  (calendar-gregorian-from-absolute (- date 3))))
70         (year (+ approx
71                  (calendar-sum y approx
72                      (>= date (calendar-absolute-from-iso (list 1 1 (1+ y))))
73                      1))))
74    (list
75     (1+ (/ (- date (calendar-absolute-from-iso (list 1 1 year))) 7))
76     (% date 7)
77     year)))
78
79(defun calendar-iso-date-string (&optional date)
80  "String of ISO date of Gregorian DATE.
81Defaults to today's date if DATE is not given."
82  (let* ((d (calendar-absolute-from-gregorian
83             (or date (calendar-current-date))))
84         (day (% d 7))
85         (iso-date (calendar-iso-from-absolute d)))
86    (format "Day %s of week %d of %d"
87            (if (zerop day) 7 day)
88            (extract-calendar-month iso-date)
89            (extract-calendar-year iso-date))))
90
91(defun calendar-print-iso-date ()
92  "Show equivalent ISO date for the date under the cursor."
93  (interactive)
94  (message "ISO date: %s"
95           (calendar-iso-date-string (calendar-cursor-to-date t))))
96
97(defun calendar-iso-read-args (&optional dayflag)
98  "Interactively read the arguments for an iso date command."
99  (let* ((today (calendar-current-date))
100         (year (calendar-read
101                "ISO calendar year (>0): "
102                '(lambda (x) (> x 0))
103                (int-to-string (extract-calendar-year today))))
104         (no-weeks (extract-calendar-month
105                    (calendar-iso-from-absolute
106                     (1-
107                      (calendar-dayname-on-or-before
108                       1 (calendar-absolute-from-gregorian
109                          (list 1 4 (1+ year))))))))
110         (week (calendar-read
111                (format "ISO calendar week (1-%d): " no-weeks)
112                '(lambda (x) (and (> x 0) (<= x no-weeks)))))
113         (day (if dayflag (calendar-read
114                           "ISO day (1-7): "
115                           '(lambda (x) (and (<= 1 x) (<= x 7))))
116                1)))
117    (list (list week day year))))
118
119(defun calendar-goto-iso-date (date &optional noecho)
120  "Move cursor to ISO DATE; echo ISO date unless NOECHO is t."
121  (interactive (calendar-iso-read-args t))
122  (calendar-goto-date (calendar-gregorian-from-absolute
123                       (calendar-absolute-from-iso date)))
124  (or noecho (calendar-print-iso-date)))
125
126(defun calendar-goto-iso-week (date &optional noecho)
127  "Move cursor to ISO DATE; echo ISO date unless NOECHO is t.
128Interactively, goes to the first day of the specified week."
129  (interactive (calendar-iso-read-args))
130  (calendar-goto-date (calendar-gregorian-from-absolute
131                       (calendar-absolute-from-iso date)))
132  (or noecho (calendar-print-iso-date)))
133
134(defun diary-iso-date ()
135  "ISO calendar equivalent of date diary entry."
136  (format "ISO date: %s" (calendar-iso-date-string date)))
137
138(provide 'cal-iso)
139
140;;; arch-tag: 3c0154cc-d30f-4981-9f60-42bdf7a468f6
141;;; cal-iso.el ends here
142