lisp/calendar/cal-iso.el

   1 ;;; cal-iso.el --- calendar functions for the ISO calendar
   2
   3 ;; Copyright (C) 1995, 1997, 2001, 2002, 2003, 2004, 2005, 2006
   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 ;; Comments, corrections, and improvements should be sent to
  38 ;;  Edward M. Reingold               Department of Computer Science
  39 ;;  (217) 333-6733                   University of Illinois at Urbana-Champaign
  40 ;;  reingold@cs.uiuc.edu             1304 West Springfield Avenue
  41 ;;                                   Urbana, Illinois 61801
  42
  43 ;;; Code:
  44
  45 (defvar date)
  46
  47 (require 'calendar)
  48
  49 (defun calendar-absolute-from-iso (date)
  50   "The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
  51 The `ISO year' corresponds approximately to the Gregorian year, but
  52 weeks start on Monday and end on Sunday.  The first week of the ISO year is
  53 the first such week in which at least 4 days are in a year.  The ISO
  54 commercial DATE has the form (week day year) in which week is in the range
  55 1..52 and day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 =
  56 Sunday).  The Gregorian date Sunday, December 31, 1 BC is imaginary."
  57   (let* ((week (extract-calendar-month date))
  58          (day (extract-calendar-day date))
  59          (year (extract-calendar-year date)))
  60     (+ (calendar-dayname-on-or-before
  61         1 (+ 3 (calendar-absolute-from-gregorian (list 1 1 year))))
  62        (* 7 (1- week))
  63        (if (= day 0) 6 (1- day)))))
  64
  65 (defun calendar-iso-from-absolute (date)
  66   "Compute the `ISO commercial date' corresponding to the absolute DATE.
  67 The ISO year corresponds approximately to the Gregorian year, but weeks
  68 start on Monday and end on Sunday.  The first week of the ISO year is the
  69 first such week in which at least 4 days are in a year.  The ISO commercial
  70 date has the form (week day year) in which week is in the range 1..52 and
  71 day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 = Sunday).  The
  72 absolute date is the number of days elapsed since the (imaginary) Gregorian
  73 date Sunday, December 31, 1 BC."
  74   (let* ((approx (extract-calendar-year
  75                   (calendar-gregorian-from-absolute (- date 3))))
  76          (year (+ approx
  77                   (calendar-sum y approx
  78                       (>= date (calendar-absolute-from-iso (list 1 1 (1+ y))))
  79                       1))))
  80     (list
  81      (1+ (/ (- date (calendar-absolute-from-iso (list 1 1 year))) 7))
  82      (% date 7)
  83      year)))
  84
  85 (defun calendar-iso-date-string (&optional date)
  86   "String of ISO date of Gregorian DATE.
  87 Defaults to today's date if DATE is not given."
  88   (let* ((d (calendar-absolute-from-gregorian
  89              (or date (calendar-current-date))))
  90          (day (% d 7))
  91          (iso-date (calendar-iso-from-absolute d)))
  92     (format "Day %s of week %d of %d"
  93             (if (zerop day) 7 day)
  94             (extract-calendar-month iso-date)
  95             (extract-calendar-year iso-date))))
  96
  97 (defun calendar-print-iso-date ()
  98   "Show equivalent ISO date for the date under the cursor."
  99   (interactive)
 100   (message "ISO date: %s"
 101            (calendar-iso-date-string (calendar-cursor-to-date t))))
 102
 103 (defun calendar-iso-read-args (&optional dayflag)
 104   "Interactively read the arguments for an iso date command."
 105   (let* ((today (calendar-current-date))
 106          (year (calendar-read
 107                 "ISO calendar year (>0): "
 108                 '(lambda (x) (> x 0))
 109                 (int-to-string (extract-calendar-year today))))
 110          (no-weeks (extract-calendar-month
 111                     (calendar-iso-from-absolute
 112                      (1-
 113                       (calendar-dayname-on-or-before
 114                        1 (calendar-absolute-from-gregorian
 115                           (list 1 4 (1+ year))))))))
 116          (week (calendar-read
 117                 (format "ISO calendar week (1-%d): " no-weeks)
 118                 '(lambda (x) (and (> x 0) (<= x no-weeks)))))
 119          (day (if dayflag (calendar-read
 120                            "ISO day (1-7): "
 121                            '(lambda (x) (and (<= 1 x) (<= x 7))))
 122                 1)))
 123     (list (list week day year))))
 124
 125 (defun calendar-goto-iso-date (date &optional noecho)
 126   "Move cursor to ISO DATE; echo ISO date unless NOECHO is t."
 127   (interactive (calendar-iso-read-args t))
 128   (calendar-goto-date (calendar-gregorian-from-absolute
 129                        (calendar-absolute-from-iso date)))
 130   (or noecho (calendar-print-iso-date)))
 131
 132 (defun calendar-goto-iso-week (date &optional noecho)
 133   "Move cursor to ISO DATE; echo ISO date unless NOECHO is t.
 134 Interactively, goes to the first day of the specified week."
 135   (interactive (calendar-iso-read-args))
 136   (calendar-goto-date (calendar-gregorian-from-absolute
 137                        (calendar-absolute-from-iso date)))
 138   (or noecho (calendar-print-iso-date)))
 139
 140 (defun diary-iso-date ()
 141   "ISO calendar equivalent of date diary entry."
 142   (format "ISO date: %s" (calendar-iso-date-string date)))
 143
 144 (provide 'cal-iso)
 145
 146 ;;; arch-tag: 3c0154cc-d30f-4981-9f60-42bdf7a468f6
 147 ;;; cal-iso.el ends here