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, 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 3, 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.
  45 The `ISO year' corresponds approximately to the Gregorian year, but
  46 weeks start on Monday and end on Sunday.  The first week of the ISO year is
  47 the first such week in which at least 4 days are in a year.  The ISO
  48 commercial DATE has the form (week day year) in which week is in the range
  49 1..52 and day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 =
  50 Sunday).  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.
  61 The ISO year corresponds approximately to the Gregorian year, but weeks
  62 start on Monday and end on Sunday.  The first week of the ISO year is the
  63 first such week in which at least 4 days are in a year.  The ISO commercial
  64 date has the form (week day year) in which week is in the range 1..52 and
  65 day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 = Sunday).  The
  66 absolute date is the number of days elapsed since the (imaginary) Gregorian
  67 date 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.
  81 Defaults 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.
 128 Interactively, 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