Re: holidays.el

[Ed Reingold]

That's a good idea--there are similar things elsewhere that should be changed. 
I did not write it that way originally (nearly 20 years ago!) because it would 
have been too slow; now hardware is so fast that the difference in time would 
be negligible.  Feel free to fix this.

> I was thinking that if the function holiday-easter-etc looked
> something like this:
> 
> (defun holiday-easter-etc (n string)
>   "Holiday on Nth day after Easter called STRING."
>   (let* ((century (1+ (/ displayed-year 100)))
>        (shifted-epact        ;; Age of moon for April 5...
>         (% (+ 14 (* 11 (% displayed-year 19));;     ...by Nicaean rule
>               (-           ;; ...corrected for the Gregorian century rule
>                (/ (* 3 century) 4))
>               (/    ;; ...corrected for Metonic cycle inaccuracy.
>                (+ 5 (* 8 century)) 25)
>               (* 30 century));;              Keeps value positive.
>            30))
>        (adjusted-epact       ;;  Adjust for 29.5 day month.
>         (if (or (= shifted-epact 0)
>                 (and (= shifted-epact 1) (< 10 (% displayed-year 19))))
>             (1+ shifted-epact)
>           shifted-epact))
>        (paschal-moon       ;; Day after the full moon on or after March 21.
>         (- (calendar-absolute-from-gregorian (list 4 19 displayed-year))
>            adjusted-epact))
>        (abs-easter (calendar-dayname-on-or-before 0 (+ paschal-moon 7)))
>        (mandatory
>         (list
>          (list (calendar-gregorian-from-absolute (+ abs-easter n))
>                string)))
>        (output-list
>         (filter-visible-calendar-holidays mandatory)))
>     output-list))
> 
> and the function holiday-advent looked something like this:
> 
> (defun holiday-advent (n string)
>   "Holiday on Nth day after Advent called STRING."
>   (let ((year displayed-year)
>         (month displayed-month))
>     (increment-calendar-month month year -1)
>     (let ((advent (calendar-gregorian-from-absolute
>                  (+ n
>                     (calendar-dayname-on-or-before 0
>                      (calendar-absolute-from-gregorian
>                       (list 12 3 year)))))))
>       (if (calendar-date-is-visible-p advent)
>           (list (list advent string))))))
> 
> then the variable christian-holidays (in calendar.el) could look like
> this:
> 
> ;;;###autoload
> (put 'christian-holidays 'risky-local-variable t)
> ;;;###autoload
> (defcustom christian-holidays
>   '((if all-christian-calendar-holidays
>         (holiday-fixed 1 6 "Epiphany"))
>     (holiday-easter-etc 0 "Easter Sunday")
>     (holiday-easter-etc -2 "Good Friday")
>     (holiday-easter-etc -46 "Ash Wednesday")
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -63 "Septuagesima Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -56 "Sexagesima Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -49 "Shrove Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -48 "Shrove Monday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -47 "Shrove Tuesday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -14 "Passion Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -7 "Palm Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc -3 "Maundy Thursday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc 35 "Rogation Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc 39 "Ascension Day"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc 49 "Pentecost (Whitsunday)"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc 50 "Whitmonday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc 56 "Trinity Sunday"))
>     (if all-christian-calendar-holidays
>         (holiday-easter-etc 60 "Corpus Christi"))
>     (if all-christian-calendar-holidays
>         (holiday-greek-orthodox-easter))
>     (if all-christian-calendar-holidays
>         (holiday-fixed 8 15 "Assumption"))
>     (if all-christian-calendar-holidays
>         (holiday-advent 0 "Advent"))
>     (holiday-fixed 12 25 "Christmas")
>     (if all-christian-calendar-holidays
>         (holiday-julian 12 25 "Eastern Orthodox Christmas")))
>   "*Christian holidays.
> See the documentation for `calendar-holidays' for details."
>   :type 'sexp
>   :group 'holidays)
> 
> and be much easier to customize. Is this a good/bad idea?
> 
> 
> 
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