zeller.clj

;exercise from http://programmingpraxis.com/2010/10/08/zellers-congruence/
;formula from http://en.wikipedia.org/wiki/Zeller%27s_congruence

(defn month-id [month]
  (+ 3 
    (mod (+ month 9) 12)))

(defn year-of-century [year]
  (mod year 100))

(defn century [year]
  (quot year 100))

(defn int-week-day [day month year]
  (let [q day
        m (month-id month)
        k (year-of-century year)
  	j (century year)]
	
	(mod 
	  (+ q 
	     (quot (* (+ m 1) 26) 10) 
	     k 
	     (quot k 4) 
	     (quot j 4) 
	     (- (* 2 j)))
	  7)))
        
(defn week-day [day month year]
  (+ 1
    (mod 
      (+ 5 (int-week-day day month year)) 
      7)))


;usage (week-day 12 10 2010)