date-truncate.clj

;; Doesn't work...
;; TODO: allow (truncate n (t/minutes 10)): "2014-06-27 12:34:56.789" -> "2014-06-27 12:30:00.000"

(defmacro mutator [method val]
  `(fn [^MutableDateTime mdt#] (~method mdt# val)))

(defn truncate [^DateTime dt units]
  (loop [^MutableDateTime mdt (.toMutableDateTime dt)
         [[unit method] & more] [[t/years   nil]
                                 [t/months  (mutator .setMonthOfYear 1)]
                                 [t/days    (mutator .setDayOfMonth 1)]
                                 [t/hours   (mutator .setHourOfDay  0)]
                                 [t/minutes (mutator .setMinuteOfHour 0)]
                                 [t/seconds (mutator .setSecondOfMinute 0)]
                                 [nil       (mutator .setMillisOfSecond 0)]]
         zero? false]
    (when zero? (method mdt))
    (if unit
      (recur mdt more (or zero? (= (unit) units)))
      (if zero?
        (.toDateTime mdt)
        (throw (Exception. "Unrecognised unit"))))))

(do ;comment
  (let [n (t/now)]
    (println n (truncate n (t/years)))
    (println n (truncate n (t/months)))
    (println n (truncate n (t/days)))
    (println n (truncate n (t/hours)))
    (println n (truncate n (t/minutes)))
    (println n (truncate n (t/seconds)))))