date.clj

;;; A date library that follows principle of least surprise
(ns clojure.contrib.date)

(import 'java.util.Calendar)
(import 'java.util.TimeZone)

(def #^{:doc "Conversion of Calendar weekdays to keywords"}
     weekday-map
     {Calendar/SUNDAY :sunday
      Calendar/MONDAY :monday
      Calendar/TUESDAY :tuesday
      Calendar/WEDNESDAY :wednesday
      Calendar/THURSDAY :thursday
      Calendar/FRIDAY :friday
      Calendar/SATURDAY :saturday})

(defn- make-calendar
  "Given some date values, create a Java Calendar object with only
that data."
  ([] (doto (Calendar/getInstance)
        (.clear)
        (.setLenient true)))
  ([year month day]
     (doto (make-calendar)
       (.set year (dec month) day)))
  ([year month day hours minutes]
     (doto (make-calendar)
       (.set year (dec month) day hours minutes)))
  ([year month day hours minutes seconds]
     (doto (make-calendar)
       (.set year (dec month) day hours minutes seconds))))

(defn- time-zone-object
  "Gets a Java TimeZone object from a string ID.
If no ID is given, use the default time zone for the current locale."
  ([] (TimeZone/getDefault))
  ([id] (TimeZone/getTimeZone id)))

(defn- time-zone-id
  "Gets the string ID of a Java TimeZone object"
  [tz-obj]
  (.getID tz-obj))

(defn date-dispatcher
  "Gets a type keyword for a Date object. Uses ::Calendar for Java
Calendar objects, and the symbol in the :type slot for Clojure dates."
  [x]
  (if (instance? Calendar x)
    ::Calendar
    (:type x)))

(derive ::Date ::Instant)
(derive ::Time ::Instant)

(defmulti to-date date-dispatcher)

(defmethod to-date ::Calendar [cal]
  (let [d {:year (.get cal Calendar/YEAR)
           :month (inc (.get cal Calendar/MONTH))
           :day (.get cal Calendar/DAY_OF_MONTH)
           :zone (time-zone-id (.getTimeZone cal))}
        h (.get cal Calendar/HOUR_OF_DAY)
        m (.get cal Calendar/MINUTE)
        s (.get cal Calendar/SECOND)]
    (if (= 0 h m s)
      (assoc d :type ::Date)
      (assoc d
        :type ::Time
        :hour h
        :minute m
        :second s))))

(defmulti to-calendar date-dispatcher)

(defmethod to-calendar ::Date [date]
  (doto (Calendar/getInstance)
    (.clear)
    (.set (:year date)
          (dec (:month date))
          (:day date))
    (.setTimeZone (time-zone-object (:zone date)))))

(defmethod to-calendar ::Instant [date]
  (doto (Calendar/getInstance)
    (.clear)
    (.set (:year date)
          (dec (:month date))
          (:day date)
          (:hour date)
          (:minute date)
          (:second date))
    (.setTimeZone (time-zone-object (:zone date)))))

(defn date
  "Creates a Date or Time object with exactly the given information."
  [& args]
  (to-date (apply make-calendar args)))

(defn now
  "Creates a Time object with the current date and time."
  []
  (to-date (Calendar/getInstance)))

(defn today
  "Creates a Date object with the current date."
  []
  (assoc (dissoc (now) :hour :minute :second)
    :type ::Date))

(defn day-of-week [date]
  (weekday-map (.get (to-calendar date) Calendar/DAY_OF_WEEK)))