comparator composition

comparator_composition.clj

user> (defn multi-comparator [& comps]
        (reify java.util.Comparator
          (compare [this x y]
            (reduce (fn [r ^java.util.Comparator c]
                      (if (zero? r) (.compare c x y) r))
                    0
                    comps))))
#'user/multi-comparator

;;; earlier versions, last-to-first

user> (defn multi-comparator [& comps]
        (reify java.util.Comparator
          (compare [this x y]
            (reduce (fn [r c]
                      (if (zero? r) (c x y) r))
                    0
                    comps))))
#'user/multi-comparator
user> (def test-set (sorted-set-by (multi-comparator (fn [x y] (compare (count x) (count y))) compare)))
#'user/test-set
user> ((into test-set [[1 2 3] [4 5 6] [7 8 9] [10 11 12] [13 14 15]]) [7 8 9])
[7 8 9]

user> (defn multi-comparator [& comps]
        (reify java.util.Comparator
          (compare [this x y]
            (loop [comps comps r 0]
              (if-let [c (and (zero? r) (first comps))]
                (recur (next comps) (c x y))
                r)))))
#'user/multi-comparator
user> (def test-set (sorted-set-by (multi-comparator (fn [x y] (compare (count x) (count y))) compare)))
#'user/test-set
user> ((into test-set [[1 2 3] [4 5 6] [7 8 9] [10 11 12] [13 14 15]]) [7 8 9])
[7 8 9]

user> (defn comparator* [& comps]
        (fn [x y]
          (reduce (fn [r c] (if (zero? r) (c x y) r))
                  0
                  comps)))
#'user/comparator*
user> (def test-set (sorted-set-by (comparator* (fn [x y] (compare (count x) (count y))) compare)))
#'user/test-set
user> ((into test-set [[1 2 3] [4 5 6] [7 8 9] [10 11 12] [13 14 15]]) [7 8 9])
[7 8 9]

user> (defn comparator** [& preds]
        (fn [x y]
          (reduce (fn [r p] (if (zero? r) (cond (p x y) -1 (p y x) 1 :else 0) r))
                  0
                  preds)))
#'user/comparator**
user> (def test-set2 (sorted-set-by (comparator** (fn [x y] (< (count x) (count y))) (fn [x y] (some true? (map < x y))))))
#'user/test-set2
user> ((into test-set2 [[1 2 3] [4 5 6] [7 8 9] [10 11 12] [13 14 15]]) [7 8 9])
[7 8 9]