set-test.clj

(ns set-test
  (:require [criterium.core :refer [bench quick-bench]])
  (:import [clojure.lang IPersistentSet]))

(defn- bubble-max-key
  "Move a maximal element of coll according to fn k (which returns a
  number) to the front of coll."
  [k coll]
  (let [max (apply max-key k coll)]
    (cons max (remove #(identical? max %) coll))))

(defn union
  "Return a set that is the union of the input sets"
  {:added "1.0"}
  ([] #{})
  ([s1] s1)
  ([s1 s2]
     (if (< (count s1) (count s2))
       (reduce conj s2 s1)
       (reduce conj s1 s2)))
  ([s1 s2 & sets]
     (let [bubbled-sets (bubble-max-key count (conj sets s2 s1))]
       (reduce into (first bubbled-sets) (rest bubbled-sets)))))

(defn union-checking
  "Return a set that is the union of the input sets"
  {:added "1.0"}
  ([] #{})
  ([s1] 
   (assert (instance? IPersistentSet s1) "First argument must be a set")
   s1)
  ([s1 s2]
      (assert (instance? IPersistentSet s1) "First argument must be a set")
      (assert (instance? IPersistentSet s2) "Second argument must be a set")
     (if (< (count s1) (count s2))
       (reduce conj s2 s1)
       (reduce conj s1 s2)))
  ([s1 s2 & sets]
     (let [bubbled-sets (bubble-max-key count (conj sets s2 s1))]
       (reduce into (first bubbled-sets) (rest bubbled-sets)))))

(let [a #{1 2 3}
      b #{2 3 4}]
  (println "Benching Unchecked")
  (flush)
  (bench
    (union a b))
  (println "Benching Checked")
  (flush)
  (bench
    (union-checking a b)))