mapmap.clj

(defn repeat
  ([x] (iterate identity x))
  ([n x] (take n (repeat x))))

(def bar (doall (repeat 100 5)))

(defn mapmap [f coll]
  (zipmap coll (map f coll)))

;; => (time (dotimes [_ 10000] (mapmap (partial * 10) bar)))
;; => "Elapsed time: 759.188944 msecs"

(defn mapmap [f coll]
  (persistent!
    (reduce #(assoc! %1 %2 (f %2)) (transient {}) coll)))

;; => (time (dotimes [_ 10000] (mapmap (partial * 10) bar)))
;; => "Elapsed time: 543.236477 msecs"

(defn mapcachemap [f coll]
  (reduce #(assoc %1 %2 (or (%1 %2) (f %2))) {} coll))

;; => (time (dotimes [_ 10000] (mapcachemap (partial * 10) bar)))
;; => "Elapsed time: 246.671207 msecs"

(defn mapcachemap [f coll]
  (reduce #(if (%1 %2) %1 (assoc %1 %2 (f %2))) {} coll))

;; ___

(defn mapmap
  ([f coll] (into {} (map (juxt identity f) coll)))
  ([f coll & more] (into {} (apply map (juxt vector f) coll more))))

;; => (mapmap + [1 2 3] [4 5 6])
;; => {[1 4] 5, [2 5] 7, [3 6] 9}