Implement group-by with reducers

group_by_reducers.clj

(ns group-by-reducers.core
  (:require [clojure.core.reducers :as r :only [fold reduce map]])
  (:require [criterium.core :as c]))

(defn group-by-naive [f coll]
  (reduce
    (fn [groups a]
      (assoc groups (f a) (conj (get groups a []) a)))
    {}
    coll))

(defn group-by-red [f coll]
  (letfn [(reduce-groups
            ([] {})
            ([g1 g2]
              (merge-with concat g1 g2)))]
    (r/fold reduce-groups (r/map #(hash-map (f %) [%]) coll))))

(defn -main
  [& args]
  (c/quick-bench (group-by #(mod % 29) (range 10000)))
  (c/quick-bench (group-by-naive #(mod % 29) (range 10000)))
  (c/quick-bench (group-by-red #(mod % 29) (range 10000))))