fast-max.clj

user> (defn fast-max [k x & more]
                    (first
                     (reduce (fn [[x x-key :as orig] y]
                               (let [[_ y-key :as new] [y (k y)]]
                                 (if (> x-key y-key)
                                   orig
                                   new)))
                             [x (k x)]
                             more)))
        
#'user/fast-max
user> (time (apply fast-max #(do (Thread/sleep 100) %) (range 100)))
"Elapsed time: 10007.125451 msecs"
99
user> (time (apply max-key #(do (Thread/sleep 100) %) (range 100)))
"Elapsed time: 19811.947463 msecs"
99

Any interest in adding something like this to contrib? It shouldn't replace max-key, since the extra bookkeeping slows things down if k is cheap to apply, but for expensive keys this is better.