partition-key-by: Clojure transducer which emits the value of `(f x)` every time it changes. Useful when you want to use distinct on a collection you know is already sorted, or group-by but only care about the keys.

partition-key-by.cljc

(defn partition-key-by
  "Return a transducer which emits the value of `(f x)` every time it changes.
  
  This is the same as the following, except far more efficient because it will
  not retain large intermediate collections:
  
      (comp
        (map f)
        (partition-all)
        (map first))
  
  This function is chiefly useful with sorted collections."
  [f]
  (fn [xf]
    (let [last (volatile! (Object.))]
      (fn
        ([] (xf))
        ([r] (vreset! last nil) (xf r))
        ([r x]
         (let [k (f x)]
           (if (= k @last)
             r
             (do
               (vreset! last k)
               (xf r k)))))))))