Find BPM by tapping. Belongs to this blog post: http://multimethods.blogspot.se/2013/08/small-app-for-finding-bpms-by-key-tap.html

bpm.clj

(use 'seesaw.core)

;; the state, when was the last keypress?
(def last-tap (atom nil))

(defn nanoseconds-diff-to-bpm [now then]
  (let [nanosecs-in-a-minute (* 60 1000 1000 1000.0)  
        diff                 (- now then)]
    (/ nanosecs-in-a-minute diff)))

(def ze-label (label "o"))
(def ze-frame (frame :title "BPM-counter" :content ze-label))
(-> ze-frame pack! show!)

;; looking for changes of the state, process and presentes the BPM from this info
(add-watch last-tap :calculate-bpm-and-put-it-to-label 
           (fn [_ _ last-time now] 
             (if last-time 
               (config! ze-label :text (nanoseconds-diff-to-bpm now last-time)))))

;; attempts to put the current time in nanoseconds into the state
(defn tap! [] (reset! last-tap (System/nanoTime)))

;;when any key is pressed the window triggers fn tap!
(listen ze-frame :key-pressed (fn [e] (future (tap!))))