map-with-state.clj

(def entries [{:id 1 :value 88}
              {:id 2 :value 99}
              {:id 1 :value 98}
              {:id 3 :value 5}
              {:id 2 :value 100}
              {:id 3 :value 10}
              {:id 1 :value 150}])

(defn make-running-diff-map []
  (let [state (atom {})]
    ;; Map function
    (fn [{:as entry :keys [id value]}]
      (get (swap! state (fn [s]
                      (update s id (fn [{:as p
                                         pvalue :value}]
                                     (if p
                                       ;; Add a :diff
                                       (assoc entry :diff (- value pvalue))
                                       ;; no previous entry
                                       entry)
                                     ))))
           id))))

(map (make-running-diff-map) entries)
;; => ({:id 1, :value 88} {:id 2, :value 99} {:id 1, :value 98, :diff 10} {:id 3, :value 5} {:id 2, :value 100, :diff 1} {:id 3, :value 10, :diff 5} {:id 1, :value 150, :diff 52})