When was Yale's 483rd reunion?

gistfile1.clj

(ns reunions.core)

(defn- next-year
  [years reunion-counts]
  (let [this-year (inc (last (keys reunion-counts)))
        reunion-counts (assoc reunion-counts this-year 0)
        add-reunion (fn [counts year]
                      (let [abs-year (- this-year year)]
                        (if (contains? counts abs-year)
                          (update-in counts [abs-year] inc)
                          counts)))]
    (reduce add-reunion reunion-counts years)))

(defn reunions
  [year-established years]
  (iterate (partial next-year years)
           (sorted-map year-established 0)))

(defn when-was-yales-483rd-reunion?
  [years]
  (last (keys (first (filter #(>= (apply + (vals %)) 483)
                               (reunions 1701 years))))))

; (core/when-was-yales-483rd-reunion? #{5 10 20})
;=> 1873