flengyel-4clojure-solution84.clj

;; flengyel's solution to Transitive Closure
;; https://4clojure.com/problem/84

(fn [rel]
  (reduce clojure.set/union
    (take (count rel)
       (iterate
         (partial 
           (fn [R S]
             (set (let [left (lazy-seq (map #(% 0) R))
              right (lazy-seq (map #(% 1) S))
              field (lazy-cat (map #(% 1) R) (map #(% 0) S))]
              (keep (fn [[i k j]] (if (and (contains? R [i k]) 
                                           (contains? S [k j])) 
                                      [i j] 
                                      nil)) 
                     (for [i left j right k field] [i k j])))))
           rel) rel))))