find-all-cycles.clj

(defn find-all-cycles [a b]
  "finds all possible cycles in the parents"
  (loop [idx 0
         cyl (order-preserving-set)]
    (let [c (find-cycle a b idx)]
      (if (= idx (count a))
        (vec (seq cyl))
        (recur (inc idx) (into cyl [c]))))))

(defn find-cycle [a b seed]
  "finds a single cycle in the parents
   starting at the seed index"
  (loop [coll (mapv vector a b)
         cyl  #{}
         cur  (get-in coll [seed 0])]
    (let [idx (first (keep-indexed #(if (= (first %2) cur) %1) coll))]
      (if (nil? idx)
        (vec cyl)
        (recur (vec-remove coll idx)
               (into cyl (nth coll idx))
               (last (nth coll idx)))))))