Levenshtein distance that short circuits

gistfile1.txt

(defn levenshtein [thr str1 str2]
        "Clojure Levenshtein distance which short circuits when the distance is > thr"
        (let [n (count str1) 
              m (count str2)]
          (cond 
            (= 0 n) m
            (= 0 m) n
            :else
            (last
             (reduce (fn [prev-col i]
                       (let [nextrow (reduce (fn [col j]
                                               (let [lev (fn [i j]
                                                           (min (inc (nth col j))
                                                                (inc (nth prev-col (inc j)))
                                                                (+ (get prev-col j)
                                                                   (if (= (nth str1 i) (nth str2 j)) 0 1))))]
                                                 (conj col (lev i j)))) ; update col[1..m]
                                             [(inc i)]
                                             (range m))]
                         (if (<= (apply min nextrow) thr)
                           nextrow
                           (reduced [:big]))))
                     (vec (range (inc m)))
                     (vec (range n)))) ; initialization for the first column.
            )))