project_euler_problem_18_67.clj

(use '[clojure.contrib.duck-streams :only (reader read-lines)])

(defn build-sum-mtx [triangle]
  (let [len (count triangle)]
    (loop [mtx [] i 0]
      (cond
       (= i len) mtx
       :else (let [row (get mtx (dec i))
                   sum (fn [n j]
                         (let [a (or (get row (dec j)) 0)
                               b (or (get row j) 0)]
                           (+ (max a b) n)))]
               (recur (conj mtx
                            ;; converting back lazyseq from map to a vector
                            ;; since we would access it (on line 8) in the next iteration
                            (vec (map sum (get triangle i) (range 0 (inc i)))))

                      (inc i)))))))

(defn max-sum [mtx] (apply max (last mtx)))

(defn problem-67 []
  (max-sum
   (build-sum-mtx
    ;; reduce below builds a triangle as a vector of vectors
    (reduce
     conj []
     (map (fn [line]
            (vec (map #(Integer. %)
                      (.split line " "))))
          (read-lines (reader "http://projecteuler.net/project/triangle.txt")))))))