problem-008.lisp

;;;; Problem 8


(defun problem-008 ()
  (let ((series "73167176531330624919225119674426574742355349194934
                 96983520312774506326239578318016984801869478851843
                 85861560789112949495459501737958331952853208805511
                 12540698747158523863050715693290963295227443043557
                 66896648950445244523161731856403098711121722383113
                 62229893423380308135336276614282806444486645238749
                 30358907296290491560440772390713810515859307960866
                 70172427121883998797908792274921901699720888093776
                 65727333001053367881220235421809751254540594752243
                 52584907711670556013604839586446706324415722155397
                 53697817977846174064955149290862569321978468622482
                 83972241375657056057490261407972968652414535100474
                 82166370484403199890008895243450658541227588666881
                 16427171479924442928230863465674813919123162824586
                 17866458359124566529476545682848912883142607690042
                 24219022671055626321111109370544217506941658960408
                 07198403850962455444362981230987879927244284909188
                 84580156166097919133875499200524063689912560717606
                 05886116467109405077541002256983155200055935729725
                 71636269561882670428252483600823257530420752963450"))
    (largest-product-in-series series)))



(defun largest-product-in-series (series)
  "Returns the greatest product of five consecutive digits in series."
  (let* ((numbers (map 'vector
                       #'digit-char-p
                       (remove-if-not #'digit-char-p
                                      (coerce series 'list))))
         (cursor (make-array 5
                             :adjustable t
                             :displaced-to numbers
                             :displaced-index-offset 0)))
    (apply #'max
           (loop for i upto (- (length numbers)
                               (length cursor))
              collecting (progn
                           (adjust-array cursor
                                         5
                                         :displaced-to numbers
                                         :displaced-index-offset i)
                           (reduce #'* cursor))))))

;; (PROBLEM-008)
;; took 784 microseconds (0.000784 seconds) to run.
;; During that period, and with 8 available CPU cores,
;;      784 microseconds (0.000784 seconds) were spent in user mode
;;        4 microseconds (0.000004 seconds) were spent in system mode
;;  93,792 bytes of memory allocated.