euler008.lisp

;;;;
;;;; Project Euler - Problem 8
;;;;

(setq *suppress-similar-constant-redefinition-warning* t)
(defconstant *euler008-numbers*
  '(
    "73167176531330624919225119674426574742355349194934"
    "96983520312774506326239578318016984801869478851843"
    "85861560789112949495459501737958331952853208805511"
    "12540698747158523863050715693290963295227443043557"
    "66896648950445244523161731856403098711121722383113"
    "62229893423380308135336276614282806444486645238749"
    "30358907296290491560440772390713810515859307960866"
    "70172427121883998797908792274921901699720888093776"
    "65727333001053367881220235421809751254540594752243"
    "52584907711670556013604839586446706324415722155397"
    "53697817977846174064955149290862569321978468622482"
    "83972241375657056057490261407972968652414535100474"
    "82166370484403199890008895243450658541227588666881"
    "16427171479924442928230863465674813919123162824586"
    "17866458359124566529476545682848912883142607690042"
    "24219022671055626321111109370544217506941658960408"
    "07198403850962455444362981230987879927244284909188"
    "84580156166097919133875499200524063689912560717606"
    "05886116467109405077541002256983155200055935729725"
    "71636269561882670428252483600823257530420752963450"
    ))

(defun euler008 ()
  (let ((number-string "")
        (greatest-product 0))
    (dolist (num *euler008-numbers*)
      (setf number-string (concatenate 'string number-string num)))
    (do ((i 0 (1+ i)))
        ((= i 996))
      (let ((digit1 (parse-integer (subseq number-string i (+ i 1))))
            (digit2 (parse-integer (subseq number-string (+ i 1) (+ i 2))))
            (digit3 (parse-integer (subseq number-string (+ i 2) (+ i 3))))
            (digit4 (parse-integer (subseq number-string (+ i 3) (+ i 4))))
            (digit5 (parse-integer (subseq number-string (+ i 4) (+ i 5)))))
        (if (> (* digit1 digit2 digit3 digit4 digit5) greatest-product)
            (setf greatest-product (* digit1 digit2 digit3 digit4 digit5)))))
    greatest-product))