euler8.lisp

;;;; Problem 8 - Largest product in a series

;;; The four adjacent digits in the 1000-digit number that have the
;;; greatest product are 9 × 9 × 8 × 9 = 5832.
;;; 
;;; 73167176531330624919225119674426574742355349194934
;;; 96983520312774506326239578318016984801869478851843
;;; 85861560789112949495459501737958331952853208805511
;;; 12540698747158523863050715693290963295227443043557
;;; 66896648950445244523161731856403098711121722383113
;;; 62229893423380308135336276614282806444486645238749
;;; 30358907296290491560440772390713810515859307960866
;;; 70172427121883998797908792274921901699720888093776
;;; 65727333001053367881220235421809751254540594752243
;;; 52584907711670556013604839586446706324415722155397
;;; 53697817977846174064955149290862569321978468622482
;;; 83972241375657056057490261407972968652414535100474
;;; 82166370484403199890008895243450658541227588666881
;;; 16427171479924442928230863465674813919123162824586
;;; 17866458359124566529476545682848912883142607690042
;;; 24219022671055626321111109370544217506941658960408
;;; 07198403850962455444362981230987879927244284909188
;;; 84580156166097919133875499200524063689912560717606
;;; 05886116467109405077541002256983155200055935729725
;;; 71636269561882670428252483600823257530420752963450
;;; 
;;; Find the thirteen adjacent digits in the 1000-digit number that have
;;; the greatest product. What is the value of this product?

(defparameter bignumber
  7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450)

(defparameter bigstring (write-to-string bignumber))

(defun sum-ints-string (i)
  (loop for p from 0 to (- (length i) 1) 
	collect (parse-integer (subseq i p (+ p 1)) :end 1)))

(defun trawl-big-number (number chunksize)
  (let ((result 0)
	(biggest-result 0))
    (loop for i from 0 to (- 1000 chunksize) do
      (setf result (apply #'* (sum-ints-string (subseq number i (+ i chunksize)))))
      (if (> result biggest-result)
	  (setf biggest-result result)))
    biggest-result))

;; (trawl-big-number bigstring 13)
;; 23514624000