guile-project-euler-solutions/008/solution.scm

;; https://projecteuler.net/problem=8

;; Largest product in a series

;; Problem 8

;; 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?



(import
  (except (rnrs base) let-values)
  (only (guile) lambda* λ))


(define %number-string
  "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450")


(define char->number
  (λ (c)
    (string->number (list->string (list c)))))


(define calculate-product
  (λ (digits-as-string)
    (string-fold (λ (a b)
                   (* (char->number a) b))
                 1
                 digits-as-string)))


(define find-largest-product
  (λ (number-as-string digit-count)
    (let ([num-len (string-length number-as-string)])
      ;; Can this be made more readable / simpler?
      (let loop ([pos 0] [previous-maximum 0])
        (cond
         [(>= (+ pos digit-count) num-len)
          previous-maximum]
         [else
          (let ([digits-as-string
                 (substring number-as-string
                            pos
                            (+ pos digit-count))])
            (cond
             [(string-contains digits-as-string "0")
              (loop (+ pos 1) previous-maximum)]
             [else
              (display (simple-format #f "position: ~a\n" pos))
              (display (simple-format #f "digits: ~a\n" digits-as-string))
              (display (simple-format #f "product: ~a\n" (calculate-product digits-as-string)))
              (loop (+ pos 1)
                    (max previous-maximum
                         (calculate-product digits-as-string)))]))])))))


(find-largest-product %number-string 13)  ; 23514624000