20c03264a55e

--- a/euler.lisp	Sun Apr 10 14:30:17 2016 +0000
+++ b/euler.lisp	Sun Apr 10 19:17:38 2016 +0000
@@ -128,6 +128,13 @@
 (defun problem-7 ()
   (nth-prime 10001))
 
+(defun problem-8 ()
+ (let ((digits (map 'list #'digit-char-p
+                    "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450")))
+   (loop :for window :in (n-grams 13 digits)
+         :maximize (apply #'* window))))
+
+
 ;;;; Tests
 (def-suite :euler)
 (in-suite :euler)
@@ -139,5 +146,7 @@
 (test p5 (is (= 232792560 (problem-5))))
 (test p6 (is (= 25164150 (problem-6))))
 (test p7 (is (= 104743 (problem-7))))
+(test p8 (is (= 23514624000 (problem-8))))
+
 
 ; (run! :euler)

--- a/make-utilities.lisp	Sun Apr 10 14:30:17 2016 +0000
+++ b/make-utilities.lisp	Sun Apr 10 19:17:38 2016 +0000
@@ -7,5 +7,6 @@
                :while
                :ensure-boolean
                :with-gensyms
+               :n-grams
                )
   :package "EULER.QUICKUTILS")

--- a/quickutils.lisp	Sun Apr 10 14:30:17 2016 +0000
+++ b/quickutils.lisp	Sun Apr 10 19:17:38 2016 +0000
@@ -2,7 +2,7 @@
 ;;;; See http://quickutil.org for details.
 
 ;;;; To regenerate:
-;;;; (qtlc:save-utils-as "quickutils.lisp" :utilities '(:DEFINE-CONSTANT :SWITCH :WHILE :ENSURE-BOOLEAN :WITH-GENSYMS) :ensure-package T :package "EULER.QUICKUTILS")
+;;;; (qtlc:save-utils-as "quickutils.lisp" :utilities '(:DEFINE-CONSTANT :SWITCH :WHILE :ENSURE-BOOLEAN :WITH-GENSYMS :N-GRAMS) :ensure-package T :package "EULER.QUICKUTILS")
 
 (eval-when (:compile-toplevel :load-toplevel :execute)
   (unless (find-package "EULER.QUICKUTILS")
@@ -15,7 +15,8 @@
 (when (boundp '*utilities*)
   (setf *utilities* (union *utilities* '(:DEFINE-CONSTANT :STRING-DESIGNATOR
                                          :WITH-GENSYMS :EXTRACT-FUNCTION-NAME
-                                         :SWITCH :UNTIL :WHILE :ENSURE-BOOLEAN))))
+                                         :SWITCH :UNTIL :WHILE :ENSURE-BOOLEAN
+                                         :TAKE :N-GRAMS))))
 
   (defun %reevaluate-constant (name value test)
     (if (not (boundp name))
@@ -165,8 +166,29 @@
     "Convert `x` into a Boolean value."
     (and x t))
   
+
+  (defun take (n sequence)
+    "Take the first `n` elements from `sequence`."
+    (subseq sequence 0 n))
+  
+
+  (defun n-grams (n sequence)
+    "Find all `n`-grams of the sequence `sequence`."
+    (assert (and (plusp n)
+                 (<= n (length sequence))))
+    
+    (etypecase sequence
+      ;; Lists
+      (list (loop :repeat (1+ (- (length sequence) n))
+                  :for seq :on sequence
+                  :collect (take n seq)))
+      
+      ;; General sequences
+      (sequence (loop :for i :to (- (length sequence) n)
+                      :collect (subseq sequence i (+ i n))))))
+  
 (eval-when (:compile-toplevel :load-toplevel :execute)
   (export '(define-constant switch eswitch cswitch while ensure-boolean
-            with-gensyms with-unique-names)))
+            with-gensyms with-unique-names n-grams)))
 
 ;;;; END OF quickutils.lisp ;;;;