32aa6dc56935

--- a/src/euler.lisp	Fri Feb 10 21:11:38 2017 +0000
+++ b/src/euler.lisp	Fri Feb 10 21:30:11 2017 +0000
@@ -162,6 +162,65 @@
   ;; Find the sum of all the primes below two million.
   (sum (primes-below 2000000)))
 
+(defun problem-11 ()
+  ;; In the 20×20 grid below, four numbers along a diagonal line have been marked
+  ;; in red.
+  ;;
+  ;; The product of these numbers is 26 × 63 × 78 × 14 = 1788696.
+  ;;
+  ;; What is the greatest product of four adjacent numbers in the same direction
+  ;; (up, down, left, right, or diagonally) in the 20×20 grid?
+  (let ((grid
+          #2A((08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08)
+              (49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00)
+              (81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65)
+              (52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91)
+              (22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80)
+              (24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50)
+              (32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70)
+              (67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21)
+              (24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72)
+              (21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95)
+              (78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92)
+              (16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57)
+              (86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58)
+              (19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40)
+              (04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66)
+              (88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69)
+              (04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36)
+              (20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16)
+              (20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54)
+              (01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48))))
+    (max
+      ;; horizontal
+      (iterate (for-nested ((row :from 0 :below 20)
+                            (col :from 0 :below 16)))
+               (maximize (* (aref grid row (+ 0 col))
+                            (aref grid row (+ 1 col))
+                            (aref grid row (+ 2 col))
+                            (aref grid row (+ 3 col)))))
+      ;; vertical
+      (iterate (for-nested ((row :from 0 :below 16)
+                            (col :from 0 :below 20)))
+               (maximize (* (aref grid (+ 0 row) col)
+                            (aref grid (+ 1 row) col)
+                            (aref grid (+ 2 row) col)
+                            (aref grid (+ 3 row) col))))
+      ;; backslash \
+      (iterate (for-nested ((row :from 0 :below 16)
+                            (col :from 0 :below 16)))
+               (maximize (* (aref grid (+ 0 row) (+ 0 col))
+                            (aref grid (+ 1 row) (+ 1 col))
+                            (aref grid (+ 2 row) (+ 2 col))
+                            (aref grid (+ 3 row) (+ 3 col)))))
+      ;; slash /
+      (iterate (for-nested ((row :from 3 :below 20)
+                            (col :from 0 :below 16)))
+               (maximize (* (aref grid (- row 0) (+ 0 col))
+                            (aref grid (- row 1) (+ 1 col))
+                            (aref grid (- row 2) (+ 2 col))
+                            (aref grid (- row 3) (+ 3 col))))))))
+
 
 ;;;; Tests --------------------------------------------------------------------
 (def-suite :euler)
@@ -177,6 +236,7 @@
 (test p8 (is (= 23514624000 (problem-8))))
 (test p9 (is (= 31875000 (problem-9))))
 (test p10 (is (= 142913828922 (problem-10))))
+(test p11 (is (= 70600674 (problem-11))))
 
 
 ; (run! :euler)