Closure, coin sums, & number spiral generator

1 files changed, 27 insertions, 0 deletions

diff --git a/number-spiral-diagonals/nsd.scm b/number-spiral-diagonals/nsd.scm
new file mode 100644
index 0000000..9d4bfe3
--- /dev/null
+++ b/number-spiral-diagonals/nsd.scm
@@ -0,0 +1,27 @@
+(define-module (nsd)
+  #:export (nsd
+            nsd-generator
+            calculate-seq-length))
+
+
+(define (nsd n)
+  "Given dimension N of a square, calculates and sums
+an appropriately sized sequence."
+  (apply + (nsd-generator (calculate-seq-length n))))
+
+(define (nsd-generator k)
+  "Generates a sequence of numbers corresponding to the
+diagonals in a square. See README for more details on how
+this is used to solve the problem."
+  (let loop ((i 0)
+             (lst '(1)))
+    (if (>= (1+ i) k)
+        (reverse lst)
+        (loop (1+ i)
+              (cons (+ (car lst) (* 2 (1+ (quotient i 4)))) lst)))))
+
+(define (calculate-seq-length n)
+  "Calculates the size of the sequence. This is equal to
+the length of the two diagonals, minus one for the overlapping
+center."
+  (1- (* 2 n)))