Data commit

Task/Bell-numbers/Racket/bell-numbers.rkt

@@ -0,0 +1,26 @@
+#lang racket
+
+(define (build-bell-row previous-row)
+  (define seed (last previous-row))
+  (reverse
+   (let-values (((reversed _) (for/fold ((acc (list seed)) (prev seed))
+                                        ((pprev previous-row))
+                                (let ((n (+ prev pprev))) (values (cons n acc) n)))))
+     reversed)))
+
+(define reverse-bell-triangle
+  (let ((memo (make-hash '((0 . ((1)))))))
+    (λ (rows) (hash-ref! memo
+                         rows
+                         (λ ()
+                           (let ((prev (reverse-bell-triangle (sub1 rows))))
+                             (cons (build-bell-row (car prev)) prev)))))))
+
+(define bell-triangle (compose reverse reverse-bell-triangle))
+
+(define bell-number (compose caar reverse-bell-triangle))
+
+(module+ main
+  (map bell-number (range 15))
+  (bell-number 50)
+  (bell-triangle 10))