2016 Update

Task/Hamming-numbers/Racket/hamming-numbers-2.rkt

@@ -0,0 +1,27 @@
+#lang racket
+(require racket/stream)
+(define first stream-first)
+(define rest  stream-rest)
+
+(define (hamming)
+  (define (merge s1 s2)
+    (let ([x1 (first s1)]
+          [x2 (first s2)])
+      (if (< x1 x2) ; don't have to handle duplicate case
+          (stream-cons x1 (merge (rest s1) s2))
+          (stream-cons x2 (merge s1 (rest s2))))))
+  (define (smult m s) ; faster than using map (* m)
+    (define (smlt ss)
+      (stream-cons (* m (first ss)) (smlt (rest ss))))
+    (smlt s))
+  (define (u s n)
+    (if (stream-empty? s) ; checking here more efficient than in merge
+        (letrec ([r (smult n (stream-cons 1 r))])
+          r)
+        (letrec ([r (merge s (smult n (stream-cons 1 r)))])
+          r)))
+  (stream-cons 1 (stream-fold u empty-stream '(5 3 2))))
+
+(for/list ([i 20] [x (hamming)]) x) (newline)
+(stream-ref (hamming) 1690) (newline)
+(stream-ref (hamming) 999999) (newline)