Data commit

Task/Perfect-shuffle/EchoLisp/perfect-shuffle-1.l

@@ -0,0 +1,19 @@
+;; shuffler : a permutation vector which interleaves both halves of deck
+(define (make-shuffler n)
+	(let ((s (make-vector n)))
+		(for ((i (in-range 0 n 2))) (vector-set! s i (/ i 2)))
+		(for ((i (in-range 0 n 2))) (vector-set! s (1+ i) (+ (/ n 2) (vector-ref s i))))
+	 s))
+	
+;; output : (n . # of shuffles needed to go back)
+(define (magic-shuffle n)
+		(when (odd? n) (error "magic-shuffle:odd input" n))
+		(let [(deck (list->vector (iota n))) ;; (0 1 ... n-1)
+			(dock (list->vector (iota n))) ;; keep trace or init deck
+			(shuffler (make-shuffler n))]
+		
+		(cons n (1+
+		(for/sum ((i Infinity)) ; (in-naturals missing  in EchoLisp v2.9)
+			(vector-permute! deck shuffler) ;; permutes in place
+		    #:break (eqv? deck dock) ;; compare to first
+			1)))))

Task/Perfect-shuffle/EchoLisp/perfect-shuffle-2.l

@@ -0,0 +1,12 @@
+map magic-shuffle '(8 24 52 100 1020 1024 10000))
+    → ((8 . 3) (24 . 11) (52 . 8) (100 . 30) (1020 . 1018) (1024 . 10) (10000 . 300))
+
+;; Let's look in the On-line Encyclopedia of Integer Sequences
+;; Given a list of numbers, the (oeis ...) function looks for a sequence
+
+(lib 'web)
+Lib: web.lib loaded.
+map magic-shuffle (range 2 18 2))
+    → ((2 . 1) (4 . 2) (6 . 4) (8 . 3) (10 . 6) (12 . 10) (14 . 12) (16 . 4))
+(oeis '(1 2 4 3 6 10 12 4))
+→ Sequence A002326 found