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Task/Hailstone-sequence/EchoLisp/hailstone-sequence-1.echolisp

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+(lib 'hash)
+(lib 'sequences)
+(lib 'compile)
+
+(define (hailstone n)
+(when (> n 1)
+	(if (even? n) (/ n 2) (1+ (* n 3)))))
+	
+(define H (make-hash))
+
+;; (iterator/f seed f) returns seed, (f seed) (f(f seed)) ...
+
+(define (hlength seed)
+	(define collatz (iterator/f hailstone seed))
+	(or
+	   (hash-ref H seed) ;; known ?
+	   (hash-set H seed
+	      (for ((i (in-naturals)) (h collatz))
+              ;; add length of subsequence if already known
+	      #:break (hash-ref H h) => (+ i (hash-ref H h))
+	      (1+ i)))))
+	
+(define (task (nmax 100000))
+	(for ((n [1 .. nmax])) (hlength n)) ;; fill hash table
+
+	(define hmaxlength (apply max (hash-values H)))
+	(define hmaxseed (hash-get-key H hmaxlength))
+	(writeln 'maxlength= hmaxlength 'for hmaxseed))