Data commit

Task/Stern-Brocot-sequence/Common-Lisp/stern-brocot-sequence.lisp

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+(defun stern-brocot (numbers)
+  (declare ((or null (vector integer)) numbers))
+  (cond ((null numbers)
+         (setf numbers (make-array 2 :element-type 'integer :adjustable t :fill-pointer t
+                                     :initial-element 1)))
+        ((zerop (length numbers))
+         (vector-push-extend 1 numbers)
+         (vector-push-extend 1 numbers))
+        (t
+         (assert (evenp (length numbers)))
+         (let* ((considered-index (/ (length numbers) 2))
+                (considered (aref numbers considered-index))
+                (precedent  (aref numbers (1- considered-index))))
+           (vector-push-extend (+ considered precedent) numbers)
+           (vector-push-extend considered numbers))))
+  numbers)
+
+(defun first-15 ()
+  (loop for input = nil then seq
+        for seq = (stern-brocot input)
+        while (< (length seq) 15)
+        finally (format t "First 15: ~{~A~^ ~}~%" (coerce (subseq seq 0 15) 'list))))
+
+(defun first-1-to-10 ()
+  (loop with seq = (stern-brocot nil)
+        for i from 1 to 10
+        for index = (loop with start = 0
+                          for pos = (position i seq :start start)
+                          until pos
+                          do (setf start (length seq)
+                                   seq   (stern-brocot seq))
+                          finally (return (1+ pos)))
+        do (format t "First ~D at ~D~%" i index)))
+
+(defun first-100 ()
+  (loop for input = nil then seq
+        for start = (length input)
+        for seq = (stern-brocot input)
+        for pos = (position 100 seq :start start)
+        until pos
+        finally (format t "First 100 at ~D~%" (1+ pos))))
+
+(defun check-gcd ()
+  (loop for input = nil then seq
+        for seq = (stern-brocot input)
+        while (< (length seq) 1000)
+        finally (if (loop for i from 0 below 999
+                          always (= 1 (gcd (aref seq i) (aref seq (1+ i)))))
+                    (write-line "Correct.  The GCDs of all the two consecutive numbers are 1.")
+                    (write-line "Wrong."))))
+
+(defun main ()
+  (first-15)
+  (first-1-to-10)
+  (first-100)
+  (check-gcd))