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@oantolin
Last active May 11, 2020
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(declaim (optimize (speed 3) (safety 0)))
(defmacro with-type (type expr)
(if (atom expr)
expr
(let ((op (car expr)))
(reduce
(lambda (x y)
`(the ,type
(,op ,@(if x (list x) '())
(with-type ,type ,y))))
(cdr expr)
:initial-value nil))))
(defun layer (x y z n)
(declare (fixnum x y z n))
; (with-type fixnum
(logand #.(1- (ash 1 (integer-length most-positive-fixnum)))
(+ (* 2 (+ (* x y) (* y z) (* x z)))
(* 4 (+ x y z n -2) (1- n)))))
(defun cubes (n)
(declare (fixnum n))
(let ((count (make-array (+ n 1) :element-type 'fixnum)))
(loop for x of-type fixnum from 1 while (<= (layer x x x 1) n) do
(loop for y of-type fixnum from x while (<= (layer x y y 1) n) do
(loop for z of-type fixnum from y while (<= (layer x y z 1) n) do
(loop for k of-type fixnum from 1 while (<= (layer x y z k) n) do
(incf (elt count (layer x y z k)))))))
count))
(defun first-time (x)
(declare (fixnum x))
(loop for n of-type fixnum = 1000 then (* 2 n)
for k = (position x (cubes n))
until k
finally (return k)))
(time (print (first-time 1000)))
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