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  1. ; closest two-sum to zero
  2.  
  3. (define (range . args)
  4.   (case (length args)
  5.     ((1) (range 0 (car args) (if (negative? (car args)) -1 1)))
  6.     ((2) (range (car args) (cadr args) (if (< (car args) (cadr args)) 1 -1)))
  7.     ((3) (let ((le? (if (negative? (caddr args)) >= <=)))
  8.            (let loop ((x(car args)) (xs '()))
  9.              (if (le? (cadr args) x)
  10.                  (reverse xs)
  11.                  (loop (+ x (caddr args)) (cons x xs))))))
  12.     (else (error 'range "unrecognized arguments"))))
  13.  
  14. (define rand #f)
  15. (define randint #f)
  16. (let ((two31 #x80000000) (a (make-vector 56 -1)) (fptr #f))
  17.   (define (mod-diff x y) (modulo (- x y) two31)) ; generic version
  18.   ; (define (mod-diff x y) (logand (- x y) #x7FFFFFFF)) ; fast version
  19.   (define (flip-cycle)
  20.     (do ((ii 1 (+ ii 1)) (jj 32 (+ jj 1))) ((< 55 jj))
  21.       (vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
  22.     (do ((ii 25 (+ ii 1)) (jj 1 (+ jj 1))) ((< 55 ii))
  23.       (vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
  24.     (set! fptr 54) (vector-ref a 55))
  25.   (define (init-rand seed)
  26.     (let* ((seed (mod-diff seed 0)) (prev seed) (next 1))
  27.       (vector-set! a 55 prev)
  28.       (do ((i 21 (modulo (+ i 21) 55))) ((zero? i))
  29.         (vector-set! a i next) (set! next (mod-diff prev next))
  30.         (set! seed (+ (quotient seed 2) (if (odd? seed) #x40000000 0)))
  31.         (set! next (mod-diff next seed)) (set! prev (vector-ref a i)))
  32.       (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle)))
  33.   (define (next-rand)
  34.     (if (negative? (vector-ref a fptr)) (flip-cycle)
  35.       (let ((next (vector-ref a fptr))) (set! fptr (- fptr 1)) next)))
  36.   (define (unif-rand m)
  37.     (let ((t (- two31 (modulo two31 m))))
  38.       (let loop ((r (next-rand)))
  39.         (if (<= t r) (loop (next-rand)) (modulo r m)))))
  40.   (init-rand 19380110) ; happy birthday donald e knuth
  41.   (set! rand (lambda seed
  42.     (cond ((null? seed) (/ (next-rand) two31))
  43.           ((eq? (car seed) 'get) (cons fptr (vector->list a)))
  44.           ((eq? (car seed) 'set) (set! fptr (caadr seed))
  45.                                  (set! a (list->vector (cdadr seed))))
  46.           (else (/ (init-rand (modulo (numerator
  47.                   (inexact->exact (car seed))) two31)) two31)))))
  48.   (set! randint (lambda args
  49.     (cond ((null? (cdr args))
  50.             (if (< (car args) two31) (unif-rand (car args))
  51.               (floor (* (next-rand) (car args)))))
  52.           ((< (car args) (cadr args))
  53.             (let ((span (- (cadr args) (car args))))
  54.               (+ (car args)
  55.                  (if (< span two31) (unif-rand span)
  56.                    (floor (* (next-rand) span))))))
  57.           (else (let ((span (- (car args) (cadr args))))
  58.                   (- (car args)
  59.                      (if (< span two31) (unif-rand span)
  60.                        (floor (* (next-rand) span))))))))))
  61.  
  62. (define (rand-list k n)
  63.   (map (lambda (r) (randint (- 1 n) n)) (range k)))
  64.  
  65. (define (brute-force xs)
  66.   (let ((min-sum (apply max xs)) (min-pair (list)))
  67.     (do ((ys xs (cdr ys))) ((null? ys) min-pair)
  68.       (do ((zs (cdr ys) (cdr zs))) ((null? zs))
  69.         (let ((s (abs (+ (car ys) (car zs)))))
  70.           (when (< s min-sum)
  71.             (set! min-sum s)
  72.             (set! min-pair (list (car ys) (car zs)))))))))
  73.  
  74. (define (sort-and-scan xs)
  75.   (define (lt? x y) (< (abs x) (abs y)))
  76.   (let loop ((xs (sort xs lt?))
  77.             (min-sum (apply max xs))
  78.             (min-pair (list)))
  79.     (if (null? (cdr xs)) min-pair
  80.       (let ((s (abs (+ (car xs) (cadr xs)))))
  81.         (if (< s min-sum)
  82.             (loop (cdr xs) s (list (car xs) (cadr xs)))
  83.             (loop (cdr xs) min-sum min-pair))))))
  84.  
  85. (let ((xs (rand-list 7 100)))
  86.   (display xs) (newline)
  87.   (display (brute-force xs)) (newline)
  88.   (display (sort-and-scan xs)) (newline))
Success #stdin #stdout 0.01s 7272KB
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(-75.0 85.0 43.0 15.0 -22.0 -99.0 -3.0)
(15.0 -22.0)
(15.0 -22.0)
https://ideone.com/s2iFaQ
language:
()
created:
8 years ago
visibility:
public

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