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Project: programmingpraxis
Link: http://programmingpraxis.codepad.org/ZmUkqk5Y    [ raw code | output | fork ]

programmingpraxis - Scheme, pasted on Nov 26:
; amazon interview question

(define (square x) (* x x))

(define rand #f)
(define randint #f)
(let ((two31 #x80000000) (a (make-vector 56 -1)) (fptr #f))
  (define (mod-diff x y) (modulo (- x y) two31)) ; generic version
  ; (define (mod-diff x y) (logand (- x y) #x7FFFFFFF)) ; fast version
  (define (flip-cycle)
    (do ((ii 1 (+ ii 1)) (jj 32 (+ jj 1))) ((< 55 jj))
      (vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
    (do ((ii 25 (+ ii 1)) (jj 1 (+ jj 1))) ((< 55 ii))
      (vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
    (set! fptr 54) (vector-ref a 55))
  (define (init-rand seed)
    (let* ((seed (mod-diff seed 0)) (prev seed) (next 1))
      (vector-set! a 55 prev)
      (do ((i 21 (modulo (+ i 21) 55))) ((zero? i))
        (vector-set! a i next) (set! next (mod-diff prev next))
        (set! seed (+ (quotient seed 2) (if (odd? seed) #x40000000 0)))
        (set! next (mod-diff next seed)) (set! prev (vector-ref a i)))
      (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle)))
  (define (next-rand)
    (if (negative? (vector-ref a fptr)) (flip-cycle)
      (let ((next (vector-ref a fptr))) (set! fptr (- fptr 1)) next)))
  (define (unif-rand m)
    (let ((t (- two31 (modulo two31 m))))
      (let loop ((r (next-rand)))
        (if (<= t r) (loop (next-rand)) (modulo r m)))))
  (init-rand 19380110) ; happy birthday donald e knuth
  (set! rand (lambda seed
    (cond ((null? seed) (/ (next-rand) two31))
          ((eq? (car seed) 'get) (cons fptr (vector->list a)))
          ((eq? (car seed) 'set) (set! fptr (caadr seed))
                                 (set! a (list->vector (cdadr seed))))
          (else (/ (init-rand (modulo (numerator
                  (inexact->exact (car seed))) two31)) two31)))))
  (set! randint (lambda args
    (cond ((null? (cdr args))
            (if (< (car args) two31) (unif-rand (car args))
              (floor (* (next-rand) (car args)))))
          ((< (car args) (cadr args))
            (let ((span (- (cadr args) (car args))))
              (+ (car args)
                 (if (< span two31) (unif-rand span)
                   (floor (* (next-rand) span))))))
          (else (let ((span (- (car args) (cadr args))))
                  (- (car args)
                     (if (< span two31) (unif-rand span)
                       (floor (* (next-rand) span))))))))))

(define heap #f)

(define (closest points k)
  (set! heap (make-vector k (list -1 0 0)))
  (do ((points points (cdr points)))
      ((null? points) heap)
    (let ((dist (+ (square (caar points)) (square (cadar points)))))
      (when (lt? (vector-ref heap 0) (list dist))
        (vector-set! heap 0 (cons dist (car points)))
        (siftdown k)))))

(define (siftdown len)
  (let loop ((parent 0))
    (let ((child (+ parent parent 1)))
      (when (< child len)
        (if (and (< (+ child 1) len) ; (+ child 1) is sibling
                 (lt? (vector-ref heap (+ child 1))
                      (vector-ref heap child)))
            (set! child (+ child 1))) ; the least child
        (when (lt? (vector-ref heap child)
                   (vector-ref heap parent))
          (let ((temp (vector-ref heap child)))
            (vector-set! heap child (vector-ref heap parent))
            (vector-set! heap parent temp))
          (loop child))))))

(define (lt? pt1 pt2)
  (not 
    (if (negative? (car pt1)) #f
      (if (negative? (car pt2)) #t
        (< (car pt1) (car pt2))))))

(define points
  (let loop ((n 20) (pts (list)))
    (if (zero? n) pts
      (loop (- n 1) (cons (list (randint -1000 1000)
                                (randint -1000 1000)) pts)))))

(for-each
  (lambda (p)
    (display p) (newline))
  (vector->list
    (closest points 10)))


Output:
1
2
3
4
5
6
7
8
9
10
(480573 18 693)
(462376 530 -426)
(192596 -436 50)
(221341 -210 421)
(458330 -677 -1)
(158089 380 -117)
(153722 29 -391)
(62057 109 224)
(34064 -92 -160)
(290132 -526 116)


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