; 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)))