; selection
(define rand
(let* ((a 3141592653) (c 2718281829)
(m (expt 2 35)) (x 5772156649)
(next (lambda ()
(let ((x-prime (modulo (+ (* a x) c) m)))
(set! x x-prime) x-prime)))
(k 103)
(v (list->vector (reverse
(let loop ((i k) (vs (list x)))
(if (= i 1) vs
(loop (- i 1) (cons (next) vs)))))))
(y (next))
(init (lambda (s)
(set! x s) (vector-set! v 0 x)
(do ((i 1 (+ i 1))) ((= i k))
(vector-set! v i (next))))))
(lambda seed
(cond ((null? seed)
(let* ((j (quotient (* k y) m))
(q (vector-ref v j)))
(set! y q)
(vector-set! v j (next)) (/ y m)))
((eq? (car seed) 'get) (list a c m x y k v))
((eq? (car seed) 'set)
(let ((state (cadr seed)))
(set! a (list-ref state 0))
(set! c (list-ref state 1))
(set! m (list-ref state 2))
(set! x (list-ref state 3))
(set! y (list-ref state 4))
(set! k (list-ref state 5))
(set! v (list-ref state 6))))
(else (init (modulo (numerator
(inexact->exact (car seed))) m))
(rand))))))
(define (randint . args)
(cond ((null? (cdr args))
(floor (* (rand) (car args))))
((< (car args) (cadr args))
(+ (floor (* (rand) (- (cadr args) (car args)))) (car args)))
(else (+ (ceiling (* (rand) (- (cadr args) (car args)))) (car args)))))
(define (shuffle x)
(do ((v (list->vector x)) (n (length x) (- n 1)))
((zero? n) (vector->list v))
(let* ((r (randint n)) (t (vector-ref v r)))
(vector-set! v r (vector-ref v (- n 1)))
(vector-set! v (- n 1) t))))
(define (partition xs)
(let ((x (car xs)))
(let loop ((xs xs) (lt '()) (gt '()))
(cond ((null? xs) (values x lt gt))
((< (car xs) x)
(loop (cdr xs) (cons (car xs) lt) gt))
(else (loop (cdr xs) lt (cons (car xs) gt)))))))
(define (select k xs)
(if (<= (length xs) k)
(error 'select "out of range")
(let loop ((k k) (xs (shuffle xs)))
(let-values (((x lt gt) (partition xs)))
(cond ((< k (length lt)) (loop k lt))
((< (length lt) k) (loop (- k (length lt)) gt))
(else x))))))
(display (select 2 '(1 2 3 4 5 6 7)))