; reservoir sampling
(define (take n xs)
(let loop ((n n) (xs xs) (ys '()))
(if (or (zero? n) (null? xs))
(reverse ys)
(loop (- n 1) (cdr xs)
(cons (car xs) ys)))))
(define (drop n xs)
(let loop ((n n) (xs xs))
(if (or (zero? n) (null? xs)) xs
(loop (- n 1) (cdr xs)))))
(define rand ; knuth random number generator with shuffle box
(let* ((a 69069) (c 1234567) (m 4294967296) (k 32) ; 32-bit
; (a 6364136223846793005) (c 1442695040888963407)
; (m 18446744073709551616) (k 256) ; 64-bit
(seed (current-seconds))
(next (lambda ()
(set! seed (modulo (+ (* a seed) c) m)) seed))
(init (lambda (seed) (let ((box (make-vector k)))
(do ((j 0 (+ j 1))) ((= j k) box)
(vector-set! box j (next))))))
(box (init seed)))
(lambda args
(when (pair? args)
(set! seed (modulo (car args) m)) (set! box (init seed)))
(let* ((j (quotient (* k seed) m)) (n (vector-ref box j)))
(set! seed (next)) (vector-set! box j seed) (/ n m)))))
(define (randint . args)
(let ((lo (if (pair? (cdr args)) (car args) 0))
(hi (if (pair? (cdr args)) (cadr args) (car args))))
(+ lo (floor (* (rand) (- hi lo))))))
(define (sample k xs)
(let ((resv (list->vector (take k xs))))
(do ((xs (drop k xs) (cdr xs)) (i k (+ i 1))) ((null? xs) resv)
(let ((j (randint i)))
(when (< j k) (vector-set! resv j (car xs)))))))
(define xs '(a b c d e f g h i j k l m n o p q r s t u v w x y z))
(display (sample 3 xs)) (newline)
(display (sample 3 xs)) (newline)
(display (sample 3 xs)) (newline)