(define (primes n)
(let* ((max-index (quotient (- n 3) 2))
(v (make-vector (+ 1 max-index) #t)))
(let loop ((i 0) (ps '(2)))
(let ((p (+ i i 3)) (startj (+ (* 2 i i) (* 6 i) 3)))
(cond ((>= (* p p) n)
(let loop ((j i) (ps ps))
(cond ((> j max-index) (reverse ps))
((vector-ref v j)
(loop (+ j 1) (cons (+ j j 3) ps)))
(else (loop (+ j 1) ps)))))
((vector-ref v i)
(let loop ((j startj))
(if (<= j max-index)
(begin (vector-set! v j #f)
(loop (+ j p)))))
(loop (+ 1 i) (cons p ps)))
(else (loop (+ 1 i) ps)))))))
(display (length (primes 15485863)))