Fast inverse square root in R6RS

fast-inv-sqrt.scm

#!r6rs
(import (rnrs))

(define (fast-inv-sqrt f)
  (let ((bv (make-bytevector 4)))
    (bytevector-ieee-single-native-set! bv 0 f)
    (let* ((i0 (bitwise-arithmetic-shift (bytevector-s32-native-ref bv 0) -1))
	   (i1 (- #x5f3759df i0)))
      (bytevector-s32-native-set! bv 0 i1)
      (let ((y (bytevector-ieee-single-native-ref bv 0)))
	(* y (- 1.5 (* 0.5 f y y)))))))

(let loop ()
  (let ((line (read (current-input-port))))
    (unless (eof-object? line)
      (display (fast-inv-sqrt line)) (newline)
      (loop))))