Do you know how to write this Scheme function?

inline unsigned int step(unsigned int i)
{
    return (i&0x1)*(i*3+1)+((i+1)&0x1)*(i>>1);
}

unsigned int collatz(unsigned int i)
{
    unsigned int cur = i;
    unsigned steps = 0;
    while((cur=step(cur))!=1) steps++;
    return steps;
}

(define (listfrom a b)
  (if (= a b)
      (cons a empty)
      (cons a (listfrom (+ 1 a) b))))

(define (max-collatz a b)
  (foldl max 0 (map collatz-loop (listfrom a b))))

(define (max-collatz-num a b)
  (foldl (lambda (c r)
           (if (> (collatz-loop c) (collatz-loop r)) c r))
         a
         (listfrom a b)))

(define (collatz x)
  (if (even? x)
      (/ x 2)
      (+ (* x 3) 1)))

(define (collatz-loop x)
  (if (= x 1) 1
      (+ (collatz-loop (collatz x)) 1)))

(define (collatz-loop x)
  (define (internal x counter)
    (if (= x 1) counter
    (internal (collatz x) (+ counter 1))))
  (internal x 1))

(define (collatz-range a b)
  (if (= a b)
      (cons a (collatz-loop a))
      (let ((this (collatz-loop a))
        (rest (collatz-range (+ a 1) b)))
        (if (< this (cdr rest)) rest
            (cons a this)))))

(collatz-range 1 20) ; returns (18 . 21), which means that (collatz-loop 18) returns 21

(define (collatz-range a b)
  (define (internal a best)
    (if (< b a) best
        (internal (+ a 1)
        (let ((x (collatz-loop a)))
          (if (< x (cdr best))
              best
              (cons a x))))))
  (internal a (cons -1 -1)))