(define (string-find-char str char start)
  (let ((end (string-length str)))
    (cond
      ((= start end) #f)
      ((char=? (string-ref str start) char) start)
      (else (string-find-char str char (+ start 1))))))

(define (%string-split str sep found-sep? start res)
  (let ((next (string-find-char str sep start))
        (end (string-length str)))
    (cond
      (next (%string-split str sep #t (+ next 1) (cons (substring str start next) res)))
      (found-sep? (%string-split str sep #f end (cons (substring str start end) res)))
      (else res))))

(define (string-split str sep)
  (reverse (%string-split str sep #f 0 '())))