;from Gambit utils

; Sort a list using mergesort
;
; (sort '(6 2 8 4 1) <) ==> (1 2 4 6 8)

(define (LIST-SORT l <?)

  (define (mergesort l)

    (define (merge l1 l2)
      (cond ((null? l1) l2)
            ((null? l2) l1)
            (else
             (let ((e1 (car l1)) (e2 (car l2)))
               (if (<? e1 e2)
                 (cons e1 (merge (cdr l1) l2))
                 (cons e2 (merge l1 (cdr l2))))))))

    (define (split l)
      (if (or (null? l) (null? (cdr l)))
        l
        (cons (car l) (split (cddr l)))))

    (if (or (null? l) (null? (cdr l)))
      l
      (let* ((l1 (mergesort (split l)))
             (l2 (mergesort (split (cdr l)))))
        (merge l1 l2))))

  (mergesort l))