Function: ROUND-HALF-EVEN

Documentation

Round towards the nearest value allowed with the current precision. If the current value is exactly halfway between two legal values round towards the nearest even value.

Source

(defun round-half-even (number &optional (precision *precision*))
  "Round towards the nearest value allowed with the current
precision. If the current value is exactly halfway between two legal
values round towards the nearest even value."
  (multiple-value-bind (value discarded)
      (floor (* number precision))
    (cond
     ((< discarded 1/2) ;; down
      (/ value precision))
     ((= discarded 1/2) ;; goto even
      (if (evenp value)
	  (/ value precision)
	  (/ (1+ value) precision)))
     (t ;; (>= discarded 1/2)
      (/ (1+ value) precision)))))