gistfile1.lisp

(defun blerp (x y)
  "A closure that provides a linear interpolation of a function y = f(x). Attempts to evaluate f
  at values of x below the minimum value of x result in zero, and attempts above the maximum 
  value of x result in the last value of y."
  (assert (vectorp x))
  (assert (vectorp y))
  (let ((min-x    (aref x 0))
        (max-x    (aref x (1- (length x))))
        (interp-f (cl-numlib:make-interpolating-function x y)))
    (lambda (x)
      (cond
        ((eq x :min) min-x)
        ((eq x :max) max-x)
        (t           (cond    
                       ((< x min-x) 0.0)
                       ((> x max-x) (funcall interp-f max-x))
                       (t           (funcall interp-f x))))))))


(defparameter f (blerp (vector 1 2 3 4 5) (vector 1 0 3 4 9)))

(funcall f :min)    ; => 1

(funcall f 0)       ; => 0
(funcall f 2.3)     ; => 0.899
(funcall f 100)     ; => 9