; vec-search-binary? : (any -> boolean) (vectorof any) -> boolean
; Purpose: Returns whether any element of the vector meets the given
;          predicate.
; Examples:
;   (vec-search-binary? 4 (vector 0 2 3 4 5 8 10 11 15 20 40)) = true
;   (vec-search-binary? 3 (vector))                            = false

(define (vec-search-binary? n a-vec)
  (local [;; search : natnum natnum -> boolean
	  ;; Purpose: Return whether n is any element lo..hi-1 of a-vec.
	  (define (search lo hi)
	    (local [(define mid (truncate (/ (+ lo hi) 2)))]
	      (cond
		[(= lo hi)
		 false]
		[(= n (vector-ref a-vec mid))
		 true]
		[(< n (vector-ref a-vec mid))
		 (search lo (sub1 mid))]
		[(> n (vector-ref a-vec mid))
		 (search (add1 mid) hi)])))]
    (search 0 (vector-length a-vec))))

; This is just one way to write binary search.