; vec-search-down? : (any -> boolean) (vectorof any) -> boolean
; Purpose: Returns whether any element of the vector meets the given
;          predicate.
; Examples:
;   (vec-search-down? number? (vector true false))   = false
;   (vec-search-down? number? (vector true 3 false)) = true
;   (vec-search-down? number? (vector))              = false

(define (vec-search-down1? pred? a-vec)
  (local [;; search : natnum -> boolean
	  ;; Purpose: Returns whether any element 0..num_elts-1 of a-vec
	  ;;          meets predicate pred?.
	  (define (search num_elts)
	    (cond
	      [(zero? num_elts)
	       false]
	      [else
		(or (pred? (vector-ref a-vec (sub1 num_elts)))
		    (search (sub1 num_elts)))]))]
    (search (vector-length a-vec))))

(define (vec-search-down2? pred? a-vec)
  (local [;; search : natnum -> boolean
	  ;; Purpose: Returns whether any element 0..index of a-vec
	  ;;          meets predicate pred?.
	  (define (search index)
	    (cond
	      [(zero? index)
	       (pred? (vector-ref a-vec index))]
	      [else
		(or (pred? (vector-ref a-vec index))
		    (search (sub1 index)))]))]
    (search (sub1 (vector-length a-vec)))))

; Note:  vec-search-down2? does NOT work with zero-length vectors,
; i.e., (vector).  To make it work, you need to change the base case,
; e.g., to
;   [(= -1 index)  false]
; or
;   [(>= 0 index)  false]
; That would result in essentially the first version, except that
; the meaning of "index" and "num_elts" would differ by 1.
; Since that base case is slightly more complicated than that of
; vec-search-down1?, we slightly prefer vec-search-down1?.