Clojure Needle Search

needle.clj

(defmacro if-nonneg [a b]
  "Eval the 1st arg, and if it's nonnegative, return it without evaling the 2nd arg.                                                        
   If the 1st arg evals to negative, eval and return the value of the 2nd arg."
  `(let [v# ~a] (if (>= v# 0) v# ~b)))

(defn inc-if-nonneg [x]
  "Return a value that is 1 more than x if x is nonnegative; otherwise return x."
  (if (>= x 0) (inc x) x))

(defn find-needle-end [n h]
  "Find the END of the first occurrence of needle n in haystack h -- i.e. the                                                               
   index of the position in h immediately after the last character in the first                                                                 
   occurrence of n.  If n does not appear in h, return -1."
  (cond
   (empty? n) 0  ; empty needle found at front of haystack (also works for empty haystack)                                                     
   (empty? h) -1 ; empty haystack contains no (nonempty) needles                                                                            
   ; if this :else clause is invoked, we know that neither n nor h is empty:                                                                                        
   :else      (if-nonneg
               (if (= (first n) (first h)) ; if first chars match...                                                                        
                 ; search for end of needle tail in haystack tail, and if found,                                                            
                 ; return that position + 1                                                                                                 
                 (inc-if-nonneg (find-needle-end (rest n) (rest h)))
                 -1)
                 ; otherwise, search for (entire) needle in haystack tail                                                                   
                 (inc-if-nonneg (find-needle-end n (rest h))))))

(defn clamp [x v]
  "Clamp all negative values to v -- i.e. returns x if x >= 0, otherwise returns v."
  (if (< x 0) v x))

(defn find-needle [n h]
  "Return the index of the (beginning of the) first occurrence of needle n                                                                  
   in haystack h.  If n does not appear in h, return -1."
  (clamp (- (find-needle-end n h) (count n)) -1))

(defn run-tests []
  [                                      ;0123456789                                                                                        
   (=   0  (find-needle "ha"             "hahaystack"))
   (=   2  (find-needle "hay"            "hahaystack"))
   (=  -1  (find-needle "halt"           "hahaystack"))
   (=   3  (find-needle "ayst"           "hahaystack"))
   (=   5  (find-needle "stack"          "hahaystack"))
   (=  -1  (find-needle "stacky"         "hahaystack"))
   (=  -1  (find-needle "acky"           "hahaystack"))
   (=   0  (find-needle "haha"           "hahaystack"))
   (=   2  (find-needle "hays"           "hahaystack"))
   (=   2  (find-needle "hayst"          "hahaystack"))
   (=  -1  (find-needle "haste"          "hahaystack"))
   (=   9  (find-needle "k"              "hahaystack"))
   (=   0  (find-needle ""               "hahaystack"))
   (=   0  (find-needle "hahaystack"     "hahaystack"))
   (=  -1  (find-needle "hahaystackify"  "hahaystack"))
   (=  -1  (find-needle "needle"         "hahaystack"))
   (=   2  (find-needle "haystack"       "hahaystack"))
   (=   0  (find-needle ""               ""))
   (=  -1  (find-needle "needle"         ""))
   ])