zip-against.cljc

(defn zip-against [values & {:keys [context test] :or {test =}}]
  "Conditionally zip against a second sequence."
  (loop [result []
         index 0
         value (first values)
         remain (rest values)
         context-item (first context)
         context-rest (rest context)]
    (cond
      (empty? context-rest) nil
      (test value context-item) (if (empty? remain)
                                  (conj result [index value])
                                  (recur (conj result [index value])
                                         (+ 1 index)
                                         (first remain)
                                         (rest remain)
                                         (first context-rest)
                                         (rest context-rest)))
      :else (recur result (+ index 1) value remain (first context-rest) (rest context-rest)))))

(deftest zip-against-test
  (is (= (zip-against '(e l o w l) :context '(h e l l o w o r l d))
         [[1 'e] [2 'l] [4 'o] [5 'w] [8 'l]]))
  (is (= (zip-against '(e l o w l z) :context '(h e l l o w o r l d))
         nil)))