map-indexed with start

map-indexed-with-start.clj

(defn map-indexed
  "Returns a lazy sequence consisting of the result of applying f to 0
  and the first item of coll, followed by applying f to 1 and the second
  item in coll, etc, until coll is exhausted. Thus function f should
  accept 2 arguments, index and item."
  {:added "1.2"}
  ([f coll] (map-indexed f 0 coll))
  ([f start coll] 
      (letfn [(mapi [idx coll]
	    (lazy-seq
	      (when-let [s (seq coll)]
		 (if (chunked-seq? s)
		   (let [c (chunk-first s)
			 size (int (count c))
			 b (chunk-buffer size)]
		     (dotimes [i size]
			(chunk-append b (f (+ idx i) (.nth c i))))
		     (chunk-cons (chunk b) (mapi (+ idx size) (chunk-rest s))))
		   (cons (f idx (first s)) (mapi (inc idx) (rest s)))))))]
   (mapi start coll))))