Ordered paths

ordered_paths.clj

(defn ordered-paths
  [coll]
  (let [len (count coll)
        end (dec len)]
    (cond
      (zero? len) coll
      (zero? end) coll
      :else
      (loop [start 0
             curr  1
             paths []]
        (if (and (= start end) (> curr end)) paths
          (do
            (recur
              (if (= curr end) (inc start) start)
              (if (= curr end) (+ start 2) (inc curr))
              (conj paths [(nth coll start) (nth coll curr)]))))))))

(ordered-paths [])
;; []

(ordered-paths [4])
;; [4]

(ordered-paths [4 1 2 8 9])
;; [[4 1] [4 2] [4 8] [4 9] [1 2] [1 8] [1 9] [2 8] [2 9] [8 9]]