page-offset.clj

(defn page-offset
  "Return [next-group, next-offset] in sequence
   given offset of first item in required range.
   group-fn should not return nil."
  [group-fn offset items]
  {:pre [(<= 0 offset)]}
  ;; (Do not force too much items from source sequence here)
  (let [split-pos offset]
    (when (seq (drop split-pos items))
      (->> (take (inc split-pos) items)
           (map group-fn)
           (reduce
             (fn [[v idx] x]
               (if (= v x)
                 [v (inc idx)]
                 [x 0]))
             [nil 0])))))

(facts "page-offset basic"
  (fact (api/page-offset div10 2 [0 1])
    => nil)
  (fact (api/page-offset div10 1 [])
    => nil)
  (fact (api/page-offset div10 2 [0 1 10 11])
    => [1 0])
  (fact (api/page-offset div10 3 [0 11 22 23 33])
    => [2 1]))