gistfile1.clj

(deftest changes-at-test
  (let [frames [[2 :a] [4 :b] [6 :c]]]
    (is (= (changes-at frames 0) nil))
    (is (= (changes-at frames 1) nil))
    (is (= (changes-at frames 2) :a))
    (is (= (changes-at frames 3) :a))
    (is (= (changes-at frames 4) :a))
    (is (= (changes-at frames 5) :a))
    (is (= (changes-at frames 6) :b))
    (is (= (changes-at frames 7) :b))
    (is (= (changes-at frames 11) :b))
    (is (= (changes-at frames 12) :c))
    (is (= (changes-at frames 13) :c))))

; take 1

(defn changes-at [frames seconds]
  (last
    (first
      (reduce
        (fn [[_ t :as acc] [delay _ :as frame]]
          (let [t1 (+ t delay)]
            (if (> t1 seconds) (reduced acc) [frame t1])))
        [nil 0]
        frames))))

; take 2

(defn changes-at [frames seconds]
  (let [frames-with-abs-time (reductions (fn [[t _] [delay changes]] [(+ t delay) changes]) frames)
        prev-frames (take-while (comp (partial >= seconds) first) frames-with-abs-time)]
    (last (last prev-frames))))