arthur/frontend/test/arthur/clock_test.cljs

(ns arthur.clock-test
  "The clock is four lines of arithmetic and the whole take's sync depends on
  them, so they are asserted rather than eyeballed. A drift bug is invisible for
  the first second and unmistakable by the tenth, which is the worst possible
  shape for a bug to have."
  (:require [cljs.test :refer [deftest is testing]]
            [arthur.clock :as clock]))

(defn- fake
  "Stands in for the audio element. The clock only ever reads `currentTime`,
  `paused`, `ended` and `playbackRate` and writes `currentTime`, which is the
  entire coupling — so the whole of it can be asserted in node."
  [& {:keys [t paused? ended? rate duration]
      :or   {t 0 paused? true ended? false rate 1.0 duration 7.6}}]
  #js {:currentTime t :paused paused? :ended ended?
       :playbackRate rate :duration duration})

(deftest the-frame-is-derived-from-the-audio-not-counted
  ;; frame = ⌊currentTime · fps⌋. The whole point: nothing accumulates, so
  ;; nothing can drift.
  (clock/attach! (fake :t 0.0))
  (is (= 0 (clock/frame 30 229)))
  (doseq [[t want] [[0.0 0] [0.033 0] [0.034 1] [1.0 30] [1.999 59] [2.0 60]]]
    (clock/attach! (fake :t t))
    (is (= want (clock/frame 30 229)) (str t "s at 30fps"))))

(deftest a-dropped-frame-lands-where-the-audio-already-is
  ;; THE property the derivation buys. A loop that stalled for a third of a
  ;; second resumes at the frame the audio reached, not a third of a second
  ;; behind it — the failure is a visible stutter rather than an invisible slide
  ;; out of sync, and those are very different bugs to own.
  (clock/attach! (fake :t 1.0))
  (let [before (clock/frame 30 229)]
    (clock/attach! (fake :t 1.5))           ; fifteen frames' worth of stall
    (is (= 45 (clock/frame 30 229)))
    (is (= 30 before) "and nothing was counted in between")))

(deftest the-frame-is-clamped-into-the-clip
  ;; Audio is 7.601s and the clip is 229 frames at 30fps = 7.633s, so the last
  ;; fraction of a second has no audio and the end of the audio has no frame
  ;; past the last. Neither may produce an out-of-range index.
  (clock/attach! (fake :t 99.0))
  (is (= 228 (clock/frame 30 229)))
  (clock/attach! (fake :t -1.0))
  (is (= 0 (clock/frame 30 229))))

(deftest a-seek-round-trips
  ;; Seeking to the START of a frame rather than its middle is what makes this
  ;; idempotent: seek to f, read back f, at every f.
  (let [a (fake)]
    (clock/attach! a)
    (doseq [f [0 1 57 114 171 228]]
      (clock/seek! 30 229 f)
      (is (= f (clock/frame 30 229)) (str "seek to " f)))))

(deftest a-seek-past-either-end-is-clamped-before-it-reaches-the-element
  (let [a (fake)]
    (clock/attach! a)
    (clock/seek! 30 229 9999)
    (is (= 228 (clock/frame 30 229)))
    (clock/seek! 30 229 -5)
    (is (= 0 (clock/frame 30 229)))
    (is (>= (.-currentTime a) 0) "and currentTime is never negative")))

(deftest rate-does-not-enter-the-frame-calculation
  ;; ½× and ¼× are playbackRate and nothing else. If rate appeared here as well
  ;; it would be applied twice — the picture would have one rate and the sound
  ;; another, which is precisely the desync the audio clock exists to prevent.
  (doseq [r [1.0 0.5 0.25]]
    (clock/attach! (fake :t 2.0 :rate r))
    (is (= 60 (clock/frame 30 229)) (str "at " r "x, 2.0s is still frame 60"))))

(deftest playing-follows-the-element
  (clock/attach! (fake :paused? true))
  (is (not (clock/playing?)))
  (clock/attach! (fake :paused? false))
  (is (clock/playing?))
  (testing "and a finished file is not playing, whatever `paused` says"
    (clock/attach! (fake :paused? false :ended? true))
    (is (not (clock/playing?)))))

(deftest the-audio-length-is-reported-rather-than-assumed
  ;; A clip longer than its audio is a legitimate thing to be told about and not
  ;; a thing to silently truncate.
  (clock/attach! (fake :duration 7.601))
  (is (= 229 (clock/duration-frames 30)))
  (is (= 92 (clock/duration-frames 12)))
  (testing "and an unloaded element has no opinion"
    (clock/attach! (fake :duration js/NaN))
    (is (nil? (clock/duration-frames 30)))))

(deftest exposure-is-applied-to-the-derived-frame
  ;; The transport shows which frame the grid holds the playhead back onto, so
  ;; that `exposure 2` is visibly doing something rather than only inside the
  ;; scene.
  (is (= [0 0 2 2 4 4] (mapv #(clock/exposed-frame % 2) (range 6))))
  (is (= [0 1 2 3 4 5] (mapv #(clock/exposed-frame % 1) (range 6)))))

(deftest with-no-element-attached-nothing-explodes
  ;; The loop starts before the :ref has fired, so every reader has to be safe
  ;; on a clock that has not been handed its element yet.
  (clock/attach! nil)
  (is (= 0 (clock/frame 30 229)))
  (is (not (clock/playing?)))
  (is (= 1.0 (clock/rate)))
  (is (nil? (clock/duration-frames 30)))
  (is (nil? (clock/seek! 30 229 5)))
  (is (nil? (clock/pause!))))