Fix rounding resulting in choppy timing animations

Summary: Casting to long too early here and dropping some precision, resulting in skipped (not dropped) frames.

Reviewed By: sahrens

Differential Revision: D3819153

fbshipit-source-id: 83676cf4c9129638348890c74d563db121049e4a

ReactAndroid/src/main/java/com/facebook/react/animated/FrameBasedAnimationDriver.java

@@ -19,6 +19,9 @@
  */
 class FrameBasedAnimationDriver extends AnimationDriver {
 
+  // 60FPS
+  private static final double FRAME_TIME_MILLIS = 1000d / 60d;
+
   private long mStartFrameTimeNanos = -1;
   private final double[] mFrames;
   private final double mToValue;
@@ -40,10 +43,8 @@ public void runAnimationStep(long frameTimeNanos) {
       mStartFrameTimeNanos = frameTimeNanos;
       mFromValue = mAnimatedValue.mValue;
     }
-    long timeFromStartNanos = (frameTimeNanos - mStartFrameTimeNanos);
-    // frames are calculated at 60FPS, to get index by a given time offset from the start of the
-    // animation, we take the time diff in millisecond and divide it by 60 frames per 1000ms.
-    int frameIndex = (int) (timeFromStartNanos / 1000000L * 60L / 1000L);
+    long timeFromStartMillis = (frameTimeNanos - mStartFrameTimeNanos) / 1000000;
+    int frameIndex = (int) (timeFromStartMillis / FRAME_TIME_MILLIS);
     if (frameIndex < 0) {
       throw new IllegalStateException("Calculated frame index should never be lower than 0");
     } else if (mHasFinished) {