Single line Pendulum stroke algorythm

doPendulum.js

var inc = 1,
    slideCount = 30,
    slideIndex = 1;

// PENDULUM ANIMATION 
// -------------------------------
function doPendulum() {
  slideIndex += (inc *= (((slideIndex >= slideCount-1 || slideIndex <= 0 ) && -1) || 1));
}
//  where:
// inc - slide index increment (-1 or 1)
// slideIndex - index of current slide, E=>[0..slideIndex)
// slideCount - index of last slide (0.png, 1.png, ..., slideCount.png)