in js/reveal.js [3113:3151]
function getSlidePastCount() {
var horizontalSlides = toArray( dom.wrapper.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ) );
// The number of past slides
var pastCount = 0;
// Step through all slides and count the past ones
mainLoop: for( var i = 0; i < horizontalSlides.length; i++ ) {
var horizontalSlide = horizontalSlides[i];
var verticalSlides = toArray( horizontalSlide.querySelectorAll( 'section' ) );
for( var j = 0; j < verticalSlides.length; j++ ) {
// Stop as soon as we arrive at the present
if( verticalSlides[j].classList.contains( 'present' ) ) {
break mainLoop;
}
pastCount++;
}
// Stop as soon as we arrive at the present
if( horizontalSlide.classList.contains( 'present' ) ) {
break;
}
// Don't count the wrapping section for vertical slides
if( horizontalSlide.classList.contains( 'stack' ) === false ) {
pastCount++;
}
}
return pastCount;
}