window.addEventListener('load', init);

let slides;
let currentSlide;

function init() {
	slides = Array.from(document.querySelectorAll('article'));
	for (let i = 0; i < slides.length; i++) {
		slides[i].id = `slide${i}`;
	}
	resumeOrGoToStart();
	document.addEventListener('keydown', onKeyPressed);
}

function resumeOrGoToStart() {
	let urlParts = window.location.href.split('#');
	if (urlParts.length > 1) {
		goToSlide(parseInt(urlParts[1].replace('slide', '')))
	} else {
		goToSlide(0)
	}
}

function goToSlide(index) {
	if (index >= 0 && index < slides.length) {
		currentSlide = index;
		window.location.href = window.location.href.split('#')[0] + `#slide${index}`;
	}
}

function goToPreviousSlide() {
	if (!showPreviousFragment()) {
		goToSlide(currentSlide - 1);
	}
}

function goToNextSlide() {
	if (!showNextFragment()) {
		goToSlide(currentSlide + 1);
	}
}

function goToFirstSlide() {
	goToSlide(0);
}

function goToLastSlide() {
	goToSlide(slides.length - 1);
}

function onKeyPressed(event) {
	switch (event.keyCode) {
		case 34: // page down
		case 40: // arrow down
		case 39: // arrow right
			goToNextSlide();
			break;
		case 33: // page up
		case 38: // arrow up
		case 37: // arrow left
			goToPreviousSlide();
			break;
		case 36: // pos1
			goToFirstSlide();
			break;
		case 35: // end
			goToLastSlide();
			break;
	}
}

function showNextFragment() {
	let fragments = [... slides[currentSlide].querySelectorAll('.fragment')]
	let visible = [... slides[currentSlide].querySelectorAll('.fragment.visible')]

	if (fragments.length == visible.length) {
		return false;
	} else {
		fragments[visible.length].classList.add('visible')
		return true;
	}
}

function showPreviousFragment() {
	let visible = [...slides[currentSlide].querySelectorAll('.fragment.visible')]

	if (visible.length == 0) {
		return false;
	} else {
		visible[visible.length - 1].classList.remove('visible')
		return true;
	}
}