/**
 * Update results based on form data.
 * @param   Event  event
 * @return  void
 */
function update(event) {	
	var output = '😵 Whoops! I screwed up. Would you leave a comment to let me know? Thanks!';
	
	// Get form values
	var currentPage = parseInt(form.elements.current_page.value, 10);
	var totalPages = parseInt(form.elements.total_pages.value, 10);
	var method = form.elements.method.value;
	var pagesPerDay = parseInt(form.elements.pages_per_day.value, 10);
	var finishDate = form.elements.finish_date.value;
	
	// Common variables
	var pagesLeft = totalPages - currentPage;

	// Calculate - Pages per day
	// NOTE: Round up so we always return full days
	var daysToRead = Math.ceil(pagesLeft / pagesPerDay);
		
	// Calculate - Finish by date
	// NOTE: Round up so we always return full days
	var today = new Date();
	var daysLeft = Math.ceil((Date.parse(finishDate) - today.getTime()) / (1000 * 3600 * 24));
	// NOTE: Round up so we always return full pages
	var newPagesPerDay = Math.ceil(pagesLeft / daysLeft);

	// Update CSS classes
	form.className = form.className.replace(/form--\w+/, 'form--' + method);
	
	// Output - Finish by date
	if (method === 'date') {
		output = 'You’ll need to read <strong>' + newPagesPerDay + ' pages per day</strong> to finish by ' + finishDate + '.';
	// Output - Pages per day
	} else {
		output = 'If you read ' + pagesPerDay + ' pages per day, you’ll finish your book in <strong>' + daysToRead + ' days</strong>, on ' + getDateFromNow(daysToRead) + '.';
	}
	
	// Save data to localStorage
	localStorage.setItem('readingPlanner', JSON.stringify({
		current_page: currentPage,
		total_pages: totalPages,
		method: method,
		pages_per_day: pagesPerDay,
		finish_date: finishDate,
	}));
	
	result.innerHTML = output;
}

/**
 * Load form data from localStorage.
 * @return void
 */
function loadData() {
	var data = JSON.parse(localStorage.getItem('readingPlanner') || 'false');
	
	if (!data) {
		return;
	}
	
	// Set data for each form input
	Object.entries(data).forEach(function ([key, value]) {
		form.elements[key].value = value;
	});
}

/**
 * Get date N days from now. Defaults to today.
 * @param   Number  Number of days
 * @return  String  Date in mm/dd/yyyy
 */
function getDateFromNow(days) {
	var date = new Date();
	
	// Increment days
	date.setDate(date.getDate() + (days || 0));
	
	// Return formatted string
	return date
		.toISOString()
		.replace(/(\d+-\d+-\d+).*/, '$1');
}

// Init default values
window['input-finish-date'].value = getDateFromNow(30);
loadData();
update();

// Init event listeners
form.addEventListener('input', update);
form.addEventListener('submit', function (event) {
	event.preventDefault();
	update();
});