/**
 * Progressive enhancement for the student registration form.
 *
 * Two independent behaviours, both optional — without JS every panel stays
 * visible and the single submit still works:
 *
 * 1. **Two steps.** When account-signup questions are configured the form
 *    renders two panels (`[data-step="1"]` account details, `[data-step="2"]`
 *    the questions) inside a form marked `data-steps="1"`. Step two is hidden
 *    behind a "Next" button that only advances once step one passes native
 *    validation.
 * 2. **Parent/guardian.** The children section is hidden until the
 *    parent/guardian box is ticked, and "Add another child" clones the child
 *    block. Ticking the box also takes the guardian's *own* question panel out
 *    of play — in guardian mode the questions are asked per child, so the
 *    server ignores those answers and the browser must not demand them.
 */
(function () {
	'use strict';

	function enhanceSteps(form) {
		var step1 = form.querySelector('[data-step="1"]');
		var step2 = form.querySelector('[data-step="2"]');
		var next = form.querySelector('.us-reg-next');
		var back = form.querySelector('.us-reg-back');

		if (!step1 || !step2 || !next) {
			return null;
		}

		function show(step) {
			step1.hidden = step !== 1;
			step2.hidden = step !== 2;
		}

		show(1);

		next.addEventListener('click', function () {
			var fields = step1.querySelectorAll('input, select, textarea');

			for (var i = 0; i < fields.length; i++) {
				if (!fields[i].checkValidity()) {
					fields[i].reportValidity();
					return;
				}
			}

			show(2);
		});

		if (back) {
			back.addEventListener('click', function () {
				show(1);
			});
		}

		return {
			step2: step2,
			next: next,
			earlySubmit: form.querySelector('.us-reg-submit-early'),
		};
	}

	/**
	 * Rewrite a cloned child block's `children[0][…]` names and ids to the new
	 * index, and clear the values carried over from the block it was cloned from.
	 */
	function reindex(block, index) {
		block.setAttribute('data-child-index', String(index));

		var fields = block.querySelectorAll('input, select, textarea');
		for (var i = 0; i < fields.length; i++) {
			var field = fields[i];

			if (field.name) {
				field.name = field.name.replace(/^children\[\d+\]/, 'children[' + index + ']');
			}

			var oldId = field.id;
			if (oldId) {
				field.id = oldId.replace(/^us-child-\d+-/, 'us-child-' + index + '-');

				var label = block.querySelector('label[for="' + oldId + '"]');
				if (label) {
					label.setAttribute('for', field.id);
				}
			}

			if (field.type === 'checkbox' || field.type === 'radio') {
				field.checked = false;
			} else {
				field.value = '';
			}
		}
	}

	function enhanceGuardian(form, steps) {
		var toggle = form.querySelector('#us-is-guardian');
		var children = form.querySelector('#us-children');

		if (!toggle || !children) {
			return;
		}

		var addButton = children.querySelector('.us-add-child');
		var nextIndex = 1;

		// The guardian's own question panel is only meaningful when they are
		// registering for themselves. Disabling it (rather than hiding it) is what
		// stops a `required` question the server will ignore from blocking submit.
		function sync() {
			children.hidden = !toggle.checked;

			// Each student's name and birth year are required, but only once the
			// block is in play: a `required` field inside a hidden container makes
			// the form unsubmittable with no way to reach the offending control, so
			// the attribute goes on and comes off with the block itself. The server
			// enforces the same rule either way.
			var required = children.querySelectorAll('[data-us-child-required]');
			for (var r = 0; r < required.length; r++) {
				required[r].required = toggle.checked;
			}

			if (!steps) {
				return;
			}

			var fields = steps.step2.querySelectorAll('input, select, textarea');
			for (var i = 0; i < fields.length; i++) {
				fields[i].disabled = toggle.checked;
			}

			// With the questions out of play there is no second step to advance to,
			// so "Next" would be a dead end — swap it for the submit.
			steps.next.hidden = toggle.checked;

			if (steps.earlySubmit) {
				steps.earlySubmit.hidden = !toggle.checked;
			}
		}

		toggle.addEventListener('change', sync);
		sync();

		if (addButton) {
			addButton.addEventListener('click', function () {
				var blocks = children.querySelectorAll('.us-child');
				var clone = blocks[blocks.length - 1].cloneNode(true);

				reindex(clone, nextIndex);
				nextIndex += 1;

				children.insertBefore(clone, addButton.parentNode);

				// The clone carries the data attribute but not necessarily the
				// current required state, so settle it the same way as the rest.
				sync();
			});
		}
	}

	document.addEventListener('DOMContentLoaded', function () {
		var forms = document.querySelectorAll('.us-register-form form');

		for (var i = 0; i < forms.length; i++) {
			var steps = forms[i].getAttribute('data-steps') === '1'
				? enhanceSteps(forms[i])
				: null;

			enhanceGuardian(forms[i], steps);
		}
	});
})();