// Makes the server-rendered TodoMVC behave like the reference app.
//
// Every change is still the form POST the page uses with JavaScript off.
// This module sends that same POST in the background with an
// X-Todo-Fragment header, and the server answers with fresh HTML for the
// list and footer instead of a redirect. The server stays the only place
// that decides what the page shows.

const app = document.querySelector('.todoapp');
const newTodo = app.querySelector('.new-todo');

// Requests run one at a time, so their answers land in the order they
// were made.
let queue = Promise.resolve();
let swapping = false;

function send(form) {
	const body = new URLSearchParams(new FormData(form));
	const url = form.action;
	queue = queue
		.then(async () => {
			const res = await fetch(url, {
				method: 'POST',
				body,
				headers: { 'X-Todo-Fragment': '1' },
			});
			if (!res.ok) throw new Error(`${res.status} from ${url}`);
			swap(await res.text());
		})
		.catch((err) => {
			// Something is out of step (an expired list, a lost connection).
			// The server's page is the truth, so load it.
			console.error(err);
			location.reload();
		});
}

// Replaces section.main and footer.footer with the server's HTML, keeping
// keyboard focus on the same control where it still exists, and keeping
// any edit still in progress (one not yet being saved) open with its
// draft intact.
function swap(html) {
	const again = focusSelector(document.activeElement);
	const drafts = [...app.querySelectorAll('.todo-list li.editing:not([data-saving])')].map((li) => {
		const input = li.querySelector('.edit');
		return {
			id: li.dataset.id,
			value: input.value,
			focused: input === document.activeElement,
			start: input.selectionStart,
			end: input.selectionEnd,
		};
	});
	// Removing a focused field can fire focusout; that isn't the visitor
	// leaving the field, so it mustn't save.
	swapping = true;
	for (const el of app.querySelectorAll(':scope > .main, :scope > .footer')) el.remove();
	swapping = false;
	const t = document.createElement('template');
	t.innerHTML = html;
	app.append(t.content);
	for (const d of drafts) {
		const li = app.querySelector(`.todo-list li[data-id="${CSS.escape(d.id)}"]`);
		if (!li) continue; // the item is gone, so is its edit
		const input = li.querySelector('.edit');
		li.classList.add('editing');
		input.value = d.value;
		if (d.focused) {
			input.focus();
			input.setSelectionRange(d.start, d.end);
		}
	}
	if (again && !drafts.some((d) => d.focused)) app.querySelector(again)?.focus();
}

function focusSelector(el) {
	if (!el || !app.contains(el) || el === newTodo) return null;
	if (el.id) return `#${CSS.escape(el.id)}`;
	const li = el.closest('li[data-id]');
	if (li && el.classList.length) {
		return `li[data-id="${CSS.escape(li.dataset.id)}"] .${CSS.escape(el.classList[0])}`;
	}
	return null;
}

// Adding, deleting, clearing completed, and saving an edit with Enter all
// arrive here as ordinary form submissions.
app.addEventListener('submit', (e) => {
	const form = e.target;
	e.preventDefault();
	const edit = form.querySelector('.edit');
	if (edit) {
		saveEdit(edit);
		return;
	}
	send(form);
	if (form.contains(newTodo)) newTodo.value = '';
});

// The checkboxes have no submit button of their own: a change submits the
// form they sit in.
app.addEventListener('change', (e) => {
	if (e.target.matches('.toggle, .toggle-all')) send(e.target.form);
});

// Double-click a title to edit it.
app.addEventListener('dblclick', (e) => {
	const label = e.target.closest('.todo-list label');
	if (!label) return;
	const li = label.closest('li');
	const input = li.querySelector('.edit');
	li.classList.add('editing');
	input.focus();
	input.setSelectionRange(input.value.length, input.value.length);
});

// Escape cancels the edit; leaving the field saves it.
app.addEventListener('keydown', (e) => {
	if (e.key !== 'Escape' || !e.target.matches('.edit')) return;
	const input = e.target;
	input.value = input.defaultValue;
	input.closest('li').classList.remove('editing');
});

app.addEventListener('focusout', (e) => {
	if (!swapping && e.target.matches('.edit')) saveEdit(e.target);
});

// Saves an edit once, however many ways it was finished (Enter, then the
// blur that follows when the item is redrawn).
function saveEdit(input) {
	const li = input.closest('li');
	if (!li.classList.contains('editing') || li.dataset.saving) return;
	li.dataset.saving = '1';
	if (input.value.trim() === input.defaultValue) {
		// Unchanged: nothing to send, just leave edit mode.
		input.value = input.defaultValue;
		li.classList.remove('editing');
		delete li.dataset.saving;
		return;
	}
	send(input.form);
}