
(() => {
	"use strict";

	// prefix for event & document body class name, e.g. "ghmo:container"
	const prefix = "ghmo";
	const disableAttr = `data-${prefix}-disable`;
	const debounceInterval = 250;
	const targets = {
		// pjax container (covers general, repo & gists)
		// .news = newsfeed layout
		// .repository-content = file code (code folding)
		// body = global changes
		"[data-pjax-container], .news, .repository-content, body": {
			count: 0,
			name: "container"
		},
		// comment preview active
		".js-preview-body": {
			count: 0,
			name: "preview"
		},
		// .js-discussion = wrapper for progressively loaded comments;
		// "# items not shown" example: https://github.com/isaacs/github/issues/18
		// .discussion-item = issue status changed (github-issue-show-status)
		// #progressive-timeline-item-container = load hidden items (old?)
		// #js-progressive-timeline-item-container = load hidden items
		// markdown-toolbar = issue comments
		".js-discussion, .discussion-item, .toolbar-item, #progressive-timeline-item-container, #js-progressive-timeline-item-container, markdown-toolbar": {
			count: 0,
			name: "comments"
		},
		// progressively loaded content (diff files)
		".js-diff-progressive-container, .data.blob-wrapper, .js-diff-load-container, .diff-table tbody, .diff-progressive-loader": {
			count: 0,
			name: "diff"
		},
		// issues/pr sidebar & timeline sections: e.g. form actions, commit
		// references, deployment state & PR checks container
		".js-updatable-content, .js-updatable-content-preserve-scroll-position": {
			count: 0,
			name: "updatable"
		},
		// user profile menu (loads on hover)
		"details-menu": {
			count: 0,
			name: "menu"
		}
	};
	const list = Object.keys(targets);
	let queue = new Set();
	let timer;

	function fireEvents() {
		list.forEach(selector => {
			if (targets[selector].count > 0) {
				// event => "ghmo:container", "ghmo:comments"
				const event = new Event(prefix + ":" + targets[selector].name);
				document.dispatchEvent(event);
				targets[selector].count = 0;
			}
		});
	}

	function process() {
		clearTimeout(timer);
		// avoiding use of forEach loops for performance reasons
		for (const target of queue) {
			if (target) {
				for (const item of list) {
					if (target.matches(item)) {
						targets[item].count++;
					}
				}
			}
		}
		timer = setTimeout(() => fireEvents(), debounceInterval);
		queue.clear();
	}

	function init() {
		// prevent error when library is loaded at document-start & no body exists
		const container = document.querySelector("body");
		/* document.body attribute used to disable updates; it *should not*
		 * be used regularly as multiple scripts may enable or disable the
		 * observers at inappropriate times. It is best that each script handles
		 * the mutation events triggered by this library on its own
		 */
		if (container.getAttribute(disableAttr)) {
			return;
		}
		// prevent script from installing more than once
		if (container && !container.classList.contains(prefix + "-enabled")) {
			container.classList.add(prefix + "-enabled");
			// http://stackoverflow.com/a/39332340/145346
			new MutationObserver(mutations => {
				if (!queue.size) {
					requestAnimationFrame(process);
				}
				for (const mutation of mutations) {
					queue.add(mutation.target);
				}
			}).observe(document, {
				childList: true,
				subtree: true,
			});
		}
	}

	if (document.readyState === "loading") {
		document.addEventListener("DOMContentLoaded", () => init);
	} else {
		init();
	}
})();
