var Preloader = /** @constructor */ function() { // eslint-disable-line no-unused-vars

	const loadXHR = function(resolve, reject, file, tracker, attempts) {
		const xhr = new XMLHttpRequest();
		tracker[file] = {
			total: 0,
			loaded: 0,
			final: false,
		};
		xhr.onerror = function() {
			if (attempts <= 1) {
				reject(new Error("Failed loading file '" + file + "'"));
			} else {
				setTimeout(function () {
					loadXHR(resolve, reject, file, tracker, attempts - 1);
				}, 1000);
			}
		};
		xhr.onabort = function() {
			tracker[file].final = true;
			reject(new Error("Loading file '" + file + "' was aborted."));
		};
		xhr.onloadstart = function(ev) {
			tracker[file].total = ev.total;
			tracker[file].loaded = ev.loaded;
		};
		xhr.onprogress = function(ev) {
			tracker[file].loaded = ev.loaded;
			tracker[file].total = ev.total;
		};
		xhr.onload = function() {
			if (xhr.status >= 400) {
				if (xhr.status < 500 || attempts <= 1) {
					reject(new Error("Failed loading file '" + file + "': " + xhr.statusText));
					xhr.abort();
				} else {
					setTimeout(function () {
						loadXHR(resolve, reject, file, tracker, attempts - 1);
					}, 1000);
				}
			} else {
				tracker[file].final = true;
				resolve(xhr);
			}
		};
		// Make request.
		xhr.open('GET', file);
		if (!file.endsWith('.js')) {
			xhr.responseType = 'arraybuffer';
		}
		xhr.send();
	};

	const DOWNLOAD_ATTEMPTS_MAX = 4;
	const loadingFiles = {};
	const lastProgress = { loaded: 0, total: 0 };
	let progressFunc = null;

	const animateProgress = function() {

		var loaded = 0;
		var total = 0;
		var totalIsValid = true;
		var progressIsFinal = true;

		Object.keys(loadingFiles).forEach(function(file) {
			const stat = loadingFiles[file];
			if (!stat.final) {
				progressIsFinal = false;
			}
			if (!totalIsValid || stat.total === 0) {
				totalIsValid = false;
				total = 0;
			} else {
				total += stat.total;
			}
			loaded += stat.loaded;
		});
		if (loaded !== lastProgress.loaded || total !== lastProgress.total) {
			lastProgress.loaded = loaded;
			lastProgress.total = total;
			if (typeof progressFunc === 'function')
				progressFunc(loaded, total);
		}
		if (!progressIsFinal)
			requestAnimationFrame(animateProgress);
	}

	this.animateProgress = animateProgress;

	this.setProgressFunc = function(callback) {
		progressFunc = callback;
	}


	this.loadPromise = function(file) {
		return new Promise(function(resolve, reject) {
			loadXHR(resolve, reject, file, loadingFiles, DOWNLOAD_ATTEMPTS_MAX);
		});
	}

	this.preloadedFiles = [];
	this.preload = function(pathOrBuffer, destPath) {
		let buffer = null;
		if (typeof pathOrBuffer === 'string') {
			var me = this;
			return this.loadPromise(pathOrBuffer).then(function(xhr) {
				me.preloadedFiles.push({
					path: destPath || pathOrBuffer,
					buffer: xhr.response
				});
				return Promise.resolve();
			});
		} else if (pathOrBuffer instanceof ArrayBuffer) {
			buffer = new Uint8Array(pathOrBuffer);
		} else if (ArrayBuffer.isView(pathOrBuffer)) {
			buffer = new Uint8Array(pathOrBuffer.buffer);
		}
		if (buffer) {
			this.preloadedFiles.push({
				path: destPath,
				buffer: pathOrBuffer
			});
			return Promise.resolve();
		} else {
			return Promise.reject(new Error("Invalid object for preloading"));
		}
	};
};