module.exports = async options => {

	let index = 0;
	let error = false;
	let frames = [];
	let interval;
	let seekResolve;
	let currentTime = 0;
	let extractOffsets = false;

	const isNumber = n => {
		return +n + '' === n + '';
	};

	const isTimestamp = timestamp => {
		return isNumber(timestamp) && +timestamp >= 0 && +timestamp <= video.duration;
	};

	const fallbackToDefault = (property, defaultValue) => {
		options[property] = options.hasOwnProperty(property) ? options[property] : defaultValue;
	};

	// Buffer Video Element
	const video = document.createElement('video');
	video.src = options.url;
	video.crossOrigin = 'anonymous';
	video.onseeked = async () => {
		if(seekResolve)
			seekResolve();
	};
	video.onerror = () => {
		error = true;
	};

	while((video.duration === Infinity || isNaN(video.duration)) && video.readyState < 2 && !error) {
		await new Promise(r => setTimeout(r, 100));
		video.currentTime = 10000000 * Math.random();
	}

	// Set options to default values if not set
	fallbackToDefault('format', 'image/png');
	fallbackToDefault('offsets', []);
	fallbackToDefault('startTime', 0);
	fallbackToDefault('endTime', video.duration);
	fallbackToDefault('count', 1);

	// Filter out invalid offsets
	if(options.offsets.constructor !== Array)
		options.offsets = [];
	else
		options.offsets = options.offsets.filter(offset => {
			return isTimestamp(offset);
		});

	if(options.offsets.length !== 0)
		extractOffsets = true;

	// Check if start and end times are valid
	if(!isTimestamp(options.startTime))
		options.startTime = 0;

	if(!isTimestamp(options.endTime))
		options.endTime = video.duration;

	if(options.startTime >= options.endTime) {
		options.startTime = options.endTime;
		options.count = 1;
	}

	// Convert count value to a positive integer (floor() or 0 if string)
	options.count = Math.abs(~~options.count);

	// Starting at startTime + interval and ending at endTime - interval
	interval = (options.endTime - options.startTime) / (options.count + 1);

	// Set Width and Height
	let isWidthSet = options.hasOwnProperty('width');
	let isHeightSet = options.hasOwnProperty('height');
	let videoDimensionRatio = video.videoWidth / video.videoHeight;

	// Reset Width and Height if not valid
	if(isWidthSet && !isNumber(options.width))
		isWidthSet = false;

	if(isHeightSet && !isNumber(options.height))
		isHeightSet = false;

	if(!isWidthSet && !isHeightSet) {
		// Both Width and Height not set
		options.width = 128;
		options.height = options.width / videoDimensionRatio;

	} else if(isWidthSet && !isHeightSet) {
		// Width set but Height not set
		options.height = options.width / videoDimensionRatio;

	} else if(!isWidthSet && isHeightSet) {
		// Height set but Width not set
		options.width = options.height * videoDimensionRatio;
	}

	// Float values
	options.width = +options.width;
	options.height = +options.height;

	// Buffer Canvas Element
	const canvas = document.createElement('canvas');
	const context = canvas.getContext('2d');
	canvas.width = options.width;
	canvas.height = options.height;

	return new Promise(async resolve => {

		if(error)
			resolve([]);

		while(
			(extractOffsets && index < options.offsets.length) ||
			(!extractOffsets && index < options.count)
		) {
			video.currentTime = extractOffsets ? options.offsets[index] : options.startTime + (index + 1) * interval;
			await new Promise(r => seekResolve = r);
			context.clearRect(0, 0, canvas.width, canvas.height);
			context.drawImage(video, 0, 0, canvas.width, canvas.height);
			frames.push(canvas.toDataURL(options.format));
			index ++;
		}
		resolve(frames);
	});
};