11. Stock-based Compensation

The following table presents the classification of stock-based compensation for the periods indicated:

	Three Months Ended April 30,
	2018		2017
	(in thousands)
Stock-based compensation:
Cost of revenue	$	315	$	303
Research and development		8,760		7,977
Selling, general and administrative		5,120		4,692
Total stock-based compensation	$	14,195	$	12,972

As of April 30, 2018, total unrecognized compensation cost related to unvested stock options was $5.7 million and is expected to be recognized over a weighted-average period of 2.26 years. Total unrecognized compensation cost related to unvested restricted stock units was $88.9 million and is expected to be recognized over a weighted-average period of 2.61 years. Total unrecognized compensation cost related to unvested restricted stock awards was $2.6 million and is expected to be recognized over a weighted-average period of 0.93 years.

The following table sets forth the weighted-average assumptions used to estimate the fair value of stock options and employee stock purchase plan awards for the periods indicated:

	Three Months Ended April 30,
	2018			2017
Stock Options:
Volatility		55	%		53	%
Risk-free interest rate		2.65	%		2.17	%
Expected term (years)	5.33			6.07
Dividend yield		0	%		0	%
Employee stock purchase plan awards:
Volatility		46	%		39	%
Risk-free interest rate		1.95	%		0.89	%
Expected term (years)		0.5			0.5
Dividend yield		0	%		0	%

Starting from fiscal year 2019, the Company calculates expected volatility for stock options based on its own historical stock price for a period commensurate with the expected term. In the prior fiscal year, the Company calculated expected volatility for stock options based on the weighted average of historical volatilities of its own stock price and the stock prices of similar companies that are publicly available for a period commensurate with the expected term. The Company calculates expected volatility for ESPP based on its own historical stock price for a period commensurate with the offering period.

The following table summarizes stock option activities for the period indicated:

	Option Outstanding
										Weighted-
								Total Intrinsic		Average
						Weighted-		Value Of		Remaining		Aggregate
				Weighted-		Average		Options		Contractual		Intrinsic
				Average		Grant-date		Exercised		Term		Value
	Shares			Exercise Price		Fair Value		(in thousands)		(in years)		(in thousands)
Outstanding at January 31, 2018		1,611,344		$	24.56
Granted		21,900			49.91	$	25.48
Exercised		(42,483	)		7.17			$	1,853
Forfeited		(8,921	)		55.40
Expired		(561	)		71.16
Outstanding at April 30, 2018		1,581,279			25.19					4.51		$	39,119
Exercisable at April 30, 2018		1,354,895		$	20.32						3.85	$	38,705

The intrinsic value of options outstanding and exercisable is calculated based on the difference between the fair market value of the Company’s ordinary shares on the reporting date and the exercise price. The closing price of the Company’s ordinary shares on April 30, 2018 was $46.59, as reported by The NASDAQ Global Market. The intrinsic value of exercised options is calculated by the difference between the fair market value of the Company’s ordinary shares on the exercise date and the exercise price.

The following table summarizes restricted stock and restricted stock units activities for the period indicated:

				Weighted-
				Average
				Grant-Date
	Shares			Fair Value
Unvested at January 31, 2018		2,103,281		$	56.45
Granted		28,620			49.91
Vested		(248,658	)		53.36
Forfeited		(13,097	)		54.02
Unvested at April 30, 2018		1,870,146		$	56.78

As of April 30, 2018, the aggregate intrinsic value of unvested restricted stock and restricted stock units was $87.1 million.