We recorded stock-based compensation expense related to grants of employee stock options, restricted stock units (“ZSUs”) and performance-based awards in our consolidated statements of operations as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Cost of revenue	$	397	$	564	$	681	$	995
Research and development		12,238		10,363		23,953		18,988
Sales and marketing		2,920		2,214		5,407		4,050
General and administrative		5,800		4,228		10,087		7,449
Total stock-based compensation expense	$	21,355	$	17,369	$	40,128	$	31,482

The following table shows stock option activity for the six months ended June 30, 2019 (in thousands, except weighted-average exercise price and weighted-average contractual term):

	Outstanding Options
				Weighted-		Aggregate		Weighted-
				Average		Intrinsic Value of		Average
				Exercise		Stock Options		Contractual Term
	Stock Options			Price		Outstanding		(in years)
Balance as of December 31, 2018		36,185		$	2.35	$	57,510		6.23
Granted		4,647			5.37
Forfeited, expired and cancelled		—			—
Exercised		(7,720	)		0.41
Balance as of June 30, 2019		33,112		$	3.23	$	96,299		7.57

The following table presents the weighted-average grant date fair value and the related assumptions used to estimate the fair value of our stock options:

	Six Months Ended June 30, 2019
Expected term, in years		6
Risk-free interest rates		2.53	%
Expected volatility		43	%
Dividend yield		—
Weighted-average estimated fair value of options granted	$	2.41

The following table shows a summary of ZSU activity for the six months ended June 30, 2019 (in thousands, except weighted-average grant date fair value):

	Outstanding ZSUs
				Weighted-
				Average Grant Date		Aggregate
				Fair Value		Intrinsic Value of
	Shares			(per share)		Unvested ZSUs
Unvested as of December 31, 2018		52,482		$	3.49	$	206,254
Granted		11,091			5.47
Vested		(12,577	)		3.42
Forfeited		(2,248	)		3.37
Unvested as of June 30, 2019		48,748		$	3.97	$	298,825

In April 2018, a share repurchase program was authorized for up to $200.0 million of our outstanding Class A common stock (the “2018 Share Repurchase Program”). The timing and amount of any stock repurchase will be determined based on market conditions, share price and other factors. The program does not require us to repurchase any specific number of shares of our Class A common stock and may be modified, suspended or terminated at any time without notice. The 2018 Share Repurchase Program will be funded from existing cash on hand or other sources of funding as the Company may determine to be appropriate. Share repurchases under these authorizations may be made through a variety of methods, which may include open market purchases, privately negotiated transactions, block trades, accelerated share repurchase transactions, purchases through 10b5-1 plans or by any combination of such methods. During the three and six months ended June 30, 2019, no share repurchases were made under the 2018 Share Repurchase Program.

In November 2016, we announced that our Board of Directors authorized a share repurchase program allowing us to repurchase up to $200.0 million of our outstanding shares of Class A common stock (“2016 Share Repurchase Program”). In 2018, we completed the 2016 Share Repurchase Program by repurchasing 18.2 million shares of our Class A common stock at a weighted-average price of $3.59 per share for a total of $65.4 million.

All of our stock repurchases were made through open market purchases under Rule 10b5-1 plans and subsequently retired.