Pursuant to the Company’s Fourth Amended and Restated Certificate of Incorporation, the Company stockholders authorized the issuance of up to 900,000,000 shares, consisting of 800,000,000 shares of common stock at par value of $0.00001 per share and 100,000,000 shares of preferred stock at par value $0.00001 per share.

During the year ended December 31, 2018, the Company repurchased approximately $16,000 of common stock (1,159 shares) at a price of $13.65 per share, which was approximately the fair value of common stock at such time. During the year ended December 31, 2017, the Company repurchased approximately $144,000 of common stock (11,791 shares) at $12.21 per share, which was approximately the fair value of common stock at such time.

The Company sponsors the 2018 Equity Incentive Plan (the “2018 Plan”), which was approved by stockholders on September 5, 2018. Under the 2018 Plan, the Board or a committee of the Board, may grant incentive and nonqualified stock options, stock appreciation rights, restricted or unrestricted stock awards, restricted stock units (“RSUs”), phantom stock, performance awards or other stock-based awards to employees, directors and other individuals providing services to the Company. The purpose of the 2018 Plan is to promote the long-term growth and profitability of the Company by (i) providing employees with incentives to improve stockholder value and to contribute to the growth and financial success of the Company through their future services, and (ii) enabling the Company to attract, retain and reward the best‑available persons. The options granted under the 2018 Plan, may be granted at a price not less than the fair market value on the grant date.

The Board, or a committee of the Board, has granted options with an exercise price at or which approximates the fair value on the grant date to new hires, except for the out-of-the-money options granted to certain employees as discussed below. Grants of time-based awards generally vest over a four-year period for new hires and over a three-year period for subsequent grants to existing employees. The service condition for the majority of these awards is satisfied generally over the applicable vesting period. Options expire as determined by the Board, or committee of the Board, but not more than ten years after the date of the grant. In the second quarter of 2015, the Company began granting restricted stock units that contain both a service condition and Performance Vesting Condition. Both the service condition and Performance Vesting Condition must be met in order for these awards to vest and issue. The Performance Vesting Condition occurred upon the effectiveness of the registration statement for the Company's IPO, which was September 25, 2018.

The 2018 Plan provides for annual increases in the number of shares available for issuance on the first day of each year equal to the lesser of 12,500,000 shares, 5% of the outstanding shares on the last date of the preceding year, or a lower amount determined by the plan administrator. As of December 31, 2019, 7,648,165 shares of common stock remain available for grant under the 2018 Plan.

The following is a summary of restricted stock units for the respective periods:

	Restricted Stock Units
	Number of Shares			Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Term (in years)
Unvested at December 31, 2016		8,533,775		$	12.89		3.3
Granted		2,936,437		$	11.91
Vested, gross of shares withheld for employee payroll taxes		(1,443,302	)	$	13.52
Forfeited/canceled, including shares withheld for employee payroll taxes		(1,188,629	)	$	12.71
Unvested at December 31, 2017		8,838,281		$	12.52		2.2
Granted		3,911,841		$	13.26
Vested, gross of shares withheld for employee payroll taxes		(5,883,890	)	$	12.75
Forfeited/canceled, including shares withheld for employee payroll taxes		(1,114,186	)	$	12.65
Unvested at December 31, 2018		5,752,046		$	12.77		1.2
Granted		6,270,720		$	15.23
Vested		(3,660,767	)	$	12.99
Forfeited/canceled		(1,386,005	)	$	13.53
Unvested at December 31, 2019		6,975,994		$	14.72		2.2

	Stock Options
	Number of Shares			Weighted Average Exercise Price		Aggregate Intrinsic Value (in thousands)		Weighted Average Remaining Contractual Term (in years)
Outstanding at December 31, 2016		12,028,622		$	14.53	$	9,903		8.3
Granted		4,139,372		$	16.03
Exercised		(35,352	)	$	4.35
Forfeited		(1,623,057	)	$	16.02
Expired		(700,516	)	$	14.77
Outstanding at December 31, 2017		13,809,069		$	14.82	$	9,292		7.8
Granted		5,735,643		$	13.31
Exercised		(81,849	)	$	6.01
Forfeited		(832,591	)	$	15.01
Expired		(177,897	)	$	14.08
Outstanding at December 31, 2018		18,452,375		$	14.39	$	8,482		7.5
Granted		3,394,030		$	14.17
Exercised		(3,733,401	)	$	12.77
Forfeited		(1,351,967	)	$	14.02
Expired		(948,109	)	$	15.84
Outstanding, vested and expected to vest at December 31, 2019		15,812,928		$	14.67	$	50,994		7.4
Vested and exercisable at December 31, 2019		9,976,540		$	14.86	$	30,028		6.6

The following is a summary of restricted stock awards for the respective periods:

	Restricted Stock Awards
	Number of Shares			Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Term (in years)
Unvested at December 31, 2016		930,086		$	14.38		1.6
Vested		(449,220	)	$	14.38
Unvested at December 31, 2017		480,866		$	14.38		0.6
Vested		(480,866	)	$	14.38
Unvested at December 31, 2018		—		$	—	0.0
Granted		299,798		$	18.30
Unvested at December 31, 2019		299,798		$	18.30		2.3

Stock options granted during the years ended December 31, 2019 and 2018 were at-the-money and valued using the Black-Scholes-Merton Valuation Model. The fair value of the at-the-money stock options is being amortized on a straight-line basis over the requisite service period of the awards granted. The fair value of each at-the-money stock option was estimated on their grant dates using the following assumptions:

Stock options granted during the year ended December 31, 2017 were out-of-the-money and valued using the Lattice-Binomial Option Valuation Model which estimates fair value based on the assumed changes in prices for the option grants’ underlying asset over their contractual life. The fair value of the out-of-the-money stock options is being amortized on a straight-line basis over the requisite service period of the awards granted. The fair value of each out-of-the-money stock option was estimated on their grant dates using the following assumptions:

The Company sponsors the 2018 Employee Stock Purchase Plan, as amended (the “ESPP”), which was first approved by stockholders on September 5, 2018. The ESPP provides for annual increases in the number of shares available for issuance on the first day of each year equal to the lesser of 5,346,888 shares, 1% of the outstanding shares on the last date of the preceding year, or a lower amount determined by the plan administrator.

Except for the initial offering period, the ESPP provides for 24-month offering periods beginning May 22 and November 22 of each year, and each offering period will consist of four six-month purchase periods. The initial offering period began on September 25, 2018 and will end on November 22, 2020. On each purchase date, eligible employees will purchase the shares at a price per share equal to 85% of the lesser of (1) the fair market value of the Company’s common stock on the offering date, or (2) the fair market value of its common stock on the purchase date.

During the year ended December 31, 2019, the Company’s employees purchased 505,546 shares of its common stock under the ESPP at a weighted average purchase price of $10.57 with proceeds of $5.3 million. As of December 31, 2019, 2,167,898 shares of common stock remain available for grant under the ESPP.

The Company used the Black-Scholes-Merton option pricing model to estimate the fair value of ESPP purchase rights granted using the following assumptions:

Stock-based compensation expense recognized in the consolidated financial statements is as follows:

	Year Ended December 31,
(in thousands)	2019		2018		2017
Cost of revenue	$	3,658	$	8,931	$	2,503
Research and development		21,159		48,739		9,918
Sales and marketing		11,950		19,046		8,069
General and administrative		23,478		55,054		14,496
Stock-based compensation expense, net of amounts capitalized		60,245		131,770		34,986
Capitalized stock-based compensation expense		3,503		2,609		3,119
Stock-based compensation expense	$	63,748	$	134,379	$	38,105

	Unrecognized stock-based compensation (in thousands)		Weighted average vesting period (in years)
Restricted stock units (service-based)	$	70,735		2.9
Restricted stock units (performance-based)⁽¹⁾		6,162		1.1
Stock options		34,501		2.2
Restricted stock awards		4,115		2.3
ESPP		3,649		1.3
Total unrecognized stock-based compensation	$	119,162

In the United States, the Company offers its employees a defined contribution plan that qualifies as a deferred salary arrangement under Section 401 of the U.S. Internal Revenue Code (“401(k) Plan”). Under the 401(k) Plan, participating employees may defer a portion of their pretax earnings not to exceed the maximum amount allowed by the Internal Revenue Service. The Company currently provides a matching contribution of 25% of deferrals for eligible employees. Compensation expense for the Company's matching contributions was $3.0 million, $2.3 million and $1.8 million during the years ended December 31, 2019, 2018 and 2017, respectively.