Effective October 3, 2012, the Company is authorized to issue 20.0 million shares of undesignated preferred stock with a par value of $0.001 per share. Each series of preferred stock will have such rights and preferences including dividend rights, dividend rate, conversion rights, voting rights, rights and terms of redemption (including sinking fund provisions), redemption price, and liquidation preferences as determined by the board of directors. As of December 31, 2021, and 2020, there were no issued or outstanding shares of preferred stock.

Under the 2000 Equity Incentive Plan (“2000 Plan”), the Company was authorized to grant to eligible participants either incentive stock options (“ISOs”) or non-statutory stock options (“NSOs”). The ISOs were granted at a price per share not less than the fair market value at the date of grant. The NSOs were granted at a price per share not less than 85% of the fair market value at the date of grant. Options granted generally vest over a period of up to four years, with a maximum term of ten years. The 2000 Plan was terminated in connection with the closing of the Company's initial public offering, and accordingly, no shares are currently available for grant under the 2000 Plan. The 2000 Plan continues to govern outstanding awards granted thereunder.

The 2012 Equity Incentive Plan (“2012 Plan”) was adopted and approved in September 2012 and became effective on September 26, 2012. Under the 2012 Plan, the Company is authorized to grant to eligible participant’s ISOs, NSOs, stock appreciation rights (“SARs”), restricted stock awards (“RSAs”), RSUs, performance units and performance shares. The number of shares of common stock available for issuance under the 2012 Plan is subject to an annual increase on January 1 of each year by an amount equal to the least of 3,050 thousand shares, 5% of the outstanding shares of stock as of the last day of the immediately preceding fiscal year or an amount determined by the board of directors. For the year ended December 31, 2021, 1,963 thousand shares were added to the 2012 Plan. As of December 31, 2021, a total of 17,662 thousand shares have been authorized for issuance under the 2012 Plan and 8,091 thousand shares are available for future grants. Options may be granted with an exercise price that is at least equal to the fair market value of the Company's stock at the date of grant and are exercisable when vested. Options and RSU's granted generally vest over a period of up to four years. ISOs may only be granted to employees and any subsidiary corporations' employees. All other awards may be granted to employees, directors and consultants and subsidiary corporations' employees and consultants. Options, SARs, RSUs, performance units and performance awards may be granted with vesting terms as determined by the board of directors and expire no more than ten years after the date of grant or earlier if employment or service is terminated.

On June 9, 2021, the Company’s stockholders approved the 2021 ESPP. A total of 600 thousand shares were authorized for issuance to eligible participating employees upon adoption of the ESPP, all of which are available for future purchases as of December 31, 2021. The ESPP provides for consecutive 6-month offering periods beginning on or about August 16 and February 16 of each year. Eligible employees who elect to participate can contribute from 1% to 15% of their eligible compensation through payroll withholding. During any offering period, contribution rates cannot be changed. However, eligible employees may withdraw from the current offering period. Any contributions made prior to each purchase date in the case of withdrawal or termination of employment will be refunded. On each purchase date, eligible participating employees will purchase the shares at a price per share equal to 85% of the lesser of (i) the fair market value of the Company's stock on the first trading day of the offering period or (ii) the fair market value of the Company's stock on the purchase date (i.e., the last trading day of the offering period). The first ESPP offering period commenced on August 16, 2021 and will end on February 15, 2022.

The following table shows a summary of the stock-based compensation expenses included in the consolidated statements of operations for the years ended December 31, 2021, 2020 and 2019:

The income tax benefit related to the stock-based compensation expenses was $6.2 million, $5.5 million and $5.5 million for the years ended December 31, 2021, 2020 and 2019, respectively. As of December 31, 2021, the Company had unrecognized stock-based compensation expenses of $16.7 million, $77.0 million and $0.2 million related to options, RSUs and ESPP, respectively, which are expected to be recognized over weighted-average periods of 2.8 years, 2.8 years and 0.1 years, respectively.

On December 21, 2018, the compensation committee of the Company's board of directors (“Compensation Committee”) granted the equity award for 2019 to the Company’s former chief executive officer, Philippe Courtot (“Mr. Courtot”). The first portion of the award consists of 56 thousand RSUs that were scheduled to vest in 16 quarterly increments beginning on January 1, 2019. The second portion of the award consists of a target number of 33 thousand PSUs, which were scheduled to vest at the end of the three-year performance period from January 2019 through December 2021. The actual number of PSUs eligible to vest range from 0% to 200% of the target number, depending on the level of achievement of goals related to revenue growth during the three-year performance period from January 2019 through December 2021 and Adjusted EBITDA margin for the fiscal year of 2021. The third portion of the award consists of a target number of 33 thousand PSUs, one third of which (11 thousand target PSUs) was scheduled to vest at the end of each fiscal year of 2019, 2020 and 2021. The actual number of PSUs eligible to vest at each vesting date range from 0% to 200% of the target number, depending on the level of achievement of goals related to revenue growth and Adjusted EBITDA margin for each of those years.

On November 2, 2019, the Compensation Committee granted the equity award for 2020 to Mr. Courtot. The first portion of the award consists of 49 thousand RSUs that were scheduled to vest in 16 quarterly installments beginning on December 1, 2019. The second portion of the award consists of a target number of 124 thousand PSOs, which were scheduled to vest at the end of the three-year performance period from January 2020 through December 2022. The actual number of PSOs eligible to vest range from 0% to 200% of the target number, depending on the level of achievement of goals related to revenue growth and free cash flow per share growth during the performance period.

On December 10, 2020, the Compensation Committee granted the equity award for 2021 to Mr. Courtot. The first portion of the award consists of 69 thousand RSUs that were scheduled to vest in 16 quarterly installments beginning on November 1, 2020. The second portion of the award consists of a target number of 224 thousand PSOs, which were scheduled to vest at the end of the three-year performance period from January 2021 through December 2023. The actual number of PSOs eligible to vest range from 0% to 200% of the target number, depending on the level of achievement of goals related to revenue growth and free cash flow per share growth during the performance period.

The vesting of the above awards was conditioned on Mr. Courtot’s continued service through the vesting dates or, for PSOs and PSUs, the dates that performance is certified in addition to the achievement of performance goals. If Mr. Courtot’s employment was terminated (a) by reason of death or disability or (b) by the Company for reasons other than cause or good reason within 12 months following a change in control, then 100% of any unvested portions of these awards would vest, with any vesting in connection with change in control terminations conditioned upon the effectiveness of a release of claims in favor of the Company.

In February 2021 and 2020, 22 thousand shares (representing 200% of target number of awards) and 15 thousand shares (representing 135% of target number of awards) under the equity award for 2019 for Mr. Courtot, vested as a result of the Company achieving the corresponding level of performance goals for 2020 and 2019, respectively.

On March 19, 2021, Mr. Courtot resigned from the Company due to health issues. The Compensation Committee determined that Mr. Courtot’s termination of employment was on account of disability. In accordance with the grant agreements of the equity awards for 2021, 2020 and 2019 for Mr. Courtot, all remaining outstanding RSUs, PSUs and PSOs under these grants were subject to accelerated vesting and became fully vested at 100% of the target number of awards as of the date of his termination of employment, which consist of 127 thousand RSUs, 44 thousand PSUs and 348 thousand PSOs. As a result, the Company recognized an additional $27.3 million of stock-based compensation expense due to the accelerated vesting in the consolidated statements of operations for the year ended December 31, 2021.

On April 27, 2021, the Compensation Committee granted to the Company’s current president and chief executive officer an equity award consisting of certain RSUs and a target number of 10 thousand PSUs. The PSUs are scheduled to vest at the end of the three-year performance period from January 2021 through December 2023. The actual number of the PSUs eligible to vest range from 0% to 200% of the target number, depending on the level of achievement of goals related to revenue growth and free cash flow per share growth during the performance period. If the Company's current president and chief executive officer is terminated (a) by reason of death or disability or (b) by the Company for reasons other than cause or good reason within 12 months following a change in control, then 100% of any unvested portions of the award will vest, with any vesting in connection with terminations due to change in control conditioned upon the effectiveness of a release of claims in favor of the Company.

On October 28, 2021, the Compensation Committee granted to certain executive officers of the Company equity awards consisting of certain RSUs and an aggregate number of 73 thousand PSUs. The target PSUs are scheduled to vest in three equal annual installments over a three-year period from January 2022 through December 2024. The actual number of the PSUs eligible to vest each year range from 0% to 200% of the annual target number, depending on the level of achievement of goals related to revenue growth and adjusted EBITDA margin corresponding to that year. The vesting and release of the first and second installment is capped at 100% of the target number at the end of the first and second year, respectively, with cumulative achievement over 100%, if any, to be vested and released at the end of the third year, together with the vesting of the third installment. If any of the executive officers is terminated (a) by reason of death or disability or (b) by the Company for reasons other than cause or good reason within 12 months following a change in control, any unvested PSUs eligible to vest pursuant to cumulative achievements over 100% for past installments along with any target number of unvested PSUs for any remaining installments will vest immediately.

For the years ended December 31, 2021, 2020 and 2019, stock-based compensation expenses of $13.3 million, $0.2 million and $0.3 million for PSOs, respectively, and $5.3 million, $2.8 million and $0.9 million for PSUs, respectively, were recognized.

The weighted-average grant date fair value of the Company’s stock options granted for the years ended December 31, 2021, 2020 and 2019 was $41.23, $35.49 and $34.02, respectively, using the Black-Scholes-Merton option-pricing model based on the following assumptions:

The expected term of the options is based on evaluations of historical and expected future employee exercise behavior. The risk-free interest rate is based on the U.S. Treasury rates at the date of grant with maturity dates equal to the expected term at the grant date. The volatility was estimated using the historical volatility derived from the Company's common stock. The Company has not historically declared any dividends and does not expect to in the future.

The total intrinsic value of options exercised for the years ended December 31, 2021, 2020 and 2019 was $42.5 million, $77.5 million and $52.1 million, respectively. Intrinsic value of an option is the difference between the fair value of the Company’s common stock at the time of exercise and the exercise price paid.

⁽¹⁾ Included 34 thousand shares of PSUs granted to certain executive officers in 2021 and 11 thousand additional shares of PSUs vested as a result of the Company achieving the corresponding level of performance goals for 2020.

⁽²⁾ Included 11 thousand additional shares of PSUs vested as a result of the Company achieving the corresponding level of performance goals for 2020.

⁽³⁾ Included 34 thousand shares of PSUs granted to certain executive officers in 2021.

The aggregate fair value of RSUs vested for the years ended December 31, 2021, 2020 and 2019 was $59.5 million, $46.5 million and $37.9 million, respectively.

The weighted-average grant date fair value of the Company’s ESPP for the year ended December 31, 2021 was $26.88 using the Black-Scholes-Merton option-pricing model based on the following assumptions:

The expected term of the ESPP represents the six-month offering period. The risk-free interest rate is based on the U.S. Treasury rates at the date of grant with maturity dates equal to the expected term at the grant date. The volatility was estimated using the historical volatility derived from the Company's common stock. The Company has not historically declared any dividends and does not expect to in the future.

The Company's share repurchase program was authorized by the board of directors as follows:

Shares may be repurchased from time to time on the open market in accordance with Rule 10b-18 of the Exchange Act of 1934, including pursuant to a pre-set trading plan adopted in accordance with Rule 10b5-1 under the Exchange Act. All share repurchases have been made using cash resources. Repurchased shares are retired and reclassified as authorized and unissued shares of common stock. On retirement of the repurchased shares, common stock is reduced by an amount equal to the number of shares being retired multiplied by the par value. The excess amount that is retired over its par value is first allocated as a reduction to additional paid-in capital based on the initial public offering price of the stock, with the remaining excess to retained earnings.

For the years ended December 31, 2021, 2020 and 2019, the Company repurchased 1.1 million shares, 1.3 million shares and 1.0 million shares of its common stock for $130.0 million, $126.7 million and $86.4 million, respectively. As of December 31, 2021, $271.8 million remained available for share repurchases pursuant to the Company's share repurchase program.