5. NET INCOME PER SHARE

As of December 31, 2018, 45,336,278 shares of Ambac's common stock (par value $0.01) and warrants entitling holders to acquire up to 4,877,783 shares of new common stock at an exercise price of $16.67 per share were outstanding. For the three years ended December 31, 2018, 2017 and 2016, 194, 0 and 136 warrants were exercised, respectively, resulting in an issuance of 194, 0 and 136 shares of common stock, respectively.

On June 30, 2015, the Board of Directors of Ambac authorized the establishment of a warrant repurchase program that permits the repurchase of up to $10,000 of warrants. On November 3, 2016, the Board of Directors of Ambac authorized a $10,000 increase to the warrant repurchase program. For the years ended December 31, 2018 and 2017, Ambac did not repurchase any warrants. As of December 31, 2018, Ambac had repurchased 985,331 warrants at a total cost of $8,092, (average cost of $8.21 per warrant). The remaining aggregate authorization at December 31, 2018 was $11,939. In connection with the AMPS Exchange, Ambac issued 824,307 of the repurchased warrants at a price of $9.72 per warrant on August 3, 2018. Refer to Note 1. Background and Business Description for further discussion of the AMPS Exchange.

The following table provides a reconciliation of the common shares used for basic net income per share to the diluted shares used for diluted net income per share:



Year Ended December 31,	2018	2017	2016
Basic weighted average shares outstanding	45,665,883	45,367,932	45,212,414
Effect of potential dilutive shares⁽¹⁾:
Warrants	441,104	—	312,619
Stock options	—	—	447
Restricted stock units	77,572	—	116,105
Performance stock units ⁽²⁾	375,276	—	81,939
Diluted weighted average shares outstanding	46,559,835	45,367,932	45,723,524
Anti-dilutive shares excluded from the above reconciliation
Stock options	16,667	126,667	110,000
Warrants	—	4,053,670	—
Restricted stock units	—	68,654	—
Performance stock units ⁽²⁾	—	322,943	—


(1)	For the year ended December 31, 2017, Ambac has a net loss and accordingly excluded all potentially dilutive securities from the determination of diluted loss per share as their impact was anti-dilutive.

(2)

Performance stock units that are dilutive are reflected based on the performance metrics through the balance sheet date. Performance stock units that are anti-dilutive are reflected at their target issuance amounts. Vesting of these units is contingent upon meeting certain performance metrics. Although a portion of these performance metrics have been achieved as of the respective period end, it is possible that awards may no longer meet the metric at the end of the performance period.