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INCOME TAXES - Sources (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 27, 2019	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Dec. 27, 2019	Dec. 28, 2018	Dec. 31, 2017	Dec. 29, 2017
U.S. federal statutory rate (as a percent)										21.00%	21.00%	35.00%	34.00%
One-time decrease in deferred tax assets									$ 1,300
Tax Act additional deferred tax expense											$ 200
Sources and tax effects of the differences
Computed "expected" federal income tax expense										$ 977	2,554		$ 4,655
Permanent differences										163	77		(61)
Nondeductible Executive Compensation										688
Stock options and disqualifying dispositions										(731)	(354)		(1,629)
Tax Act - federal rate change													(1,277)
Energy efficient building deduction										(1,291)	(919)
Current and deferred state income tax expense, net of federal benefit										466	815		287
Change in valuation allowances on deferred tax assets													15
Federal deferred tax adjustments										231	220		(441)
Adjustment for uncertain tax positions										(282)	61		363
Research and development tax credit										(510)	(313)		(188)
Adjustment to prior earn-out liability											198
Non-deductible transaction expenses											203
Other										27	(15)		(162)
True up income tax accounts										77
Total provision for income taxes	$ 1,188	$ (376)	$ (70)	$ (927)	$ (93)	$ 1,597	$ 869	$ (242)		$ (185)	$ 2,131		$ 1,562
Maximum
U.S. federal statutory rate (as a percent)													35.00%

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