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SEGMENT AND GEOGRAPHICAL INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 27, 2024 | Sep. 27, 2024 | Jun. 28, 2024 | Mar. 29, 2024 | Dec. 29, 2023 | Sep. 29, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Dec. 27, 2024 | Dec. 29, 2023 | Dec. 30, 2022
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue | $ 144,061 | $ 158,252 | $ 140,996 | $ 122,489 | $ 155,677 | $ 132,738 | $ 119,077 | $ 102,603 | $ 565,798 | $ 510,095 | $ 429,138
Domestic |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 565,798 | 510,095 | 429,138
Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 473,309 | 426,976 | 357,460
Energy | Domestic |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 473,309 | 426,976 | 357,460
Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 92,489 | 83,119 | 71,678
Engineering and Consulting | Domestic |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 92,489 | 83,119 | 71,678
Commercial |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 41,620 | 37,028 | 35,348
Commercial | Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 34,072 | 31,162 | 29,782
Commercial | Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 7,548 | 5,866 | 5,566
Government |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 266,774 | 236,907 | 192,463
Government | Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 182,079 | 159,935 | 126,494
Government | Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 84,695 | 76,972 | 65,969
Utilities |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 257,404 | 236,160 | 201,327
Utilities | Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 257,158 | 235,879 | 201,184
Utilities | Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 246 | 281 | 143
Time-and-materials |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 102,312 | 99,112 | 86,075
Time-and-materials | Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 34,381 | 35,582 | 32,491
Time-and-materials | Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 67,931 | 63,530 | 53,584
Unit-based |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 224,793 | 214,793 | 194,805
Unit-based | Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 205,117 | 199,040 | 180,509
Unit-based | Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 19,676 | 15,753 | 14,296
Fixed price |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 238,693 | 196,190 | 148,258
Fixed price | Energy |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | 233,811 | 192,354 | 144,460
Fixed price | Engineering and Consulting |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | $ 4,882 | $ 3,836 | $ 3,798

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