Wèile zhè ge xiàngmù, tā dānjīng-jiélǜ hǎojǐ gè yuè.
For this project he racked his brains for several months.
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From Bai Juyi's 《策林·策头》: 殚思极虑，以尽微臣献言之道乎 ('I exhaust thought and mental effort to fulfill my humble duty of offering counsel'). The idiom carries the Tang scholar-official's sense of devoted intellectual labor.
