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Income taxes - General (Details)
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Income taxes      
Statutory Tax rate 25.00% 25.00% 25.00%
Hong Kong | first HK$2 million of assessable profits earned first HK$2 million of assessable profits earned      
Income taxes      
Statutory Tax rate 8.25%    
Hong Kong | after first HK$2 million of assessable profits earned      
Income taxes      
Statutory Tax rate 16.50%    
PRC      
Income taxes      
Statutory Tax rate 25.00%    
Preferential income tax rate 15.00%    
High and new technology enterprise certificate term 3 years