question: private void computeCrcForWriting ( ) { crcengine = new CRC32 ( ) ; crcengine . update ( idbytes , 0 , 4 ) ; if ( len > 0 ) crcengine . update ( data , 0 , len ) ; PngHelperInternal . writeInt4tobytes ( ( int ) crcengine . getValue ( ) , crcval , 0 ) ; }
target:  this is called after setting data, before writing to os