question: public static int previousClearBit ( long v , int start ) { if ( start < 0 || start >= Long . SIZE ) { return - 1 ; } long cur = ~ v & ( LONG_ALL_BITS > > > start ) ; if ( cur == 0 ) { return - 1 ; } return Long . SIZE - 1 - Long . numberOfTrailingZeros ( cur ) ; }
target:  Find the previous clear bit.