mpFactorial method
Implementation
void mpFactorial( MPData ret, int x ){
bool m = false;
if( x < 0 ){
m = true;
x = 0 - x;
}
mp.str2num( ret, "1" );
MPData ii = MPData();
for( int i = 2; i <= x; i++ ){
mp.str2num( ii, "$i" );
mp.mul( ret, ret, ii );
}
if( m ){
mp.neg( ret );
}
}