problem 5.8

gistfile1.java

//assume x1 < x2
drawHorizontalLine( byte[] screen, int width, int x1, int x2, int y ) {
  	int start = y * width / 8 + ( int ) x1 / 8;
	int end = y * width / 8 + ( int ) x2 / 8;

	for ( int i = start; i <= end; i++ ) {
		screen[ i ] = 127;
	}
	if ( start == end ) {
		screen[ start ] = screen[ start ] >> ( x1 % 8 );
		byte mask = 127 << ( 8 - x2 % 8 );
		screen[ start ] = screen[ start ] ^ mask;
	}
	
	else {
		screen[ start ] = screen[ start ] >> ( x1 % 8 );
		screen[ end ] = screen[ end ] << ( 8 - x2 % 8 );
	}
}