Q5_8.java

package Chapter5;

public class Q5_8{
	
	static void drawHorizontalLine(byte[] screen, int width, int x1, int x2, int y){
		int height = screen.length/width/8;
		int startOffset = x1%8;
		int firstFullByte = x1/8;
		if(startOffset!=0)
			firstFullByte ++;
		int endOffset = x2%8;
		int lastFullByte = x2/8;
		if(endOffset !=7)
			lastFullByte --;
		for(int i = firstFullByte; i <= lastFullByte; i++){
			screen[y*width/8+i]=(byte) 0xff;
		}
		byte startMask = (byte) (0xff >> startOffset);
		byte endMask = (byte) ~(0xff>>(endOffset+1));
		
		if(x1/8 == x2/8){
			byte wholeMask = (byte) (startMask & endMask);
			screen[y*width/8+x1/8]|=wholeMask;
		}
		else if(startOffset!=0){
			screen[y*width/8+firstFullByte-1] |=  startMask;
		}
		else if(endOffset!=7)
			screen[y*width/8+lastFullByte+1] |=  endMask;
	}

}