Question5_8.java

/**
 * A monochrome screen is stored as a single array of bytes, allowing eight consecutive pixels to be stored in one byte.
 * The screen has width w, where w is divisible by 8 (that is, no byte will be split across rows).
 * The height of the screen, of course, can be derived from the length of the array and the width.
 * Implement a function drawHorizontalLine(byte[] screen, int width, int x1, int x2, int y)
 * which draws a horizontal line from (x1, y) to (x2, y).
 * 
 * Time  Complexity: O(x2 - x1)
 * Space Complexity: O(1)
 */

public class Question5_8 {
    public void drawHorizontalLine(byte[] screen, int width, int x1, int x2, int y) {
        int height = screen.length / (width / 8);
        if (y < 0 || y >= height || x1 < 0 || x1 >= screen.length * 8 || x2 < 0 || x2 >= screen.length * 8) {
            return;
        }
        
        int idx = y * (width / 8);
        int idx1 = idx + (x1 / 8);
        int idx2 = idx + (x2 / 8);
        
        for (int i = idx1 + 1; i < idx2; ++i) {
            screen[i] = (byte) 0xFF;
        }

        byte mask1 = (byte) (0xff >>> x1 % 8);
        byte mask2 = (byte) (~((1 << (7 - x2 % 8)) - 1));
        if (idx1 == idx2) {
            screen[idx1] |= (mask1 & mask2);
        } else {
            screen[idx1] |= mask1;
            screen[idx2] |= mask2;
        }
    }
    
    
    public static void main(String[] args) {
        Question5_8 question5_8 = new Question5_8();
        byte[] screen = new byte[12];
        int width = 4;
        for (int i = 0; i < screen.length / width; ++i) {
            for (int j = 0; j < width; ++j) {
                System.out.print(screen[i * width + j]);
            }
            System.out.println();
        }
        
        System.out.println();
        question5_8.drawHorizontalLine(screen, width * 8, 9, 26, 1);
        for (int i = 0; i < screen.length / width; ++i) {
            for (int j = 0; j < width; ++j) {
                System.out.print(screen[i * width + j]);
            }
            System.out.println();
        }
        
        System.out.println();
        screen = new byte[12];
        question5_8.drawHorizontalLine(screen, width * 8, 1, 5, 0);
        for (int i = 0; i < screen.length / width; ++i) {
            for (int j = 0; j < width; ++j) {
                System.out.print(screen[i * width + j]);
            }
            System.out.println();
        }
    }
}