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Suppose using Java, unless we need bound check, there's no need for 'len'

void drawLine(byte[] img, int len, int wid, int r, int x1, int x2) {
        if ( x2 < x1 ) {
              logErrorAndExit();
        }
        if ( r > len ) {
              logErrorAndExit();
        }
        // First compute from which bit to start/stop
        int startBit = (r-1) * wid + x1;
        int stopBit = (r-1) * wid + x2;
        // Then compute the corresponding in the array
        int startByte = startBit / 8;
        int stopByte = stopBit / 8;
        // Then compute from which bit of the byte to start/stop
        int byteStartBit = startBit % 8;
        int byteStopBit = stopBit % 8;
        // Then set the bits
        // Set intermediate bytes first
        for ( int i = startByte +1; i < stopByte; i++ ) {
              img[i] = img[i] | 0xFF; // suppose we are using jdk1.7
        }
        // Then set the start/end byte
        for ( int i = 0; i < byteStartBit; i++ ) {
              img[startByte] = img[startByte] | 1 << (8-i);
        }
        for ( int i = 0; i < byteEndBit; i++ ) {
              img[stopByte] = img[stopByte] | 1 << (i);
        }   
}

void drawLine(byte[] img, int len, int wid, int r, int x1, int x2) {
for(int i=wid*r+x1; i<=wid*r+x2;i++) {
 img[i]=1; //in java, we need not worry about of array index out of bounds, as jvm throws this error if i is going out of range of img array last index. Otherwise, we can add a check i<=img.length
}

length and width? Don't you mean height and width?

I don't get this qn. Can you please clarify with an example? What I make out is, byte[] img is a 2D array whose width is 8 (8 bits). In this 2D array, we have to set x1->x2 in row r as 1. Right? If this is the case, then length and width are not required. My solution for this case would be -

void drawLine(byte[] img, int len, int wid, int r, int x1, int x2) {
 byte[r] = 0; // let x1 = 3, x2 = 5;
 byte[r] << (x2-x1+2); // byte[r] = 00001000
 byte[r] = byte[r] - 1;    // byte[r] = 00000111
 byte[r] = byte[r] << (8-x2); //byte[r] = 00111000
}

public class DrawLine {
	
	static void drawLine(byte[] img, int len, int wid, int r, int x1, int x2) {
		int nbyte = wid/8;
		
		printImg(img, wid);

		for (int i = x1; i <= x2; i++) {
			int index = getIndex(i, r, nbyte);
			int bitPos = getBitPos(i, nbyte);
			img[index] = (byte) (img[index] | (0x80 >> bitPos));
		}

		System.out.println();
		printImg(img, wid);
	}
	
	static int getIndex(int x, int row, int nbyte) {
		return (row-1)*nbyte + (x >> 3); 
		
	}
	
	static int getBitPos(int x, int nbyte) {
		return (x - ((x >> 3) << 3));
	}
	
	static void printImg(byte[] img, int wid) {
		int row = img.length * 8 /wid;
		int nbyte = wid/8;
		
		for (int r = 1; r <= row; r++) {
			for (int i = 0; i< wid; i++) {
				int index = getIndex(i, r, nbyte);
				int bitPos = getBitPos(i, nbyte);

				if ((img[index] & (0x80 >> bitPos)) > 0)  System.out.print("1");
				else System.out.print("0");
			}
			System.out.println();
		}
	}
	
	public static void main(String[] args) {
		byte[] img = new byte[32];
		
		drawLine(img, 0, 32, 7, 9, 31);
	}
}

I recently gave google interview, was not able to code in one round, all othr round went good.
Wht r my chances?

len/wid parameters mean how many pixels for a horizontal/vertical line?

This Question for Graphics designer.....

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