CODEWITHSUNDEEP
java
user4650623
user4650623

Reputation: 17

trouble in understanding a method in java

The following is a method(public static byte[] bitStuff(byte[] b)) for bit stuffing of a message in java. But I can't understand the lines commented out inside the method:

 public static byte set_1_at_position(int i, byte b) {
    b |= (128 >> i);
    return b;
}

public static byte set_0_at_position(int i, byte b) {
    b &= (byte) (~(128 >> i));
    return b;
}

public static int get_bit_at_position(int pos, byte b) {
    if ((b & (byte) (128 >> pos)) == 0) {
        return 0;
    }
    return 1;
}



    public static byte[] bitStuff(byte[] b){

            byte[] temp=new byte[b.length*2];
            int size=b.length*8;
            int count_of_one=0;
            int count_of_temp=0;
            for(int i=0;i<size;i++)
            {
                int bit=get_bit_at_position(i%8, b[i/8]); //what is being done here
                if(bit==1)
                {
                    count_of_one++;
                    temp[count_of_temp/8]=set_1_at_position(count_of_temp%8, temp[count_of_temp/8]);  //what is happening in this line
                }
                else
                {
                    count_of_one=0;
                    temp[count_of_temp/8]=set_0_at_position(count_of_temp%8, temp[count_of_temp/8]);
                }

                count_of_temp++;

                if(count_of_one==5)
                {
                    count_of_one=0;
                    //bit stuffing
                    temp[count_of_temp/8]=set_0_at_position(count_of_temp%8, temp[count_of_temp/8]);  // what is happening in this line
                    count_of_temp++;
                }
            }

            byte[] ret=new byte[(count_of_temp+7)/8];
            System.arraycopy(temp, 0, ret, 0, ret.length);

    return ret;

}

can anyone explain these lines?

Upvotes: 0

Views: 54

Answers (1)

Behzad
Behzad

Reputation: 53

Note that the "size" and "i" variables, are counting the bits (not bytes). get_bit_at_position(i%8, b[i/8]); This line returns the "i"th bit value. The "i"th bit value is equivalent to the "i%8"th bit value at the "i/8"th byte. set_0_at_position(count_of_temp%8, temp[count_of_temp/8]); This is almost the inverse of the above line. It sets the "i"th bit to zero.

Upvotes: 1

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