Bitwise shift operation on a 128-bit number

Question

Lets say that I have an array of 4 32-bit integers which I use to store the 128-bit number

How can I perform left and right shift on this 128-bit number?

Tha

Answer 1

Since you mentioned you're storing your 128-bit value in an array of 4 integers, you could do the following:

void left_shift(unsigned int* array)
{   
    for (int i=3; i >= 0; i--)
    {
        array[i] = array[i] << 1;

        if (i > 0)
        {
            unsigned int top_bit = (array[i-1] >> 31) & 0x1;
            array[i] = array[i] | top_bit;
        }
    }
}

void right_shift(unsigned int* array)
{   
    for (int i=0; i < 4; i++)
    {
        array[i] = array[i] >> 1;

        if (i < 3)
        {
            unsigned int bottom_bit = (array[i+1] & 0x1) << 31;
            array[i] = array[i] | bottom_bit;
        }
    }
}