Bitwise integer cube root algorithm

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Here is a simple way to calculate an integer square root:

int isqrt(int num)
{
    int root=0;
    int b = 0x8000;
    int a=0, c=0;

    while (b) {
        c = a|b;

        if (c*c <= num)
            a |= b;   

        b >>= 1;
    }       
}

Ingeniously (thanks Wikipedia), this can be optimised like this:

int sqrt(short num)
{
    int op = num;
    int res = 0;
    int one = 1 << 30;

    while (one > op)
        one >>= 2;

    while (one != 0) {
        if (op >= res + one) {
            op -= res + one;
            res = (res >> 1) + one;
        }
        else
          res >>= 1;
        one >>= 2;
    }
    return res;
}

My question: Can a similarly optimised algorithm be written for an integer cube root? (This is to be run on a small microcontroller which prefers not to do multiplications)

2 Answers
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