Change fast inverse square root to regular square root?

[PacR]

How to edit this code to get regualr square root instead of inverse square root?

float q_qsqrt( float number)
{
    long i;
    float x2,y;
    const float threehalfs = 1.5f;

    x2 = number * 0.5f;
    y = number;
    i = * ( long * ) &y;
    i = 0x5f3759df - ( i >> 1);
    y = * ( float *) &i;
    y = y * ( threehalfs - ( x2 * y * y) );

    return y;
}

I think that magic number 0x5f3759df have to change here, but i'm not smart.
All help is much appreciated and thank you for reading.

[jonnin]

google the bablyonian method (its really simple and they did this by hand a very long time ago!). or use sqrt() in <cmath>
also prefer double and never float unless you have a requirement to use float specifically.

[PacR]

Thank you jonnin. I used approximations that depend on the floating point representation section from your link and my function look like this now:

float q_sqrt( float number)
{
    long i;
    float y;
    y = number;

    i = * ( long * ) &y;
    i = (1 << 29) + (i >> 1) - (1 << 22) + (-0x4b0d2);
    y = * ( float *) &i;

    return y;
}