Алгоритм приближённого вычисления квадратного корня

#ifndef __fast_inv_sqrt_h__
#define __fast_inv_sqrt_h__

#include <cassert>
#include <stdint.h>


/**
 * Fast approximation of the inverse square root
 *  (http://en.wikipedia.org/wiki/Fast_inverse_square_root)
 *
 * @param x
 *             32-bit floating point (IEEE 754-2008)
 * @return the approximated inverse square root of x
 */
inline float InvSqrt(float x) {
    assert( sizeof(x) == 4 );
    assert( x >= 0 );

    union {
        uint32_t i;
        float y;
    };

    y = x;
    i = 0x5f375a86 - (i >> 1);
                 /* Approximation step */

    return y * (1.5f - 0.5f * x * y * y);
                 /* One step more of the Newton's method */
}

#endif /*_fast_inv_sqrt_h__*/