A simple implementation of the Quake 2 inverse square root algorithm.

Quake2_InvSqrt.h

#ifndef QUAKE2_INVSQRT_HPP
#define QUAKE2_INVSQRT_HPP


// Fast inverse square root algorithm
// Credit: Quake III Arena
// (Obsolete now due to native CPU support)

float invsqrt (float num) {
	long i = *(long*) &y;                    // Read the bits of the float as an integer
	i = 0x5f3759df - ( i >> 1 );             // Perform some maths on the resulting int
	num = *(float *) &i;                     // Convert the number back to a float
	num *= num * ( 1.5f - (2*num*num*num) ); // Do some final maths
	return num;
}


#endif