Q_rsqrt.c

#include <stdint.h>
float Q_rsqrt (float number)
{	
	const float x2 = number * 0.5F;
	const float threehalfs = 1.5F;

	union {
		float f;
		uint32_t i;
	} conv  = { .f = number };
	conv.i  = 0x5f3759df - ( conv.i >> 1 );
	conv.f  *= threehalfs - ( x2 * conv.f * conv.f );
	return conv.f;
}

Quake3Arena_Q_rsqrt.c

float Q_rsqrt(float number)
{
	long i;
	float x2, y;
	const float threehalfs = 1.5F;

	x2 = number * 0.5F;
	y  = number;
	i  = * ( long * ) &y;                       // evil floating point bit level hacking
	i  = 0x5f3759df - ( i >> 1 );               // what the fuck? 
	y  = * ( float * ) &i;
	y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
//	y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

	return y;
}