Fast inverse square root

q_rsqrt.go

package main

import (
	"unsafe"
)

// Assumes that number always positive
// You don't want to deal with complex numbers
func Q_rsqrt(number float32) float32 {
	var i int32
	var y float32
	const threehalfs float32 = 1.5

	var x2 float32 = number * float32(0.5)
	y = number
	i = *(*int32)(unsafe.Pointer(&y)) // evil floating point bit level hacking
	i = 0x5f3759df - (i >> 1)         // what the fuck?
	y = *(*float32)(unsafe.Pointer(&i))

	y = y * (threehalfs - (x2 * y * y)) // 1st iteration
	y = y * (threehalfs - (x2 * y * y)) // 2nd iteration, this can be removed
	return y
}

q_rsqrt_test.go

package main

import (
	"math"
	"testing"
)

var EPSILON float32 = 0.0001

func floatEquals(a, b float32) bool {
	if (a-b) < EPSILON && (b-a) < EPSILON {
		return true
	}
	return false
}

func Test_Q_rsqrt(t *testing.T) {
	result16 := float32(16) * Q_rsqrt(16)
	resultFromMath16 := float32(math.Sqrt(16))

	if !floatEquals(result16, resultFromMath16) {
		t.Errorf("Floating numbers %f and %f doesn't match", result16, resultFromMath16)
	}

	result4 := float32(4) * Q_rsqrt(4)
	resultFromMath4 := float32(math.Sqrt(4))

	if !floatEquals(result4, resultFromMath4) {
		t.Errorf("Floating numbers %f and %f doesn't match", result4, resultFromMath4)
	}
}