Last active
January 23, 2023 12:03
-
-
Save juandesant/978f4e04024d57ba0f72f7a6ee0512c0 to your computer and use it in GitHub Desktop.
"Fast" Inverse Square Root based on Quake's algorithm implemented in Swift
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
//Quick inverse square root using bit patterns | |
//Might be slower nowadays than using specific functions | |
// The following is the abridged version of the Quake function | |
// See more at https://en.wikipedia.org/wiki/Fast_inverse_square_root | |
//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; | |
//} | |
// We reimplement the function above in Swift | |
func Q_rsqrt(x: Float) -> Float { | |
// get binary floating point representation of x | |
var x_bits : UInt32; // We need a UInt32 to hold the bit-pattern for the Float | |
var half_x, result : Float; | |
half_x = x*0.5; // This is used later in the iteration. See wikipedia link above. | |
x_bits = x.bitPattern; // Instead of using pointer and casting trickery, | |
// we obtain the bit-pattern directly from the | |
// Swift floating point object | |
x_bits = 0x5f3759df - (x_bits >> 1); // We do the binary operation | |
new_float = Float(bitPattern: x_bits);// We use a Float creator that uses the bitPattern | |
new_float = new_float * (1.5 - (half_float*new_float*new_float)) // See wikipedia link | |
return new_float; | |
} | |
// Normal inverse of square root for | |
func rsqrt(x: Float) -> Float { | |
return 1/(x.squareRoot()); | |
} | |
// The following allows running this algorithm in a Swift Playground, so that we can | |
// see values, or even graph them with time | |
var x :Float; | |
var y, y_, y_diff : Float; | |
for x in stride(from: 0.2, to: 64.2, by: 0.2) { | |
let y = Q_rsqrt(x: Float(x)); | |
let y_ = rsqrt(x: Float(x)); | |
let y_diff = abs(y-y_); | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment