On 19/09/2010 18:39, Bradley Mitchell wrote:
Hello,
I'm trying to implement the Quake 3 fast inverse square root algorithm which
requires casting from int to float without modifying the stored bits. A C++
reinterpret cast seems to accomplish this just fine but so far I have had no
success in D after trying quite a few different things. Here is the C++ I've
written that currently works:
#include<iostream>
float fastInvSqrt( float x )
{
const int INV_SQRT_N = 1597292357;
const float MULT = 1.000363245811462f;
float const mx = 0.5f * MULT * x;
int xi = *reinterpret_cast<int *>(&x );
xi = INV_SQRT_N - (xi>> 1);
x = *reinterpret_cast<float *>(&xi );
return x * (1.5f * MULT - mx * x * x);
}
int main()
{
float a = fastInvSqrt( 9.0f );
std::cout<< a<< std::endl;
}
And here is my D code that doesn't work yet:
module test;
import std.stdio;
float fastInvSqrt( float x )
{
const int INV_SQRT_N = 1597292357;
const float MULT = 1.000363245811462f;
float mx = 0.5f * MULT * x;
int xi = cast(int)cast(void*)x;
xi = INV_SQRT_N - (xi>> 1);
x = xi;
x = cast(float)cast(void*)xi;
return x * (1.5f * MULT - mx * x * x);
}
void main(string[] args)
{
float a = fastInvSqrt( 9.0f );
writefln("%f", a);
}
For further reference, the wikipedia article on this algorithm can be found
here:
http://en.wikipedia.org/wiki/Fast_inverse_square_root
and my C++ code is based on a slightly modified version described here:
http://www.ece.uwaterloo.ca/~dwharder/Algorithms_and_Data_Structures/Algorithms/Inverse_square_root/
As you can see, I'm not trying to convert an integer to an equivalent floating
point value but rather trying to coerce the variable to a float without
modifying the underlying bit sequence. Any help would be greatly appreciated!!
Thank you for your time,
Brad
Use a union, which is the correct and safe way.
union convert
{
float f;
uint i;
}
convert.f = 1.0f;
writeln(convert.i);
will print 1065353216 (0x3f800000)
