[Magic] [C] Fast inverse square root

[SirGod]

Ceva "magie" pentru pasionatii hack-urilor de genul. Daca vreti sa aflati ce face si cum functioneaza urmatoarea functie:

float InvSqrt(float x) {
    float xhalf = 0.5 f * x;
    int i = * (int * ) & x; // get bits for floating value
    i = 0x5f3759df - (i >> 1); // gives initial guess y0
    x = * (float * ) & i; // convert bits back to float
    x = x * (1.5 f - xhalf * x * x); // Newton step, repeating increases accuracy
    return x;
}

Puteti citi:

http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf

https://en.wikipedia.org/wiki/Fast_inverse_square_root

[Skalpel]

[REDACTED]

Edited March 27, 2017 by Skalpel

[MrGrj]

float InvSqrt(float x) {
    float xhalf = 0.5 f * x;
    int i = * (int * ) & x; // get bits for floating value
    i = 0x5f3759df - (i >> 1); // gives initial guess y0
    x = * (float * ) & i; // convert bits back to float
    x = x * (1.5 f - xhalf * x * x); // Newton step, repeating increases accuracy
    return x;
}

float xhalf =[COLOR="#FF0000"] 0.5 f[/COLOR] * x;

Nu stiu de ce, dar am impresia ca nu compileaza in forma asta

program.c:6:23: error: expected ‘,’ or ‘;’ before ‘f’

float xhalf = 0.5 f * x;

^

program.c:10:18: error: expected ‘)’ before ‘f’

x = x * (1.5 f - xhalf * x * x); // Newton step, repeating increases accuracy

Pentru cei care sunt la inceput, ar trebui sa fie:

0.5f //fara spatiu 

Edited August 21, 2015 by MrGrj