找質數程式2

改用快速開根

#include <stdio.h>

unsigned int test_number;

float SquareRootFloat(float number)
{
    long i;
    float x, y;
    const float f = 1.5F;

    x = number * 0.5F;
    y  = number;
    i  = * ( long * ) &y;
    i  = 0x5f3759df - ( i >> 1 );
    y  = * ( float * ) &i;
    y  = y * ( f - ( x * y * y ) );
    y  = y * ( f - ( x * y * y ) );
    return number * y;
}

unsigned int not_prime(unsigned int x)
{
    unsigned int q_x;
    unsigned int i;

    if ( !(x & 0x1) ) return 2;
    q_x = SquareRootFloat(x);
    for (i=3; i<=q_x; i+=2)
        if ( x % i == 0 ) return i;
    return 0;
}

void main()
{
    test_number = 0x73d58987;
    if ( test_number % 2 == 0 ) test_number--;
    while ( not_prime(test_number) )
        test_number -= 2;
    printf("%d\n", test_number);
}