Testing primality of long doubles

[psiblade]

I've been tearing my hair out over this for awhile, and I have a feeling I'm going to kick myself when someone shows me what I'm doing wrong. I've written a very basic boolian function to test input for primality (full function in the attached header). It works perfectly for all integer values I can throw at it, but when I start using floating-point numbers (which I've tried to design it to handle), I start running into weird problems.

For example, the number 9.9999999978e10, which is clearly divisible by 2, should be caught here:

  if (static_cast<int>( fmodl(n, 2) ) == 0)             // tests for divisibility by 2
   {return false;}
   else

The variable n in this case being a long double, the input to the prime() function.

As far as I know, typecasting the result of the modulus will drop anything after the decimal point, (and indeed, a simple test program running static_cast<int>(fmodl(9.9999999978e10,2)) will return 0), eliminating the weirdness that I've heard comes with comparisons with floating-point numbers.

Oddly, prime() is always returning true with non-integer numbers, and if the problem is with this comparison or somewhere else in the function, for the life of me I can't figure out why.

Any insight is much appreciated.

~Jap