Re: CTFE fmod ?

[rumbu via Digitalmars-d-learn]

On Friday, 20 November 2015 at 11:16:13 UTC, userABCabc123 wrote:
Does someone have a good CTFE fmod() function ? The problem is 
that std.math.fmod() is itself not available at CT, neither do 
floor() or similar functions necessary to get the quotient when 
the input value is two times over/under the bounds.



Any suggestion ? Or maybe this is a limit ?

Not thoroughly tested and only works for doubles, but this must 
do the trick.

double ctfe_trunc(double x) @trusted pure nothrow @nogc
{
        ulong bits = *cast(ulong*)(&x);
        auto sign = bits & 0x8000000000000000;
        long exponent = (bits >> 52) & 0x7FF;
        auto mantissa = (bits & 0xFFFFFFFFFFFFF);
        
        if (exponent == 0 && mantissa == 0)
                return 0.0;
        else if (exponent == 0x7FF && mantissa == 0)
                return sign ? -double.infinity : double.infinity;
        else if (exponent == 0x7FF)
                return double.nan;

        exponent -= 1023;
        auto target = 52 - exponent;
        if (target >= 0 && target <= 51)
        {
                auto msb = mantissa & (1UL << target);
                auto lsb = mantissa & ~((1UL << target) - 1);
                bits = sign | ((exponent + 1023)) << 52 | lsb;
                mantissa += msb;
                return *cast(double*)&bits;
        }
        else
                return sign ? -0.0 : 0.0;
}

double ctfe_fmod(double x, double y) @safe pure nothrow @nogc
{
        return x - ctfe_trunc(x / y) * y;
}