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;
}