Compile time float binary representation

[Jeremie Pelletier]

Andrei Alexandrescu Wrote:

> Jeremie Pelletier wrote:
> > Jeremie Pelletier Wrote:
> > 
> >> Andrei Alexandrescu Wrote:
> >>
> >>> Jeremie Pelletier wrote:
> >>>> Is there a way to convert a float (or double/real) to an integral number without changing its binary representation at compile time?
> >>>>
> >>>> I need to extract the sign, exponent and mantissa yet I cant use bit shifting.
> >>>> "Error: 'R' is not of integral type, it is a real" is the error I get.
> >>>>
> >>>> The usual *cast(uint*)&value wont work either at compile time.
> >>>>
> >>>> Any suggestions?
> >>> I got this to work, would it be applicable to your need?
> >>>
> >>> void main(string[] args)
> >>> {
> >>>      union A
> >>>      {
> >>>          double x;
> >>>          int y;
> >>>      }
> >>>      enum A a = A(0.3);
> >>>      writeln(a.x);
> >>>      writeln(a.y);
> >>> }
> >>>
> >>>
> >>> Andrei
> >> Good suggestion, however it doesnt work the way I need it:
> >>
> >> ---
> >> template FloatParts(real R) {
> >>     union A {
> >>         real theReal;
> >>         struct {
> >>             ushort hi;
> >>             ulong lo;
> >>         }
> >>     }
> >>     
> >>     A a = cast(A)R; // Error: cannot cast real to immutable(A)
> >>     enum A a = A(R); // members cannot be used at runtime
> >> }
> >> ---
> >>
> >> I need to gather .sign, .exp and .mantissa among others (possibly a .radix too) and use them at compile time (generic metaprogramming, or else i would just fallback to runtime).
> >>
> >> Unless there are arithmetic methods to get these, I dont see how it can be done without changes in the compiler first.
> >>
> >> To Walter: is this something possible for DMD2.32?
> > 
> > After some toying around, I managed to get some results:
> > 
> > ---
> > import std.metastrings;
> > immutable union A {
> > 	double x;
> > 	int y;
> > }
> > pragma(msg, ToString!(A(5.2).y));
> > ---
> > 
> > but it crashed dmd (2.031) instead of printing 3435973837 to stdout.
> > 
> 
> Yah, I crashed it a couple of times with similar code now too. Could you 
> please do the bugzilla honors.
> 
> (Also you may want to check std.numeric.CustomFloat. It doesn't help 
> with this particular problem, but it allows defining floating point 
> types with specific mantissa and exponent sizes.)
> 
> 
> Andrei

Thanks again for the quick reply Andrei, however, as useful as the CustomFloat template may be, I need a reverse algorithm usable entirely at compile time. If you know a way to extract this information without resorting to bitwise logic, please let me know, for the moment I'm adding a static assert(0) to my template until dmd supports compile time binary casts.

Also I created http://d.puremagic.com/issues/show_bug.cgi?id=3220.