hash for floating

[Ali Çehreli]

On 06/15/2013 03:13 AM, Paolo Invernizzi wrote:
> Hi all,
>
> How to implement a proper hash method for a struct/class having as a
> base one or more floating point?
>
> When I compile with the -w flags, dmd 2.063 tells me that the function
> must be size_t toHash() const nothrow @safe.
>
> The point is that the nothrow doesn't permit the std.conv.to
> conversions, while the @safe does not permit the cast().
>
> So how to perform a little math on the floating and then turning the
> result to a proper integer hash?
>
> Suggestions welcome! Thank!
>
> Paolo Invernizzi

I don't know whether it works correctly, but can you try calling 
typeid(f).getHash(&f):

import std.stdio;

struct S
{
     float f;
     int i;

     const hash_t toHash() nothrow @safe
     {
         return typeid(f).getHash(&f);
     }
}

void main()
{
     string[S] aa = [ S(0.5) : "half", S(0.25) : "quarter" ];

     aa[S(0.5)] = "one over two";
     aa[S(0.25)] = "one over four";

     writeln(aa);
}

Of course I have used exactly representable values 0.5 and 0.25 instead 
of any other float value. Maybe you can afford to store such a value (in 
addition to your actual float value) to be used for hashing.

Ali