Math float equivalence

[Oskar Linde]

Lucas Goss skrev:
> In many game libraries, they often have some type of float equivalence 
> function to work around rounding differences, something like:
> 
> float equal(float f1, float f2, epsilon=1e-6)
> {
>   // check for equivalence between f1 and f2
>   // where the difference between them is less than epsilon
> }
> 
> How does D check for equivalence between floats, does it account for 
> rounding errors? If not, is there a library function for this? I noticed 
> "math.feqrel" (I still loathe the names of all these functions, I have 
> no idea what feqrel is supposed to mean) which seems like it might work, 
> but I would think it would take a lot more processing than to just check 
> equivalence.

int mfeq(real x, real y, real precision)

in std.math seems to be what you are asking for. The bad news is that it 
is private. The good news is that DMD lets you use it anyway as long as 
you use the fully qualified name std.math.mfeq. ;)

int feqrel(real x, feal y) returns to what precision x and y are equal 
(number of bits) which may or may not be what you want.

feqrel is not the solution to all floating point equivalence, but is a 
very handy tool in many cases. What is equivalent is really application 
dependent. Would you consider -1e-99 to be equivalent to 1e-99 for instance?

/Oskar