BigFloat or BigDecimal?

[Paul D. Anderson]

Don Wrote:

> Paul D. Anderson wrote:
> > I have some bikeshed sort of questions regarding both naming and implementation and I'd like some feedback from the community:
> > 
> > 1) I've named the AP numbers "Decimal", but a better name is either "BigDecimal" or "BigFloat". I'm leaning toward BigDecimal. The fixed size numbers are "Dec32", "Dec64" and "Dec128". Are these names satisfactory?
> 
> Do they use base 10 ?
> Ie, is it impossible to request a significand length of (say) 2048 bits?
> 
> Name = BaseTenOnly ? "BigDecimal" : "BigFloat".
> 
> I would expect something called 'BigFloat' to be able to duplicate the 
> semantics of the built-in binary floating point types, and 'BigDecimal' 
> to duplicate the semantics of the decimal types.
> If it can do both, BigFloat is a shorter name.
> 

They are decimals. Significands are specified in decimal digits, etc.

> > 2) There are two available representations for the fixed size numbers: densely packed decimals and binary integer decimals. If anyone knows or cares about the difference do you have a preference? The binary integer decimals are much simpler to convert to other types, so I'm implementing those, but it's not much work to implement the others. If you want more info I can point you to references.
> 
> I suspect densely packed decimals are only really useful as an 
> interchange format, when you don't have hardware that uses them 
> natively. But I could be wrong.
> 

According to what I've read, there is some uncertainty as to which would or should be implemented in hardware, and the iEEE-754 is agnostic about which representation is preferred.

But this is something that can be easily incorporated later, either way.

> > 3) The current design is to back all the fixed size numbers with the AP numbers. That is, the fixed sized are converted to arbitrary precision, operated on, and coverted back. The AP numbers are based on std.bigint.BigInt, so all internal math is performed using BigInts. It is possible, however, to use uints and ulong arithmetic for Dec32 and Dec64, respectively. But the ints and longs would have to indicate overflow, etc., so it's not trivial. (I wish there were CheckedInt struct!) The first releases will use BigInts, but is it worthwhile to pursue the other?
> 
> I think that stuff belongs in a FixedInt class rather than in this code.
> 

I agree.

> > 
> > Any questions or ideas please e-mail me at the address above. (I won't be able to check my e-mail until this evening (PDT), so if you want a quicker response use this forum.)
> > 
> > Paul
> >