Re: Lua workshop: call for backup presentations.

[Sam Roberts <sroberts@...>]

On Thu, Aug 24, 2006 at 10:06:25AM +0100, Andy Stark wrote:
> > Is it worth discussing how Lua could
> >support more than one sort of number?
> 
> Would it make sense to have binary coded decimal numbers as the default
> numeric type for Lua? (Ooooh, controversial!) It's obviously much, much
> slower than onboard floating point but is still fast enough for many uses
> and it has its advantages:

How much slower is it than onboard integral arithmetic? Don't some
processors support BCD instructions?

I find the "only one numeric type" stance of lua to be hard to work
around when interfacing with C code that uses unsigned long long a lot,
and I haven't found a satisfying solution. Double is enough to contain
most integrals of a human scale, but not enough to mantain unsigned long
long without loss of value. 64 bit processors are becoming common,
uint64s in C APIs might become more common, too.

Things we've tried:

- a Uint64 class
  easy to do, hard to use (have to initialize with string literals,
  have to rework C code to check for a lua_Number or Uint64 user-data
  in lots of places, doesn't work with the %d formatting and in other
  places where lua expects a number, feels like a hack, basically)

- Kauppi's patch - seems really nice, but only does signed long long,
  and I need unsigned long long. Can keep the bit-representation, but
  printing and display would be useless for me.

- long double as the lua native type - been working on this. People
  seem horrified.... but I don't do any arithmetic to speak of other
  than comparisons, simple counting loops, etc., I just need to move
  data back and forth between C and lua accurately. I'm still working
  on this.

BCD would be a nice compromise for me. Probably adequately fast for most
things, would be a single numeric type (which fits the Lua Way, as I
understand it), but arbitrary precision. It would be quite small for
small numbers.

I'm trying to find a solution that doesn't involve creating a second
type that lua itself doesn't understand to be a number, which is what
straight-forward binding of multi-precision math libraries gives (lbc,
lmapm, etc.).

Anybody have a BCD-in-lua library? I'd be very happy to try working with
such a beast!

> Of course, we would still want the existing option of using different
> numeric types with Lua but just supply BCD as standard. And there is more

I'd be happy with BCD as an option. Actually, I'd be happy with ANYTHING
that wasn't a native C type (gnu-mp, mapm, ...), so that
9223372036854775808 in lua code became a lua_Number, with no loss.

Cheers,
Sam