Re: Integer division in Lua with numbers as longs

[ramsdell@... (John D. Ramsdell)]

> On Apr 18, 2006, at 01:26, Reuben Thomas wrote:
> 
> > On Tue, 17 Apr 2006, John D. Ramsdell wrote:
> >
> >> #define luai_numdiv(a,b) ((a)<0==(b)<0||(a)%(b)==0?(a)/(b):(a)/(b)-1)
> >
> > This is not reliable: ANSI/ISO C does not specify how integer division 
> > is rounded (to zero or to -infinity).
> 
> The 1999 C standard does.  And it specifies round towards zero (which 
> in my opinion is completely broken).
> 
> Previous versions of the C standard allowed the implementation to 
> select either style of rounding.
> 
> For personal reasons I'd love to know about any platform where someone 
> has observed C's integer division rounding towards -inf.

You are correct, ANSI C does not specify how integer division is
rounded.  Do you think these definitions will fix the problem?

#define luai_numdiv(a,b)	\
  (-1/2?(a)/(b):((a)<0==(b)<0||(a)%(b)==0?(a)/(b):(a)/(b)-1))
#define luai_nummod(a,b)	\
  (-1/2?(a)%(b):((a)<0==(b)<0||(a)%(b)==0?(a)%(b):(a)%(b)+(b)))

Thank you both for alerting me to this problem.

John