Re: gnuplot numerical errors on Intel MacBook

[Per P. <per...@ma...>]

On Sep 26, 2006, at 22:06, Ethan Merritt wrote:

> On Tuesday 26 September 2006 12:49 pm, Per Persson wrote:
>> Hi all,
>> sorry for being late to the party, but I've been busy buing a
>> house...
>
> Congratulations!

Thanks!

>
> No, it's more complicated than that.
> The problem is that the name "gamma" has historically been used
> on some systems to refer to the function that is now called
> "lgamma", and on other systems at other times to refer to a
> function that is equivalent to "tgamma".  Thus we cannot tell
> from the name exactly what its behaviour is.
>
> Gnuplot's auto-configure, as I understand it, is looking for a
> function named "gamma" and assuming that it behaves like lgamma
> with additional information about the sign of the result returned
> in extern sgngam.  I am worried that on the Mac in question,
> "gamma" is instead acting like "tgamma".  That's not broken;
> it's just a different naming convention.  My thought was that
> in this case we should test first for a function named "tgamma",
> and use it along with its slightly different API.

OK, if I understand things correctly, then we could
1) make configure check for gamma, lgamma, and tgamma *and* check  
each of them (including tgamma to be on the safe side;-) e.g.:

AC_TRY_RUN([
int main()
{
         return gamma(2.0) < 1.0;
}
]
, [ echo "gamma is GAMMA" ]
, [ echo "gamma is LOG_OF_GAMMMA" ]
)

to see if they are in reality $\ln\Gamma$ (LaTeX notation) and use  
them in order:
HAVE_LGAMMA && LGAMMA_IS_LOG_OF_GAMMA
HAVE_GAMMA && GAMMA_IS_LOG_OF_GAMMA
HAVE_TGAMMA && TGAMMA_IS_LOG_OF_GAMMA
Keeping the current code in in specfunc.c (and file a bug report with  
whoever implemented the last combo).

2) make configure check for tgamma *and* verify that it is $\Gamma$  
(LaTeX notation):
HAVE_TGAMMA && !TGAMMA_IS_LOG_OF_GAMMA
change the current code to use tgamma, and let systems without tgamma  
rely on a fallback solution in specfunc.c

If the above is correct, then I'd support (2) but I guess it boils  
down to how much work it would need.

/Per