Re: libcerf function voigt_hwhm not suitable for external use

[Joachim W. <j.w...@fz...>]

Dear Ethan,

thank you very much for the kind mail you wrote me more than a year ago.

Today, finally, I looked into the issue.  I wrote a new test that computes
voigt_hwhm for 100'000 different ratios gamma/sigma.  As expected, the Newton
algorithm did not fail a single time.  The deviation of the final result from
the initial estimate is always smaller then 1E-2.  The cases I had guarded
with messages to stderr and an exit statement will never happen.  Therefore
I converted them to assert statements.  These improvements are available
in the freshly released v1.15.

Sorry for not answering earlier.

Kind greetings, Joachim



On 25/02/2020 20.03, Ethan A Merritt wrote:
> In response to a gnuplot feature request, I added a wrapper in
> gnuplot to find the width of the Voight profile by calling libcerf function
> voigt_hwhm().
> 
> However on further evaluation this has turned out to be problematic.
> The current libcerf source for this function deals badly with out-of-range
> input.  It prints to stderr (problematic by itself) and then either
> - returns an incorrect value
> - calls exit(-1)
> 
> A call to exit() from a library routine is almost never a good thing to 
do,
> since it prevents error-handling and possible recovery by the calling
> program.  Limiting error reporting to a message on stderr also prevents
> detection and recovery by the calling program.
> 
> I suggest that it would be preferable to return NaN whenever the
> input is out of range or the function does not converge.
> 
> Alternatively libcerf could provide an error-check routine or status variable,
> but then you would run into issues of thread-safety and so on.
> 
> I am hoping that the libcerf function can be improved in a subsequent
> version, but for now I will consider implementing equivalent code
> directly in gnuplot.
> 
> 	Ethan
> 
>