[Fwd: gsl-1.5: possible bug under interpolation/accel.c module]

[Fwd: gsl-1.5: possible bug under interpolation/accel.c module]

[Gerard Jungman]

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-- 
Gerard Jungman <jungman@lanl.gov>
Los Alamos National Laboratory

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From: Paolo Montegriffo <paolo.montegriffo@bo.astro.it>
To: jungman@lanl.gov
Subject: gsl-1.5: possible bug under interpolation/accel.c module
Date: Wed, 11 May 2005 10:16:09 +0200
Message-ID: <4281BF49.2010306@bo.astro.it>

Hi

I was looking at interpolation/accel.c module grom gsl lib - version 1.5 
when I noted a possible bug
in function gsl_interp_accel_find() here reported:

> size_t
> gsl_interp_accel_find (gsl_interp_accel * a, const double xa[], size_t 
> len, doub
> le x)
> {
>   size_t x_index = a->cache;
>
>   if (x < xa[x_index])
>     {
>       a->miss_count++;
>       a->cache = gsl_interp_bsearch (xa, x, 0, x_index);
>     }
>   else if (x > xa[x_index + 1])

^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I think that here you should first test wether x_index < len - 1 to 
avoid undefined behaviour

    else if (x_index < len - 1 && x > xa[x_index + 1])

>     {
>       a->miss_count++;
>       a->cache = gsl_interp_bsearch (xa, x, x_index, len - 1);
>     }
>   else
>     {
>       a->hit_count++;
>     }
>
>   return a->cache;
> }

in this way if a_cache = len - 1 and x >= xa[len-1] the function 
correctly return (len - 1) and the
cached value is left unchanged

hope this helps

cheers

Paolo Montegriffo
INAF - Bologna Astronomical Observatory
Via Ranzani, 1        I-40127 Bologna
Italy