[GiNaC-devel] Erasing-inserting exhashmap

[Richard B. Kreckel]

Dear Vladimir,

On 19.09.19 17:06, Vladimir V. Kisil wrote:
> 	I have an issue with exhashmap, namely repeated
>   erasing-insertion of elements eventually become trapped in an infinite
>   loop in lines 487-491 of hash_map.h. Here is a sample code:
> 
> #include <ginac/ginac.h>
> using namespace GiNaC;
> 
> int main()
> {
> 	exhashmap<ex> M;
> 
> 	M[realsymbol("x")] =1;
> 	int N= 30;
> 	
> 	for (int i =0; i < N; ++i ) {
> 		realsymbol a("a");
> 		M[a]=numeric(i);
> 		M.erase(a);
> 	}
> 	return 0;
> }
> 
>   With this value of N the infinite loop appears on my computer with a
>   probability around 40%. With larger values of N an infinite loop is
>   almost certain.
> 
>   Did I miss something?

Class exhashmap<T> was a workaround for missing std::hash_map<Key, T> in
the original C++98 standard. It was put in GiNaC because map<Key, T> was
deemed too slow. Since C++11 there is std::unorderd_map<Key, T>, which
is hash-based.

Don't use that exhashmap. It should have been replaced years ago when we
switched to C++11!

The attached prelimiary patch should solve all these problems. Can you,
please, test it with whatever you're doing?

>   PS Also just to check that my patch for archiving empty lists from Sep
>   10 is not lost. 

Worry not!

  -richy.
-- 
Richard B. Kreckel
<https://in.terlu.de/~kreckel/>
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