[GiNaC-devel] Erasing-inserting exhashmap

[Vladimir V. Kisil]

	Dear Richard,

	Many thanks for the clarification. Your patch is working for me
  as the part of GiNaC. Thus I have copied it to my code to make it
  exhashmap-independent for systems with older versions of GiNaC.

  Best wishes,
  Vladimir
-- 
Vladimir V. Kisil                 http://www.maths.leeds.ac.uk/~kisilv/
  Book:     Geometry of Mobius Transformations     http://goo.gl/EaG2Vu
  Software: Geometry of cycles          http://moebinv.sourceforge.net/
  Jupyter (Colab):         https://github.com/vvkisil/MoebInv-notebooks
  Jupyter (CodeOcean):            http://doi.org/10.24433/CO.9934595.v3

>>>>> On Sun, 22 Sep 2019 02:08:36 +0200, "Richard B. Kreckel" <kreckel at in.terlu.de> said:

    RK> 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?

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

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

    RK> The attached prelimiary patch should solve all these
    RK> 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.

    RK> Worry not!

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