# [GiNaC-list] factorization bug

Ivan Vasilyev grabesstimme at gmail.com
Sat Feb 5 22:25:58 CET 2022

```Well, I did some more digging. The parser itself is fine. What matters is
the hashing order of the symbols. There's one particular sequence that
drives the factorization routine off the rails.

#include <ginac/ginac.h>
#include <iostream>
#include <algorithm>

int main()
{
GiNaC::symbol ar[5];
std::sort(std::begin(ar), std::end(ar), [](const auto &a, const auto &b)
{ return a.gethash() < b.gethash(); });
GiNaC::symbol &A = ar[4], &w = ar[0], &K = ar[1], &C = ar[2], &B =
ar[3];
A.set_name("A");
B.set_name("B");
C.set_name("C");
K.set_name("K");
w.set_name("w");
std::cout <<
factor(w*w*w*B*B*A-2*w*w*w*K*K*B*B*A+w*C*C*A+w*w*w*K*K*K*K*B*B*A) << '\n';
}

Interestingly, for the library version before 1.8.0, there were four more
permutations that produced weird output:

// GiNaC::symbol &A = ar[0], &w = ar[1], &K = ar[2], &C = ar[3], &B = ar[4];
// GiNaC::symbol &A = ar[1], &w = ar[0], &K = ar[2], &C = ar[3], &B = ar[4];
// GiNaC::symbol &A = ar[2], &w = ar[0], &K = ar[1], &C = ar[3], &B = ar[4];
// GiNaC::symbol &A = ar[3], &w = ar[0], &K = ar[1], &C = ar[2], &B = ar[4];

As Richard B. Kreckel <kreckel at in.terlu.de> wrote, this is not a bug.
So, I apologize for misreporting.
But still, the result of factorization looks somewhat counterintuitive.

With best regards,
Ivan

On Wed, Feb 2, 2022 at 12:52 AM Vladimir V. Kisil <V.Kisil at leeds.ac.uk>
wrote:

>         Hello,
>
>         It seems that the problem is not with the factor method but with
> the
>   parser. Take the following modification of your example:
>
> #include <ginac/ginac.h>
> #include <iostream>
> #include <sstream>
>
> int main()
> {
>     std::string input("w^3*B^2*A-2*w^3*K^2*B^2*A+w*C^2*A+w^3*K^4*B^2*A");
>     while (true)
>     {
>         std::ostringstream s;
>
>         s << factor(e);
>         if (s.str().length() <= input.length())
>             continue;
>         std::cout << s.str() << '\n';
>         return 0;
>     }
> }
>
>   It behaves as your example and quickly terminates with a strange
>   output. However, if the declaration "GiNaC::parser reader;" will be
>   moved out of the loop then the programme will cycle forever.
>
>   Best wishes,
> --
>   Book:      Geometry of Mobius Maps       https://doi.org/10.1142/p835
>   Soft:      Geometry of cycles         http://moebinv.sourceforge.net/
>   Jupyter notebooks:        https://github.com/vvkisil?tab=repositories
> >>>>> On Mon, 31 Jan 2022 00:01:48 +0300, Ivan Vasilyev <
> grabesstimme at gmail.com> said:
>
>     IV> Hi! I believe there's an intermittent bug in GiNaC::factor()
>     IV> function.  Sometimes it leaves uncancelled terms with seemingly
>     IV> random coefficients like:
>
>     IV>
> w*(w^2*(K^4*B^2+B^2-2*K^2*B^2)+196*w^2*K^4+196*w^2+C^2-196*w^2*(1+K^4-2*K^2)-392*w^2*K^2)*A
>
>     IV> The minimal code to reproduce the bug in GiNaC 1.8.2 is:
>
>     IV> #include <ginac/ginac.h> #include <iostream>
>
>     IV> int main() { std::string
>     IV> input("w^3*B^2*A-2*w^3*K^2*B^2*A+w*C^2*A+w^3*K^4*B^2*A"); while
>     IV> (true) { std::ostringstream s; GiNaC::parser reader; s <<
>     IV> factor(reader(input)); if (s.str().length() <= input.length())
>     IV> continue; std::cout << s.str() << '\n'; return 0; } }
>
>     IV> Also, this problem can be reproduced on
>     IV> https://daninet.github.io/ginac-wasm/ with the same expression
>     IV> being calculated several times in a row:
>
>     IV> factor(w^3*B^2*A-2*w^3*K^2*B^2*A+w*C^2*A+w^3*K^4*B^2*A);
>
>     IV> With best regards, Ivan.
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