use libGAP in MatrixGroup.as_permutation_group()

[dimpase]

Currently, one uses pexpect interface there to convert a libGAP matrix group to strings, feed it to GAP's pexpect interface, etc.

This ticket will streamline this. One still will need one conversion to pexpect GAP, at the end, to feed it to PermutationGroup().

Component: group theory

Author: Dima Pasechnik

Branch: 7dd5fa6

Reviewer: Sebastian Oehms

Issue created by migration from https://trac.sagemath.org/ticket/26889

[dimpase]

comment:1

I am marking this as a defect, as without this change some tests mysteriously fail on a new libgap interface, see #22626 and #26856.

---

New commits:

db3f63e

more libGAP in as_permutation(), less pexpect mess

[dimpase]

Commit: db3f63e

[dimpase]

Branch: u/dimpase/groups/better_as_permutation

[soehms]

comment:2

The following example which worked with the former version, is causing a string buffer overflow, right now:

sage: S = Sp(6,3)
sage: S.as_permutation_group()
Traceback (most recent call last):
...
TypeError: Gap terminated unexpectedly while reading in a large line:

My suggestion to avoid this can be seen from the following diff:

diff --git a/src/sage/groups/matrix_gps/finitely_generated.py b/src/sage/groups/matrix_gps/finitely_generated.py
index 2f5241f..4052740 100644
--- a/src/sage/groups/matrix_gps/finitely_generated.py
+++ b/src/sage/groups/matrix_gps/finitely_generated.py
@@ -613,11 +613,13 @@ class FinitelyGeneratedMatrixGroup_gap(MatrixGroup_gap):
         iso=self._libgap_().IsomorphismPermGroup()
         if algorithm == "smaller":
             iso=iso.Image().SmallerDegreePermutationRepresentation()
-        PG = PermutationGroup(map(gap, iso.Image().GeneratorsOfGroup()), \
+        PG = PermutationGroup(iso.Image().GeneratorsOfGroup().sage(), \
                        canonicalize=False) # applying gap() - as PermutationGroup is not libGAP

         def permutation_group_map(element):
-            return iso.ImageElm(element.gap())._gap_()
+            return PG(iso.ImageElm(element.gap()).sage())

         from sage.categories.homset import Hom
         self._permutation_group_morphism = Hom(self, PG)(permutation_group_map)

[soehms]

Reviewer: Sebastian Oehms

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

2d77eee	follow up pyflake hints (all but one)
7dd5fa6	reviewer's example and fix

[sagetrac-git]

Changed commit from db3f63e to 7dd5fa6

[dimpase]

comment:4

the last 2 commits add this fix, the corresponding Sp(6,3) example, and also trims the code following pyflake plugin bot hints.