#27926 closed enhancement (fixed)
Preserve backend for polytopal constructions
Reported by:  ghkliem  Owned by:  

Priority:  major  Milestone:  sage8.9 
Component:  geometry  Keywords:  polytopes, backend, normaliz 
Cc:  jipilab, vdelecroix  Merged in:  
Authors:  Jonathan Kliem  Reviewers:  Vincent Delecroix 
Report Upstream:  N/A  Work issues:  
Branch:  a3a96bc (Commits, GitHub, GitLab)  Commit:  
Dependencies:  Stopgaps: 
Description (last modified by )
Currently constructions of new polyhedra in Polyhedron_base
do not preserve the backend:
sage: Polyhedron(vertices=[[0]], backend='field').pyramid().backend() 'ppl'
This ticket fixes this.
This respects the users intentions, who might have chosen a specific backend for a good reason. Also, once #25097 is merged, this is needed to have the constructions work for polyhedra over number fields.
More precisely the constructions are:
minkowski_sum
(taken care of by@coerce_binop
and constructing fromself.parent().element_class
)minkowski_difference
(taken care of by@coerce_binop
and constructing fromself.parent().element_class
)translation
,product
,join
,subdirect_sum
,direct_sum
,dilation
,convex_hull
(taken care of by@coerce_binop
and constructing fromself.parent().element_class
),intersection
(taken care of by@coerce_binop
and constructing fromself.parent().element_class
),truncation
,face_truncation
,stack
,lawrence_extension
,lawrence_polytope
,polar
(fixed already, #25081),pyramid
,bipyramid
,prism
,face_split
,affine_hull
.
Change History (24)
comment:1 Changed 3 years ago by
 Description modified (diff)
comment:2 Changed 3 years ago by
 Branch set to public/27926
 Cc jipilab vdelecroix added
 Commit set to 716fb6975472331688cc108b524b80bebaef2360
 Status changed from new to needs_review
comment:3 Changed 3 years ago by
 Commit changed from 716fb6975472331688cc108b524b80bebaef2360 to 1635e50536aac0360a0cf40b7a99e95dbbba42a9
comment:4 Changed 3 years ago by
 Status changed from needs_review to needs_work
This is not likely to be a good idea
+ return Polyhedron(vertices=new_vertices, rays=new_rays, lines=new_lines, base_ring=new_ring, backend=self.backend())
Backends do not support all base rings. For example if you have a polytope with backend PPL and then translate with sqrt(2)
that would not work.
comment:5 followup: ↓ 6 Changed 3 years ago by
You should add backend=self.backend()
only if the base ring is identical.
comment:6 in reply to: ↑ 5 Changed 3 years ago by
I missed this instance. I'll fix it like the other methods.
There is a new function test_backend
that tests if a combination of backend
and base_ring
will work.
Replying to vdelecroix:
You should add
backend=self.backend()
only if the base ring is identical.
comment:7 Changed 3 years ago by
 Commit changed from 1635e50536aac0360a0cf40b7a99e95dbbba42a9 to 937891c8395e5905f2d2c5a074f79ffd0ff3e60e
Branch pushed to git repo; I updated commit sha1. New commits:
937891c  added backend check in translation

comment:8 Changed 3 years ago by
 Status changed from needs_work to needs_review
comment:9 Changed 3 years ago by
 Status changed from needs_review to needs_work
test_backend
is a terrible name. This function does not test the backend. It would better be does_backend_handle_base_ring
.
The result of test_backend
must be cached (with the decorator @cached_function
).
If the new_ring
is actually the same as before, you should avoid going through does_backend_handle_base_ring
.
comment:10 followup: ↓ 11 Changed 3 years ago by
I am not sure this is the right approach. The same code is copied in each single function. I think what is done with matrices is more sensible and the information about the backend is actually stored into the parents
sage: M1 = MatrixSpace(ZZ, 3, implementation='gap') sage: M1 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_gap) sage: M2 = MatrixSpace(ZZ, 3, implementation='generic') sage: M2 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_generic_dense)
The rules about promotions such as
sage: parent(M1.an_element() * M2.an_element()) Full MatrixSpace of 3 by 3 dense matrices over Integer Ring
are handled by the parents and this is what it should be done for polyhedra as well.
comment:11 in reply to: ↑ 10 ; followup: ↓ 12 Changed 3 years ago by
I guess I should work with Polyhedra_base.base_extend
.
Either I introduce a new option for backend
, such as keep
or a change the default behavior to attempt to keep the backend, if possible.
What do you think?
I find the default behavior to be strange at the moment. I have a Polyhedron, change the base ring and all the sudden I use a different backend, even though the old one would have worked.
Replying to vdelecroix:
I am not sure this is the right approach. The same code is copied in each single function. I think what is done with matrices is more sensible and the information about the backend is actually stored into the parents
sage: M1 = MatrixSpace(ZZ, 3, implementation='gap') sage: M1 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_gap) sage: M2 = MatrixSpace(ZZ, 3, implementation='generic') sage: M2 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_generic_dense)The rules about promotions such as
sage: parent(M1.an_element() * M2.an_element()) Full MatrixSpace of 3 by 3 dense matrices over Integer Ringare handled by the parents and this is what it should be done for polyhedra as well.
comment:12 in reply to: ↑ 11 ; followups: ↓ 13 ↓ 18 Changed 3 years ago by
There is no discussion about the fact that the current behavior is wrong. I tried to argue that your initial solution was not optimal. Namely, the parents already carry the backend information (Polyhedra_QQ_ppl
, Polyhedra_ZZ_ppl
, ...). Anytime a new polyhedra is constructed inside a method, the new parent should be chosen via a query to the parent.
Modifying Polyhedra_base.base_extend
is a good idea. If the new base_ring
supports the current backend, it should be kept.
Though at this point, I don't quite understand the difference between base_extend
and change_ring
... that would better be clarified.
Replying to ghkliem:
I guess I should work with
Polyhedra_base.base_extend
.Either I introduce a new option for
backend
, such askeep
or a change the default behavior to attempt to keep the backend, if possible.What do you think?
I find the default behavior to be strange at the moment. I have a Polyhedron, change the base ring and all the sudden I use a different backend, even though the old one would have worked.
Replying to vdelecroix:
I am not sure this is the right approach. The same code is copied in each single function. I think what is done with matrices is more sensible and the information about the backend is actually stored into the parents
sage: M1 = MatrixSpace(ZZ, 3, implementation='gap') sage: M1 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_gap) sage: M2 = MatrixSpace(ZZ, 3, implementation='generic') sage: M2 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_generic_dense)The rules about promotions such as
sage: parent(M1.an_element() * M2.an_element()) Full MatrixSpace of 3 by 3 dense matrices over Integer Ringare handled by the parents and this is what it should be done for polyhedra as well.
comment:13 in reply to: ↑ 12 Changed 3 years ago by
I believe change_ring
was created after base_extend
as base_extend
doesn't allow the ring to become smaller. Either way base_extend
should just call change_ring
with the coerced ring (and change_ring
should return self
if possible).
Ok. I will now proceed as follows:
 Let
change_ring
callbase_extend
. (Btw. the error checking with regard to inexact rings inPolyhedron_base.change_ring
really should be left to parent as well.)
 Create a new parent only if needed.
 Change default behavior of
None
inchange_ring
to leavebackend
if possible.
 Simplify all the methods to obtain a parent with
base_extend
and then initialize from that parent.
Replying to vdelecroix:
There is no discussion about the fact that the current behavior is wrong. I tried to argue that your initial solution was not optimal. Namely, the parents already carry the backend information (
Polyhedra_QQ_ppl
,Polyhedra_ZZ_ppl
, ...). Anytime a new polyhedra is constructed inside a method, the new parent should be chosen via a query to the parent.Modifying
Polyhedra_base.base_extend
is a good idea. If the newbase_ring
supports the current backend, it should be kept.Though at this point, I don't quite understand the difference between
base_extend
andchange_ring
... that would better be clarified.Replying to ghkliem:
I guess I should work with
Polyhedra_base.base_extend
.Either I introduce a new option for
backend
, such askeep
or a change the default behavior to attempt to keep the backend, if possible.What do you think?
I find the default behavior to be strange at the moment. I have a Polyhedron, change the base ring and all the sudden I use a different backend, even though the old one would have worked.
Replying to vdelecroix:
I am not sure this is the right approach. The same code is copied in each single function. I think what is done with matrices is more sensible and the information about the backend is actually stored into the parents
sage: M1 = MatrixSpace(ZZ, 3, implementation='gap') sage: M1 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_gap) sage: M2 = MatrixSpace(ZZ, 3, implementation='generic') sage: M2 Full MatrixSpace of 3 by 3 dense matrices over Integer Ring (using Matrix_generic_dense)The rules about promotions such as
sage: parent(M1.an_element() * M2.an_element()) Full MatrixSpace of 3 by 3 dense matrices over Integer Ringare handled by the parents and this is what it should be done for polyhedra as well.
comment:14 Changed 3 years ago by
 Commit changed from 937891c8395e5905f2d2c5a074f79ffd0ff3e60e to cbffe2867da538199933d49d4549cf03e3931ce4
Branch pushed to git repo; I updated commit sha1. New commits:
cbffe28  moved changes of ring and backend to parent

comment:15 Changed 3 years ago by
 Commit changed from cbffe2867da538199933d49d4549cf03e3931ce4 to 970c142283df9b60fc57309b3b142bb782c4bcc6
Branch pushed to git repo; I updated commit sha1. New commits:
970c142  fixed inconsistent parenthesis

comment:16 Changed 3 years ago by
 Status changed from needs_work to needs_review
comment:17 Changed 3 years ago by
 Reviewers set to Vincent Delecroix
 Status changed from needs_review to needs_work
I think the logic in change_ring
would better be simplified to
if ambient_dim is None: ambient_dim = self.ambient_dim() if base_ring == self.base_ring() and \ ambient_dim == self.ambient_dim() and \ backend is None or backend == self.backend(): return self # if not specified, try the same backend if backend is None and does_backend_handle_base_ring(base_ring, self.backend()): return Polyhedra(base_ring, ambient_dim, backend=self.backend()) return Polyhedra(base_ring, ambient_dim, backend=backend)
Also, you should add more examples to change_ring
. In particular when something has changed with respect to older versions such as
sage: Polyhedra(QQ, 3, backend='cdd').change_ring(RDF).backend() 'cdd'
comment:18 in reply to: ↑ 12 Changed 3 years ago by
Replying to vdelecroix:
Though at this point, I don't quite understand the difference between
base_extend
andchange_ring
... that would better be clarified.
base_extend
is a calque from the same function for example for matrices, that creates a new parent with an extended ring and coerces self
into this new parent and returns it.
As Jonathan said, this can only go in one direction and was not really adapted to polyhedra objects. This led to introduce change_ring
to have all the flexibility possible (and perhaps a more intuitive name than base_extend
) and because you can't really change ring without caring about the backend, change_backend
and change_ring
are bound to have fuzzy differences and may even call eachother (hopefully in only 1 direction!).
So yes, it is subtle and I am all in favor of rationalizing base_extend
, change_ring
and change_backend
(which is not all there yet).
comment:19 Changed 3 years ago by
 Commit changed from 970c142283df9b60fc57309b3b142bb782c4bcc6 to a3a96bc413727816c16705c4f03400429ee36445
Branch pushed to git repo; I updated commit sha1. New commits:
a3a96bc  small changes

comment:20 Changed 3 years ago by
 Status changed from needs_work to needs_review
comment:21 Changed 3 years ago by
 Status changed from needs_review to positive_review
comment:22 Changed 3 years ago by
 Branch changed from public/27926 to a3a96bc413727816c16705c4f03400429ee36445
 Resolution set to fixed
 Status changed from positive_review to closed
comment:23 Changed 3 years ago by
 Milestone changed from sage8.8 to sage8.9
Not in Sage 8.8. Let's please to try keep tickets' milestones related to the release in which we actually intend to include them, and in particular the release in which they were actually included, especially when closing tickets.
comment:24 Changed 3 years ago by
 Commit a3a96bc413727816c16705c4f03400429ee36445 deleted
Barycentric subdivision was forgotten... :(
Branch pushed to git repo; I updated commit sha1. New commits:
preserve backend for polytopal constructions