`AbstractLinearCode.minimum_distance` fails with GAP message for large fields

[jsrn]

The following works:

   C = LinearCode(random_matrix(GF(5^2,'a'), 2, 5)); C.minimum_distance()

while the following explodes with a GAP error

   C = LinearCode(random_matrix(GF(17^2,'a'), 2, 5)); C.minimum_distance()

It seems to be conditioned only on the size of the field: All fields with cardinality greater than 256 seem to fail.

[jfields]

The GAP algorithms which sage is calling for minimum_distance() have a (poorly) documented limitation -- field size of at most 256. I added a check on the field size and raise a "not implemented" exception in that case. So at least the user will get some explanation of the problem rather than an incomprehensible stack dump.

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New commits:

​2d47682

Added a check on the size of the finite field in minimum_distance() computation.

[tscrim]

Some minor things:

• I would put the fact that this only works for fields up to 256 in either a .. NOTE:: or .. WARNING:: block so it's more prominent in the documentation.
• You should add something just before the added doctest, something like The field must be size at most 256::. I would also split the line like this:

            NotImplementedError: The GAP algorithms that sage is using
             are limited to computing with fields of size at most 256.
  

• To be PEP8 compliant, you should align the string start points:

            raise NotImplementedError("the GAP algorithm that Sage is using "
                                      "is limited to computing with fields "
                                      "of size at most 256")
  

  I've also suggested a few minor changes to the error message. Although I might consider limiting the actual message to something like the field must have size at most 256 with a more detailed comment just before it saying the issue is in GAP for those looking at the code itself.

[git]

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

​696edb0	small fixes per tscrim's suggestions.
​4949d06	Fixed formatting of the ..warning:: block.

[git]

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

​dcaafe2

Added the sentence

[jfields]

Not sure if I did what you intended regarding adding the sentence before the doctest.

[tscrim]

I made some small tweaks. If you're good with my changes, then you can set a positive review (and don't forget to add your real name to the authors field).

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New commits:

​72f01a7	Merge branch 'u/jfields/gap_min_dist_field_size' of git://trac.sagemath.org/sage into u/jfields/gap_min_dist_field_size
​2809840	Some small reviewer changes.

[jfields]

Thanks Travis, your changes are definitely fine. I've switched the status to "positive review." Please forgive a noob question: do I need to do anything git-wise about merging your changes? It looks like I don't but I'd rather be safe than sorry...

[jfields]

Added the keyword sd75 to the ticket as this is work that began during the Sage Days 75 at INRIA Saclay.

[tscrim]

Replying to jfields:

Thanks Travis, your changes are definitely fine. I've switched the status to "positive review." Please forgive a noob question: do I need to do anything git-wise about merging your changes? It looks like I don't but I'd rather be safe than sorry...

No, there's nothing you need to do (in fact, mine are based on a later version of Sage than your previous push, so you might not want to pull them at this point).