# Introduction to Nemo development

## Relationship to AbstractAlgebra.jl

Some time in the past, Nemo was split into two packages called Nemo.jl and AbstractAlgebra.jl. The purpose was to provide a Julia only package which did some subset of what Nemo could do, albeit slower. This was requested by people in the Julia community.

Unfortunately this hasn't been terribly successful. Most Julia developers expect that AbstractAlgebra and Nemo functionality will work for Julia matrices over AbstractAlgebra/Nemo rings. This would be possible for functions that do not conflict with Base or LinearAlgebra at least when working with non-empty matrices. However, for reasons that we explain in both the Appendix to the AbstractAlgebra package and in the parent object section of the developer documentation, this is not possible even in theory for functions that would conflict with Julia's standard library or for empty matrices (except in a limited number of special cases).

Unfortunately the Julia standard library functions do not work with matrices of Nemo objects and there is little we can do about this. Moreover, some Julia functionality isn't supported by the underlying C libraries in Nemo and would be difficult or impossible to provide on the C side.

Nowadays we see AbstractAlgebra to provide three things to Nemo:

• An abstract type hierarchy
• Generic ring constructions, e.g. generic polynomials and matrices
• Generic implementations that should work for any ring implementing the required interfaces. These interfaces are documented in the AbstractAlgebra documentation.

Nemo itself is now more or less just a wrapper of four C libraries:

• Flint : polynomials and matrices over Z, Q, Z/nZ, Qp, Fq
• Arb : polynomials, matrices and special functions over balls over R and C
• Antic : algebraic number field element arithmetic
• Calcium : exact real and complex numbers, including algebraic numbers

Each ring implemented in those C libraries is wrapped in such a way as to implement the interfaces described by AbstractAlgebra.

Most of the time an AbstractAlgebra implementation will work just as well using Nemo, but the latter will usually be faster, due to the extremely performant C code (around half a million lines of it).

## Layout of files

In the src directory of Nemo are four directories flint, arb, antic and calcium, each containing the wrappers for the relevant C libraries. The test directory is similarly organised.

Within each of these directories is a set of files, one per module within the C libraries, e.g. the fmpz.jl file wraps the Flint fmpz module for multiple precision integers. The fmpz_poly.jl file wraps the Flint univariate polynomials over fmpz integers, and so on.

The fmpq prefix is for Flint rationals, fq for Flint finite fields with multiprecision characteristic, fq_nmod is the same but for single word characteristic. The padic prefix is for the field of p-adic numbers for a given p. The nmod prefix is for Z/nZ for a given n. The gfp prefix is the same as Z/nZ but where n is prime, so that we are dealing with a field.

The FlintTypes.jl file contains the implementation of all the Flint types.

In the antic directory, nf_elem is for elements of a number field.

The AnticTypes.jl file contains the Antic types.

In the arb directory the arb prefix is for arbitrary precision ball arithmetic over the reals. The acb prefix is similar but for complex numbers.

The ArbTypes.jl file contains the Arb types.

In the calcium directory the ca prefix is for Calcium's type. There is also a qqbar file for the field of algebraic numbers.

In the AbstractAlgebra.jl package the src directory contains a directory called generic. This is where the implementations of generic types, such as matrices, polynomials, series, etc. reside. Each file such as Matrix.jl corresponds to a generic group/ring/field or other algebraic construction (typically over a base ring). The files in this directory exist inside a submodule of AbstractAlgebra called Generic.

The file GenericTypes.jl is where all the generic types are implemented.

At the top level of the src directory is a file Generic.jl which is where the Generic submodule of AbstractAlgebra begins and where imports are made from AbstractAlgebra into Generic.

In the src directory we have implementations that work for every type belonging to a given abstract type, e.g. Matrix.jl has implementations that will work for any matrix type, whether from AbstractAlgebra's Generic module or even matrix types from Nemo, and so on. So long as they are implemented to provide the Matrix interface all the functions there will work for them. The same applies for Poly.jl for polynomial types, AbsSeries.jl for absolute series types, RelSeries.jl for relative series types, etc.

In the src directory is AbstractTypes.jl where all the AbstractAlgebra abstract types are defined.

Also in the src directory is a subdirectory called Julia. This is where we give our own implementations of functionality for Julia Integers and Rationals and various other basic rings implemented in terms of Julia types. These are provided so that the package will work as a pure Julia package, replacing many of the rings and fields that would be available in Flint and the other C libraries with Julia equivalents.

Note that some of the implementations we give there would conflict with Base and so are only available inside AbstractAlgebra and are not exported!

We try to keep the test directory at the top level of the source tree organised in the same manner as the other directories just discussed, though there is currently no split between tests for Generic and for the implementations in src. All tests are currently combined in test/generic..

## Git, GitHub and project workflows

The official repositories for AbstractAlgebra and Nemo are:

https://github.com/Nemocas/AbstractAlgebra.jl

https://github.com/Nemocas/Nemo.jl

If you wish to contribute to these projects, the first step is to fork them on GitHub. The button for this is in the upper right of the main project page. You will need to sign up for a free GitHub account to do this.

Once you have your own GitHub copy of our repository you can push changes to it from your local machine and this will make them visible to the world.

Before sinking a huge amount of time into a contribution, please open a ticket on the official project page on GitHub explaining what you intend to do and discussing it with the other developers.

The easiest way to get going with development on your local machine is to dev AbstractAlgebra and/or Nemo. To do this, press the ] key in Julia to enter the special package mode and type:

dev Nemo

Now you will find a local copy on your machine of the Nemo repository in

.julia/dev/Nemo

However, this will be set up to push to the official repository instead of your own, so you will need to change this. For example, if your GitHub account name is myname, edit the .git/config file in your local Nemo directory to say:

        url = https://github.com/Nemocas/Nemo.jl.git
pushurl = https://github.com/myname/Nemo.jl

It is highly recommended that you do not work in the master branch, but create a new branch for each thing you want to contribute to Nemo.

git checkout -b mynewbranch

If your contribution is small and does not take a long time to implement, everything will likely be fine if you simply commit the changes locally, then push them to your GitHub account online:

git commit -a
git push --all

However, if you are working on a much larger project it is highly recommended that you frequently pull from the official master branch and rebase your new branch on top of any changes that have been made there:

git checkout master
git pull
git checkout mynewbranch
git rebase master

Note that rebasing will try to rewrite each of your commits over the top of the branch you are rebasing on (master in this case). This process will have many steps if there are many commits and lots of conflicts. Simply follow the instructions until the process is finished.

The longer you leave it before rebasing on master the longer the rebase process will take. It can eventually become overwhelming as it is not replaying the latest state of your repository over master, but each commit that you made in order. You may have completely forgotten what those older commits were about, so this can become very difficult if not done regularly.

Once you have pushed your changes to your GitHub account, go to the official project GitHub page and you should see your branch mentioned near the top of the page. Open a pull request.

Note that we don't accept code without tests and documentation. We use Documenter.jl for our documentation, in Markdown format. See our existing code for examples of docstrings above functions in the source code and look in the docs/src directory to see how these docstrings are merged into our online documentation.

## Development list

All developers of AbstractAlgebra and Nemo are welcome to write to our development list to ask questions and discuss development:

## Reporting bugs

Bugs should be reported by opening an issue (ticket) on the official GitHub page for the relevant project. Please state the Julia version being used, the machine you are using and the version of AbstractAlgebra/Nemo you are using. The version can be found in the Project.toml file at the top level of the source tree.

AbstractAlgebra has a special roadmap ticket which lists the most important tickets that have been opened. If you want to contribute something high value this is the place to start:

https://github.com/Nemocas/AbstractAlgebra.jl/issues/492

This ticket is updated every so often.

## Binaries

Binaries of C libraries for Nemo are currently made in a separate repository:

https://github.com/JuliaPackaging/Yggdrasil

If code is added to any of the C libraries used by Nemo, this jll package must be updated first and the version updated in Nemo.jl before the new functionality can be used. Ask the core developers for help with this as various other tasks must be completed at the same time.

## Relationship to Oscar

Nemo and AbstractAlgebra are heavily used by the Oscar computer algebra system being developed in Germany by a number of universities involved in a large project known as TRR 195, funded by the DFG.

Oscar is the number one customer for Nemo. Many bugs in Nemo are found and fixed by Oscar developers and most of the key Nemo developers are part of the Oscar project.

See the Oscar website for further details:

https://oscar.computeralgebra.de/