Support for Real and Complex Numbers
------------------------------------

DOLFINX provides builds with both real and complex number support
making use of PETSc with real or complex number support.

python3-dolfinx will bring in python3-dolfinx-real (real number
support) by default. Complex number support is provided by
python3-dolfinx-complex.

In the C++ components, libdolfinx-dev brings in libdolfinx-real-dev
(real number support) by default. Complex number support is provided
by libdolfinx-complex-dev.

libdolfinx-real-dev provides dolfinx_real.pc for pkg-config.
libdolfinx-complex-dev provides dolfinx_complex.pc.

Both real and complex packages can be installed at the same time.
If not otherwise specified, the selected version of dolfinx is the one
built against the preferred version of PETSc in /usr/lib/petsc. By
default this is petsc-real, but can be controlled by
  update-alternatives --config petsc

The alternative build of DOLFINX can be accessed by setting PETSC_DIR
as an environment variable where needed,
e.g.

  PETSC_DIR=/usr/lib/petscdir/petsc-complex python3 -c "import dolfinx; print(dolfinx.__path__); print('supports complex numbers: ', dolfinx.has_petsc_complex)"

Depending on your application, you might also need to set the
SLEPC_DIR environment variable, e.g.
  SLEPC_DIR=/usr/lib/slepcdir/slepc-complex
or
  SLEPC_DIR=$( echo $PETSC_DIR | sed "s/petsc/slepc/g" )


Conditional Support for SuperLU-Dist
------------------------------------

In principle dolfin supports SuperLU-Dist. However the superlu-dist
package distributed by Debian is mostly useless since mc64, one of its
core functions, cannot legally be redistributed and so has been
removed from the Debian package. For this reason, we have given
preference to mumps over superlu_dist.

You may specify superlu_dist if desired (or if you have a local build
of superlu_dist that includes mc64), for instance with 
  solve(a == L, u, bc, solver_parameters={'linear_solver': 'superlu_dist'})

You may also successfully use superlu_dist without mc64 by specifying
C++:
  PETScOptions::set("-mat_superlu_dist_rowperm", "NATURAL");
Python:
  PETScOptions.set('-mat_superlu_dist_rowperm', 'NATURAL')
Performance will likely be reduced.

The same problem applies to superlu, from which mc64 has also been
removed. superlu still functions successfully however (mc64 is not
core for the serial version of SuperLU).

Internet availability for pusimp integration tests
--------------------------------------------------

The goal of some pusimp integration tests is to create a dirty virtualenv
in which some of dolfinx dependecies are replaced with a manually installed
package from PyPI, and verify that pusimp actually triggers an error while
trying to import dolfinx.
However, to create such virtualenv environment the build needs to be allowed
to download packages from pypi.org.

To build locally with full internet access:
- if building with dpkg-buildpackage
    export http_proxy="" https_proxy=""
  before running dpkg-buildpackage
- if building with pdebuild, in addition to exporting the two environment variables
  as above the maintainer may want to add
    USENETWORK=yes
  to their $HOME/.pbuilderrc. The pdebuild log with default USENETWORK=no contains
    I: pbuilder: network access will be disabled during build
  while the pdebuild log with customized USENETWORK=yes contains
    W: pbuilder: network will not be disabled during build!

The file test/debian-only/test_pusimp.py gracefully handles a missing network
by checking for internet availability trying to fetch www.debian.org via
python3-requests. If networks is unreachable, all tests in the file will be
skipped, and pusimp integration will only be partially tested.