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# Related codes
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The general code structure of AREPO is similar to the GADGET code, but with
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significant further developments and improvements. However, keeping in mind
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that the code architecture originates from an Tree-PM-SPH code helps to
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understand why certain things are done in certain ways.
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One of the obvious signs of this are the name of some global structures and
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functions. For example the array holding the information for gas cells only is
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called SphP. Another example would be the routines in ``src/init/density.c``,
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which used to be an SPH density calculation, but now only used for an estimate
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for the typical distance to neighbouring cells. Another aspect is the
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presence of the neighbour tree, a search-tree structure on gas cells. In
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practice this means that many of the routines that depend on the state of
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neighbouring cells, including sub-grid feedback models etc., can be based on
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this neighbour search. This makes it possible to port these models from earlier
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versions of GADGET to AREPO.
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However, it is important to realize that routines written for the GADGET code
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will in general not work directly when ported to Arepo. We encourage developers
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to carefully test that the implementation is still having the same effect.
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Apart from the differences in implementation, it is also important to realize
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that a grid-based finite volume scheme might react different to a particular to
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a sub-grid implementation.
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Differences to the development Version
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======================================
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This public version of AREPO was branched off from the development version in
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November 2017, substantially cut down, cleand up and the documentation completed
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by Rainer Weinberger. The main reason for doing this is to provide a code to the
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community that researchers are able to understand and use without the need of
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an extensive introduction by one of the authors. We belive that the public
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version, being better documented on a developer level, also provides a better
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starting point for new developments.
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The general idea for this public verson was to preserve the well-tested code
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with a limited number of changes. However, some compile-time options have been
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eliminated and to recover the same running mode **in the development version
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of AREPO**, the following compile-time flags need to be set in `Config.sh`
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* ``PERIODIC`` (has become obsolete over the years)
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* ``VORONOI`` (the development version also has an AMR mode)
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* ``VORONOI_DYNAMIC_UPDATE`` (in practice now a default)
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* ``AUTO_SWAP_ENDIAN_READIC`` (only relevant for file formats 1 and 2)
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* ``CHUNKING``
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* the switch ``EXACT_GRAVITY_FOR_PARTICLE_TYPE`` automatically activates ``NO_SELFGRAVITY_TYPE``, ``NO_GRAVITY_TYPE`` and ``EXACT_GRAVITY_REACTION`` (see src/main/allvars.h) in the public version. This is not the case in the development version where all these flags need to be set individually in Config.sh. |
