try:
    from .nbody_graph_search import Ugraph
except (SystemError, ValueError):
    # not installed as a package
    from nbody_graph_search import Ugraph


# This file defines how 3-body angle interactions are generated by moltemplate
# by default.  It can be overridden by supplying your own custom file.

#    To find 3-body "angle" interactions, we would use this subgraph:
#
#
#       *---*---*           =>  1st bond connects atoms 0 and 1
#       0   1   2               2nd bond connects atoms 1 and 2
#

bond_pattern = Ugraph([(0, 1), (1, 2)])
# (Ugraph atom indices begin at 0, not 1)


#    The next function eliminates the redundancy between 0-1-2 and 2-1-0:
def canonical_order(match):
    """
    Before defining a new interaction, we must check to see if an
    interaction between these same 3 atoms has already been created
     (perhaps listed in a different, but equivalent order).
    If we don't check for this this, we will create many unnecessary redundant
    interactions (which can slow down he simulation).
    To avoid this, I define a "canonical_order" function which sorts the atoms
    and bonds in a way which is consistent with the symmetry of the interaction
    being generated...  Later the re-ordered list of atom and bond ids will be
    tested against the list of atom/bond ids in the matches-found-so-far,
    before it is added to the list of interactions found so far.  Note that
    the energy of an angle interaction is a function of the angle between.
    three consecutively bonded atoms (referred to here as: 0,1,2).
    This angle does not change when swapping the atoms at either end (0 and 2).
    So it does not make sense to define a separate 3-body angle
    interaction between atoms 0,1,2  AS WELL AS an interaction between  2,1,0.
    So we sort the atoms and bonds so that the first atom has a always has
    a lower atomID than the third atom.  (Later we will check to see if we
    have already defined an interaction between these 3 atoms.  If not then
    we create a new one.)

    """
    # match[0][0:2] contains the ID numbers for the 3 atoms in the match
    atom0 = match[0][0]
    atom1 = match[0][1]
    atom2 = match[0][2]
    # match[1][0:1] contains the ID numbers for the 2 bonds
    bond0 = match[1][0]
    bond1 = match[1][1]
    if atom0 < atom2:
        # return ((atom0, atom1, atom2), (bond0, bond1))  same thing as:
        return match
    else:
        return ((atom2, atom1, atom0), (bond1, bond0))

[(18, 'nbody_graph_search.Ugraph', 'Ugraph', False, 'from nbody_graph_search import Ugraph\n')]