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 improper interactions are generated by moltemplate.sh
# by default.  It can be overridden by supplying your own custom file
# (for example, see "opls_imp.py")

#    To find 4-body "improper" interactions,
#    (by default, most of the time), we would use this subgraph:
#           3
#           *                  1st bond connects atoms 0 and 1
#           |              =>  2nd bond connects atoms 0 and 2
#         _.*._                3rd bond connects atoms 0 and 3
#       *'  0  `*
#      1         2
#

bond_pattern = Ugraph([(0, 1), (0, 2), (0, 3)])
# (Ugraph atom indices begin at 0, not 1)


def canonical_order(match):
    """
    Before defining a new interaction, we must check to see if an
    interaction between these same 4 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 improper interactions is a function of the improper angle.
    The improper-angle is usually defined as the angle between planes formed
    by atoms 0,1,2 & 1,2,3.  (Alternately, it is sometimes defined as the
    angle between the 0,1,2 plane and atom 3.)
    This angle does not change when swapping the MIDDLE pair of atoms (1 and 2)
    (except for a change of sign, which does not matter since the energy functions
     used are typically sign invariant.  Furthermore, neither of MIDDLE pair of atoms
     are the central atom, and there are 3!=6 ways of ordering the remaining 3 atoms.)
    Consequently it does not make sense to define a separate 4-body improper
    interaction between atoms 0,1,2,3   AS WELL AS between  0,2,1,3.
    So we sort the atoms and bonds so that the second 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 4 atoms.  If not then
    we create a new one.)

    """
    atom0 = match[0][0]
    atom1 = match[0][1]
    atom2 = match[0][2]
    atom3 = match[0][3]
    # match[1][0:2] contains the ID numbers for the 3 bonds
    bond0 = match[1][0]
    bond1 = match[1][1]
    bond2 = match[1][2]
    if atom1 <= atom2:
        # return ((atom0,atom1,atom2,atom3), (bond0,bond1,bond2))
        # But this is the same thing as:
        return match
    else:
        return ((atom0, atom2, atom1, atom3), (bond1, bond0, bond2))
