def get_dir_from_zipfile(zip_file, fallback=None):
    """Return the name of the root folder in a zip file.

    :param zip_file: a :class:`zipfile.ZipFile` instance.
    :param fallback: if `None`, the name of the zip file is used. This is
        returned if the zip file contains more than one top-level directory,
        or none at all.
    """
    fallback = fallback or zip_file.filename
    directories = [name for name in zip_file.namelist() if name.endswith("/")
                   and len(PurePath(name).parts) == 1]
    return fallback if len(directories) > 1 else directories[0]
def test_ccode_loops_multiple_contractions():
    from sympy.tensor import IndexedBase, Idx
    from sympy import symbols
    n, m, o, p = symbols('n m o p', integer=True)
    a = IndexedBase('a')
    b = IndexedBase('b')
    y = IndexedBase('y')
    i = Idx('i', m)
    j = Idx('j', n)
    k = Idx('k', o)
    l = Idx('l', p)

    s = (
        'for (int i=0; i<m; i++){\n'
        '   y[i] = 0;\n'
        '}\n'
        'for (int i=0; i<m; i++){\n'
        '   for (int j=0; j<n; j++){\n'
        '      for (int k=0; k<o; k++){\n'
        '         for (int l=0; l<p; l++){\n'
        '            y[i] = y[i] + b[%s]*a[%s];\n' % (j*o*p + k*p + l, i*n*o*p + j*o*p + k*p + l) +\
        '         }\n'
        '      }\n'
        '   }\n'
        '}'
    )
    c = ccode(b[j, k, l]*a[i, j, k, l], assign_to=y[i])
    assert c == s