[Cython] Permutation implementation

cpdef permutation_atindex(int _int, object _set, length):
    """
    Return the permutation at index '_int' for itemgetter '_set'
    with length 'length'.
    """

    # c-type declaration
    cdef int strLength, index
    cdef list items     = []

    # c-assignment
    strLength   = len(_set)
    index       = _int % strLength
    items.append(_set[index])

    for n from 1 <= n < length:
        _int  //= strLength
        index   = _int % strLength
        items.append(_set[index])

    return items

cdef class PermutationIterator:
    """
    A class that can iterate over possible permuations
    of the given 'iterable' and 'length' argument.
    """

    cpdef int current, max, length
    cpdef object iterable

    def __init__(self, object iterable, int length):
        self.length     = length
        self.current    = 0
        self.max        = len(iterable) ** length
        self.iterable   = iterable

    def __iter__(self):
        return self

    def __next__(self):
        if self.current >= self.max:
            raise StopIteration

        try:
            return permutation_atindex(self.current, self.iterable, self.length)
        finally:
            self.current   += 1