Added ordered set implementation

""" OrderedSet Python implementation.

This snippet of code is taken from http://code.activestate.com/recipes/576694/

Written by Raymond Hettinger's and licenced under the MIT Licence.

Comments::

   Runs on Py2.6 or later (and runs on 3.0 or later without any modifications).

   Implementation based on a doubly linked link and an internal dictionary.

   This design gives OrderedSet the same big-Oh running times as regular sets

   including O(1) adds, removes, and lookups as well as O(n) iteration.

"""

import collections

KEY, PREV, NEXT = range(3)

class OrderedSet(collections.MutableSet):

    def __init__(self, iterable=None):

        self.end = end = []

        end += [None, end, end]         # sentinel node for doubly linked list

        self.map = {}                   # key --> [key, prev, next]

        if iterable is not None:

            self |= iterable

    def __len__(self):

        return len(self.map)

    def __contains__(self, key):

        return key in self.map

    def add(self, key):

        if key not in self.map:

            end = self.end

            curr = end[PREV]

            curr[NEXT] = end[PREV] = self.map[key] = [key, curr, end]

    def discard(self, key):

        if key in self.map:

            key, prev, next = self.map.pop(key)

            prev[NEXT] = next

            next[PREV] = prev

    def __iter__(self):

        end = self.end

        curr = end[NEXT]

        while curr is not end:

            yield curr[KEY]

            curr = curr[NEXT]

    def __reversed__(self):

        end = self.end

        curr = end[PREV]

        while curr is not end:

            yield curr[KEY]

            curr = curr[PREV]

    def pop(self, last=True):

        if not self:

            raise KeyError('set is empty')

        key = next(reversed(self)) if last else next(iter(self))

        self.discard(key)

        return key

    def __repr__(self):

        if not self:

            return '%s()' % (self.__class__.__name__,)

        return '%s(%r)' % (self.__class__.__name__, list(self))

    def __eq__(self, other):

        if isinstance(other, OrderedSet):

            return len(self) == len(other) and list(self) == list(other)

        return set(self) == set(other)

    def __del__(self):

        self.clear()                    # remove circular references