from collections import defaultdict, OrderedDict
from itertools import (
    combinations, combinations_with_replacement, permutations,
    product, product as cartes
)
import random
from operator import gt

from sympy.core import Basic

# this is the logical location of these functions
from sympy.core.compatibility import (as_int, is_sequence, iterable, ordered)
from sympy.core.compatibility import default_sort_key  # noqa: F401

import sympy

from sympy.utilities.enumerative import (
    multiset_partitions_taocp, list_visitor, MultisetPartitionTraverser)


def is_palindromic(s, i=0, j=None):
    """return True if the sequence is the same from left to right as it
    is from right to left in the whole sequence (default) or in the
    Python slice ``s[i: j]``; else False.

    Examples
    ========

    >>> from sympy.utilities.iterables import is_palindromic
    >>> is_palindromic([1, 0, 1])
    True
    >>> is_palindromic('abcbb')
    False
    >>> is_palindromic('abcbb', 1)
    False

    Normal Python slicing is performed in place so there is no need to
    create a slice of the sequence for testing:

    >>> is_palindromic('abcbb', 1, -1)
    True
    >>> is_palindromic('abcbb', -4, -1)
    True

    See Also
    ========

    sympy.ntheory.digits.is_palindromic: tests integers

    """
    i, j, _ = slice(i, j).indices(len(s))
    m = (j - i)//2
    # if length is odd, middle element will be ignored
    return all(s[i + k] == s[j - 1 - k] for k in range(m))


def flatten(iterable, levels=None, cls=None):
    """
    Recursively denest iterable containers.

    >>> from sympy.utilities.iterables import flatten

    >>> flatten([1, 2, 3])
    [1, 2, 3]
    >>> flatten([1, 2, [3]])
    [1, 2, 3]
    >>> flatten([1, [2, 3], [4, 5]])
    [1, 2, 3, 4, 5]
    >>> flatten([1.0, 2, (1, None)])
    [1.0, 2, 1, None]

    If you want to denest only a specified number of levels of
    nested containers, then set ``levels`` flag to the desired
    number of levels::

    >>> ls = [[(-2, -1), (1, 2)], [(0, 0)]]

    >>> flatten(ls, levels=1)
    [(-2, -1), (1, 2), (0, 0)]

    If cls argument is specified, it will only flatten instances of that
    class, for example:

    >>> from sympy.core import Basic
    >>> class MyOp(Basic):
    ...     pass
    ...
    >>> flatten([MyOp(1, MyOp(2, 3))], cls=MyOp)
    [1, 2, 3]

    adapted from https://kogs-www.informatik.uni-hamburg.de/~meine/python_tricks
    """
    from sympy.tensor.array import NDimArray
    if levels is not None:
        if not levels:
            return iterable
        elif levels > 0:
            levels -= 1
        else:
            raise ValueError(
                "expected non-negative number of levels, got %s" % levels)

    if cls is None:
        reducible = lambda x: is_sequence(x, set)
    else:
        reducible = lambda x: isinstance(x, cls)

    result = []

    for el in iterable:
        if reducible(el):
            if hasattr(el, 'args') and not isinstance(el, NDimArray):
                el = el.args
            result.extend(flatten(el, levels=levels, cls=cls))
        else:
            result.append(el)

    return result


def unflatten(iter, n=2):
    """Group ``iter`` into tuples of length ``n``. Raise an error if
    the length of ``iter`` is not a multiple of ``n``.
    """
    if n < 1 or len(iter) % n:
        raise ValueError('iter length is not a multiple of %i' % n)
    return list(zip(*(iter[i::n] for i in range(n))))


def reshape(seq, how):
    """Reshape the sequence according to the template in ``how``.

    Examples
    ========

    >>> from sympy.utilities import reshape
    >>> seq = list(range(1, 9))

    >>> reshape(seq, [4]) # lists of 4
    [[1, 2, 3, 4], [5, 6, 7, 8]]

    >>> reshape(seq, (4,)) # tuples of 4
    [(1, 2, 3, 4), (5, 6, 7, 8)]

    >>> reshape(seq, (2, 2)) # tuples of 4
    [(1, 2, 3, 4), (5, 6, 7, 8)]

    >>> reshape(seq, (2, [2])) # (i, i, [i, i])
    [(1, 2, [3, 4]), (5, 6, [7, 8])]

    >>> reshape(seq, ((2,), [2])) # etc....
    [((1, 2), [3, 4]), ((5, 6), [7, 8])]

    >>> reshape(seq, (1, [2], 1))
    [(1, [2, 3], 4), (5, [6, 7], 8)]

    >>> reshape(tuple(seq), ([[1], 1, (2,)],))
    (([[1], 2, (3, 4)],), ([[5], 6, (7, 8)],))

    >>> reshape(tuple(seq), ([1], 1, (2,)))
    (([1], 2, (3, 4)), ([5], 6, (7, 8)))

    >>> reshape(list(range(12)), [2, [3], {2}, (1, (3,), 1)])
    [[0, 1, [2, 3, 4], {5, 6}, (7, (8, 9, 10), 11)]]

    """
    m = sum(flatten(how))
    n, rem = divmod(len(seq), m)
    if m < 0 or rem:
        raise ValueError('template must sum to positive number '
        'that divides the length of the sequence')
    i = 0
    container = type(how)
    rv = [None]*n
    for k in range(len(rv)):
        rv[k] = []
        for hi in how:
            if type(hi) is int:
                rv[k].extend(seq[i: i + hi])
                i += hi
            else:
                n = sum(flatten(hi))
                hi_type = type(hi)
                rv[k].append(hi_type(reshape(seq[i: i + n], hi)[0]))
                i += n
        rv[k] = container(rv[k])
    return type(seq)(rv)


def group(seq, multiple=True):
    """
    Splits a sequence into a list of lists of equal, adjacent elements.

    Examples
    ========

    >>> from sympy.utilities.iterables import group

    >>> group([1, 1, 1, 2, 2, 3])
    [[1, 1, 1], [2, 2], [3]]
    >>> group([1, 1, 1, 2, 2, 3], multiple=False)
    [(1, 3), (2, 2), (3, 1)]
    >>> group([1, 1, 3, 2, 2, 1], multiple=False)
    [(1, 2), (3, 1), (2, 2), (1, 1)]

    See Also
    ========

    multiset

    """
    if not seq:
        return []

    current, groups = [seq[0]], []

    for elem in seq[1:]:
        if elem == current[-1]:
            current.append(elem)
        else:
            groups.append(current)
            current = [elem]

    groups.append(current)

    if multiple:
        return groups

    for i, current in enumerate(groups):
        groups[i] = (current[0], len(current))

    return groups


def _iproduct2(iterable1, iterable2):
    '''Cartesian product of two possibly infinite iterables'''

    it1 = iter(iterable1)
    it2 = iter(iterable2)

    elems1 = []
    elems2 = []

    sentinel = object()
    def append(it, elems):
        e = next(it, sentinel)
        if e is not sentinel:
            elems.append(e)

    n = 0
    append(it1, elems1)
    append(it2, elems2)

    while n <= len(elems1) + len(elems2):
        for m in range(n-len(elems1)+1, len(elems2)):
            yield (elems1[n-m], elems2[m])
        n += 1
        append(it1, elems1)
        append(it2, elems2)


def iproduct(*iterables):
    '''
    Cartesian product of iterables.

    Generator of the cartesian product of iterables. This is analogous to
    itertools.product except that it works with infinite iterables and will
    yield any item from the infinite product eventually.

    Examples
    ========

    >>> from sympy.utilities.iterables import iproduct
    >>> sorted(iproduct([1,2], [3,4]))
    [(1, 3), (1, 4), (2, 3), (2, 4)]

    With an infinite iterator:

    >>> from sympy import S
    >>> (3,) in iproduct(S.Integers)
    True
    >>> (3, 4) in iproduct(S.Integers, S.Integers)
    True

    .. seealso::

       `itertools.product <https://docs.python.org/3/library/itertools.html#itertools.product>`_
    '''
    if len(iterables) == 0:
        yield ()
        return
    elif len(iterables) == 1:
        for e in iterables[0]:
            yield (e,)
    elif len(iterables) == 2:
        yield from _iproduct2(*iterables)
    else:
        first, others = iterables[0], iterables[1:]
        for ef, eo in _iproduct2(first, iproduct(*others)):
            yield (ef,) + eo


def multiset(seq):
    """Return the hashable sequence in multiset form with values being the
    multiplicity of the item in the sequence.

    Examples
    ========

    >>> from sympy.utilities.iterables import multiset
    >>> multiset('mississippi')
    {'i': 4, 'm': 1, 'p': 2, 's': 4}

    See Also
    ========

    group

    """
    rv = defaultdict(int)
    for s in seq:
        rv[s] += 1
    return dict(rv)


def postorder_traversal(node, keys=None):
    """
    Do a postorder traversal of a tree.

    This generator recursively yields nodes that it has visited in a postorder
    fashion. That is, it descends through the tree depth-first to yield all of
    a node's children's postorder traversal before yielding the node itself.

    Parameters
    ==========

    node : sympy expression
        The expression to traverse.
    keys : (default None) sort key(s)
        The key(s) used to sort args of Basic objects. When None, args of Basic
        objects are processed in arbitrary order. If key is defined, it will
        be passed along to ordered() as the only key(s) to use to sort the
        arguments; if ``key`` is simply True then the default keys of
        ``ordered`` will be used (node count and default_sort_key).

    Yields
    ======
    subtree : sympy expression
        All of the subtrees in the tree.

    Examples
    ========

    >>> from sympy.utilities.iterables import postorder_traversal
    >>> from sympy.abc import w, x, y, z

    The nodes are returned in the order that they are encountered unless key
    is given; simply passing key=True will guarantee that the traversal is
    unique.

    >>> list(postorder_traversal(w + (x + y)*z)) # doctest: +SKIP
    [z, y, x, x + y, z*(x + y), w, w + z*(x + y)]
    >>> list(postorder_traversal(w + (x + y)*z, keys=True))
    [w, z, x, y, x + y, z*(x + y), w + z*(x + y)]


    """
    if isinstance(node, Basic):
        args = node.args
        if keys:
            if keys != True:
                args = ordered(args, keys, default=False)
            else:
                args = ordered(args)
        for arg in args:
            yield from postorder_traversal(arg, keys)
    elif iterable(node):
        for item in node:
            yield from postorder_traversal(item, keys)
    yield node


def interactive_traversal(expr):
    """Traverse a tree asking a user which branch to choose. """
    from sympy.printing import pprint

    RED, BRED = '\033[0;31m', '\033[1;31m'
    GREEN, BGREEN = '\033[0;32m', '\033[1;32m'
    YELLOW, BYELLOW = '\033[0;33m', '\033[1;33m'  # noqa
    BLUE, BBLUE = '\033[0;34m', '\033[1;34m'      # noqa
    MAGENTA, BMAGENTA = '\033[0;35m', '\033[1;35m'# noqa
    CYAN, BCYAN = '\033[0;36m', '\033[1;36m'      # noqa
    END = '\033[0m'

    def cprint(*args):
        print("".join(map(str, args)) + END)

    def _interactive_traversal(expr, stage):
        if stage > 0:
            print()

        cprint("Current expression (stage ", BYELLOW, stage, END, "):")
        print(BCYAN)
        pprint(expr)
        print(END)

        if isinstance(expr, Basic):
            if expr.is_Add:
                args = expr.as_ordered_terms()
            elif expr.is_Mul:
                args = expr.as_ordered_factors()
            else:
                args = expr.args
        elif hasattr(expr, "__iter__"):
            args = list(expr)
        else:
            return expr

        n_args = len(args)

        if not n_args:
            return expr

        for i, arg in enumerate(args):
            cprint(GREEN, "[", BGREEN, i, GREEN, "] ", BLUE, type(arg), END)
            pprint(arg)
            print()

        if n_args == 1:
            choices = '0'
        else:
            choices = '0-%d' % (n_args - 1)

        try:
            choice = input("Your choice [%s,f,l,r,d,?]: " % choices)
        except EOFError:
            result = expr
            print()
        else:
            if choice == '?':
                cprint(RED, "%s - select subexpression with the given index" %
                       choices)
                cprint(RED, "f - select the first subexpression")
                cprint(RED, "l - select the last subexpression")
                cprint(RED, "r - select a random subexpression")
                cprint(RED, "d - done\n")

                result = _interactive_traversal(expr, stage)
            elif choice in ['d', '']:
                result = expr
            elif choice == 'f':
                result = _interactive_traversal(args[0], stage + 1)
            elif choice == 'l':
                result = _interactive_traversal(args[-1], stage + 1)
            elif choice == 'r':
                result = _interactive_traversal(random.choice(args), stage + 1)
            else:
                try:
                    choice = int(choice)
                except ValueError:
                    cprint(BRED,
                           "Choice must be a number in %s range\n" % choices)
                    result = _interactive_traversal(expr, stage)
                else:
                    if choice < 0 or choice >= n_args:
                        cprint(BRED, "Choice must be in %s range\n" % choices)
                        result = _interactive_traversal(expr, stage)
                    else:
                        result = _interactive_traversal(args[choice], stage + 1)

        return result

    return _interactive_traversal(expr, 0)


def ibin(n, bits=None, str=False):
    """Return a list of length ``bits`` corresponding to the binary value
    of ``n`` with small bits to the right (last). If bits is omitted, the
    length will be the number required to represent ``n``. If the bits are
    desired in reversed order, use the ``[::-1]`` slice of the returned list.

    If a sequence of all bits-length lists starting from ``[0, 0,..., 0]``
    through ``[1, 1, ..., 1]`` are desired, pass a non-integer for bits, e.g.
    ``'all'``.

    If the bit *string* is desired pass ``str=True``.

    Examples
    ========

    >>> from sympy.utilities.iterables import ibin
    >>> ibin(2)
    [1, 0]
    >>> ibin(2, 4)
    [0, 0, 1, 0]

    If all lists corresponding to 0 to 2**n - 1, pass a non-integer
    for bits:

    >>> bits = 2
    >>> for i in ibin(2, 'all'):
    ...     print(i)
    (0, 0)
    (0, 1)
    (1, 0)
    (1, 1)

    If a bit string is desired of a given length, use str=True:

    >>> n = 123
    >>> bits = 10
    >>> ibin(n, bits, str=True)
    '0001111011'
    >>> ibin(n, bits, str=True)[::-1]  # small bits left
    '1101111000'
    >>> list(ibin(3, 'all', str=True))
    ['000', '001', '010', '011', '100', '101', '110', '111']

    """
    if n < 0:
        raise ValueError("negative numbers are not allowed")
    n = as_int(n)

    if bits is None:
        bits = 0
    else:
        try:
             bits = as_int(bits)
        except ValueError:
            bits = -1
        else:
            if n.bit_length() > bits:
                raise ValueError(
                    "`bits` must be >= {}".format(n.bit_length()))

    if not str:
        if bits >= 0:
            return [1 if i == "1" else 0 for i in bin(n)[2:].rjust(bits, "0")]
        else:
            return variations(list(range(2)), n, repetition=True)
    else:
        if bits >= 0:
            return bin(n)[2:].rjust(bits, "0")
        else:
            return (bin(i)[2:].rjust(n, "0") for i in range(2**n))


def variations(seq, n, repetition=False):
    r"""Returns a generator of the n-sized variations of ``seq`` (size N).
    ``repetition`` controls whether items in ``seq`` can appear more than once;

    Examples
    ========

    ``variations(seq, n)`` will return `\frac{N!}{(N - n)!}` permutations without
    repetition of ``seq``'s elements:

        >>> from sympy.utilities.iterables import variations
        >>> list(variations([1, 2], 2))
        [(1, 2), (2, 1)]

    ``variations(seq, n, True)`` will return the `N^n` permutations obtained
    by allowing repetition of elements:

        >>> list(variations([1, 2], 2, repetition=True))
        [(1, 1), (1, 2), (2, 1), (2, 2)]

    If you ask for more items than are in the set you get the empty set unless
    you allow repetitions:

        >>> list(variations([0, 1], 3, repetition=False))
        []
        >>> list(variations([0, 1], 3, repetition=True))[:4]
        [(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1)]

    .. seealso::

       `itertools.permutations <https://docs.python.org/3/library/itertools.html#itertools.permutations>`_,
       `itertools.product <https://docs.python.org/3/library/itertools.html#itertools.product>`_
    """
    if not repetition:
        seq = tuple(seq)
        if len(seq) < n:
            return
        yield from permutations(seq, n)
    else:
        if n == 0:
            yield ()
        else:
            yield from product(seq, repeat=n)


def subsets(seq, k=None, repetition=False):
    r"""Generates all `k`-subsets (combinations) from an `n`-element set, ``seq``.

    A `k`-subset of an `n`-element set is any subset of length exactly `k`. The
    number of `k`-subsets of an `n`-element set is given by ``binomial(n, k)``,
    whereas there are `2^n` subsets all together. If `k` is ``None`` then all
    `2^n` subsets will be returned from shortest to longest.

    Examples
    ========

    >>> from sympy.utilities.iterables import subsets

    ``subsets(seq, k)`` will return the `\frac{n!}{k!(n - k)!}` `k`-subsets (combinations)
    without repetition, i.e. once an item has been removed, it can no
    longer be "taken":

        >>> list(subsets([1, 2], 2))
        [(1, 2)]
        >>> list(subsets([1, 2]))
        [(), (1,), (2,), (1, 2)]
        >>> list(subsets([1, 2, 3], 2))
        [(1, 2), (1, 3), (2, 3)]


    ``subsets(seq, k, repetition=True)`` will return the `\frac{(n - 1 + k)!}{k!(n - 1)!}`
    combinations *with* repetition:

        >>> list(subsets([1, 2], 2, repetition=True))
        [(1, 1), (1, 2), (2, 2)]

    If you ask for more items than are in the set you get the empty set unless
    you allow repetitions:

        >>> list(subsets([0, 1], 3, repetition=False))
        []
        >>> list(subsets([0, 1], 3, repetition=True))
        [(0, 0, 0), (0, 0, 1), (0, 1, 1), (1, 1, 1)]

    """
    if k is None:
        for k in range(len(seq) + 1):
            yield from subsets(seq, k, repetition)
    else:
        if not repetition:
            yield from combinations(seq, k)
        else:
            yield from combinations_with_replacement(seq, k)


def filter_symbols(iterator, exclude):
    """
    Only yield elements from `iterator` that do not occur in `exclude`.

    Parameters
    ==========

    iterator : iterable
    iterator to take elements from

    exclude : iterable
    elements to exclude

    Returns
    =======

    iterator : iterator
    filtered iterator
    """
    exclude = set(exclude)
    for s in iterator:
        if s not in exclude:
            yield s

def numbered_symbols(prefix='x', cls=None, start=0, exclude=[], *args, **assumptions):
    """
    Generate an infinite stream of Symbols consisting of a prefix and
    increasing subscripts provided that they do not occur in ``exclude``.

    Parameters
    ==========

    prefix : str, optional
        The prefix to use. By default, this function will generate symbols of
        the form "x0", "x1", etc.

    cls : class, optional
        The class to use. By default, it uses ``Symbol``, but you can also use ``Wild`` or ``Dummy``.

    start : int, optional
        The start number.  By default, it is 0.

    Returns
    =======

    sym : Symbol
        The subscripted symbols.
    """
    exclude = set(exclude or [])
    if cls is None:
        # We can't just make the default cls=Symbol because it isn't
        # imported yet.
        from sympy import Symbol
        cls = Symbol

    while True:
        name = '%s%s' % (prefix, start)
        s = cls(name, *args, **assumptions)
        if s not in exclude:
            yield s
        start += 1


def capture(func):
    """Return the printed output of func().

    ``func`` should be a function without arguments that produces output with
    print statements.

    >>> from sympy.utilities.iterables import capture
    >>> from sympy import pprint
    >>> from sympy.abc import x
    >>> def foo():
    ...     print('hello world!')
    ...
    >>> 'hello' in capture(foo) # foo, not foo()
    True
    >>> capture(lambda: pprint(2/x))
    '2\\n-\\nx\\n'

    """
    from io import StringIO
    import sys

    stdout = sys.stdout
    sys.stdout = file = StringIO()
    try:
        func()
    finally:
        sys.stdout = stdout
    return file.getvalue()


def sift(seq, keyfunc, binary=False):
    """
    Sift the sequence, ``seq`` according to ``keyfunc``.

    Returns
    =======

    When ``binary`` is ``False`` (default), the output is a dictionary
    where elements of ``seq`` are stored in a list keyed to the value
    of keyfunc for that element. If ``binary`` is True then a tuple
    with lists ``T`` and ``F`` are returned where ``T`` is a list
    containing elements of seq for which ``keyfunc`` was ``True`` and
    ``F`` containing those elements for which ``keyfunc`` was ``False``;
    a ValueError is raised if the ``keyfunc`` is not binary.

    Examples
    ========

    >>> from sympy.utilities import sift
    >>> from sympy.abc import x, y
    >>> from sympy import sqrt, exp, pi, Tuple

    >>> sift(range(5), lambda x: x % 2)
    {0: [0, 2, 4], 1: [1, 3]}

    sift() returns a defaultdict() object, so any key that has no matches will
    give [].

    >>> sift([x], lambda x: x.is_commutative)
    {True: [x]}
    >>> _[False]
    []

    Sometimes you will not know how many keys you will get:

    >>> sift([sqrt(x), exp(x), (y**x)**2],
    ...      lambda x: x.as_base_exp()[0])
    {E: [exp(x)], x: [sqrt(x)], y: [y**(2*x)]}

    Sometimes you expect the results to be binary; the
    results can be unpacked by setting ``binary`` to True:

    >>> sift(range(4), lambda x: x % 2, binary=True)
    ([1, 3], [0, 2])
    >>> sift(Tuple(1, pi), lambda x: x.is_rational, binary=True)
    ([1], [pi])

    A ValueError is raised if the predicate was not actually binary
    (which is a good test for the logic where sifting is used and
    binary results were expected):

    >>> unknown = exp(1) - pi  # the rationality of this is unknown
    >>> args = Tuple(1, pi, unknown)
    >>> sift(args, lambda x: x.is_rational, binary=True)
    Traceback (most recent call last):
    ...
    ValueError: keyfunc gave non-binary output

    The non-binary sifting shows that there were 3 keys generated:

    >>> set(sift(args, lambda x: x.is_rational).keys())
    {None, False, True}

    If you need to sort the sifted items it might be better to use
    ``ordered`` which can economically apply multiple sort keys
    to a sequence while sorting.

    See Also
    ========

    ordered

    """
    if not binary:
        m = defaultdict(list)
        for i in seq:
            m[keyfunc(i)].append(i)
        return m
    sift = F, T = [], []
    for i in seq:
        try:
            sift[keyfunc(i)].append(i)
        except (IndexError, TypeError):
            raise ValueError('keyfunc gave non-binary output')
    return T, F


def take(iter, n):
    """Return ``n`` items from ``iter`` iterator. """
    return [ value for _, value in zip(range(n), iter) ]


def dict_merge(*dicts):
    """Merge dictionaries into a single dictionary. """
    merged = {}

    for dict in dicts:
        merged.update(dict)

    return merged


def common_prefix(*seqs):
    """Return the subsequence that is a common start of sequences in ``seqs``.

    >>> from sympy.utilities.iterables import common_prefix
    >>> common_prefix(list(range(3)))
    [0, 1, 2]
    >>> common_prefix(list(range(3)), list(range(4)))
    [0, 1, 2]
    >>> common_prefix([1, 2, 3], [1, 2, 5])
    [1, 2]
    >>> common_prefix([1, 2, 3], [1, 3, 5])
    [1]
    """
    if any(not s for s in seqs):
        return []
    elif len(seqs) == 1:
        return seqs[0]
    i = 0
    for i in range(min(len(s) for s in seqs)):
        if not all(seqs[j][i] == seqs[0][i] for j in range(len(seqs))):
            break
    else:
        i += 1
    return seqs[0][:i]


def common_suffix(*seqs):
    """Return the subsequence that is a common ending of sequences in ``seqs``.

    >>> from sympy.utilities.iterables import common_suffix
    >>> common_suffix(list(range(3)))
    [0, 1, 2]
    >>> common_suffix(list(range(3)), list(range(4)))
    []
    >>> common_suffix([1, 2, 3], [9, 2, 3])
    [2, 3]
    >>> common_suffix([1, 2, 3], [9, 7, 3])
    [3]
    """

    if any(not s for s in seqs):
        return []
    elif len(seqs) == 1:
        return seqs[0]
    i = 0
    for i in range(-1, -min(len(s) for s in seqs) - 1, -1):
        if not all(seqs[j][i] == seqs[0][i] for j in range(len(seqs))):
            break
    else:
        i -= 1
    if i == -1:
        return []
    else:
        return seqs[0][i + 1:]


def prefixes(seq):
    """
    Generate all prefixes of a sequence.

    Examples
    ========

    >>> from sympy.utilities.iterables import prefixes

    >>> list(prefixes([1,2,3,4]))
    [[1], [1, 2], [1, 2, 3], [1, 2, 3, 4]]

    """
    n = len(seq)

    for i in range(n):
        yield seq[:i + 1]


def postfixes(seq):
    """
    Generate all postfixes of a sequence.

    Examples
    ========

    >>> from sympy.utilities.iterables import postfixes

    >>> list(postfixes([1,2,3,4]))
    [[4], [3, 4], [2, 3, 4], [1, 2, 3, 4]]

    """
    n = len(seq)

    for i in range(n):
        yield seq[n - i - 1:]


def topological_sort(graph, key=None):
    r"""
    Topological sort of graph's vertices.

    Parameters
    ==========

    graph : tuple[list, list[tuple[T, T]]
        A tuple consisting of a list of vertices and a list of edges of
        a graph to be sorted topologically.

    key : callable[T] (optional)
        Ordering key for vertices on the same level. By default the natural
        (e.g. lexicographic) ordering is used (in this case the base type
        must implement ordering relations).

    Examples
    ========

    Consider a graph::

        +---+     +---+     +---+
        | 7 |\    | 5 |     | 3 |
        +---+ \   +---+     +---+
          |   _\___/ ____   _/ |
          |  /  \___/    \ /   |
          V  V           V V   |
         +----+         +---+  |
         | 11 |         | 8 |  |
         +----+         +---+  |
          | | \____   ___/ _   |
          | \      \ /    / \  |
          V  \     V V   /  V  V
        +---+ \   +---+ |  +----+
        | 2 |  |  | 9 | |  | 10 |
        +---+  |  +---+ |  +----+
               \________/

    where vertices are integers. This graph can be encoded using
    elementary Python's data structures as follows::

        >>> V = [2, 3, 5, 7, 8, 9, 10, 11]
        >>> E = [(7, 11), (7, 8), (5, 11), (3, 8), (3, 10),
        ...      (11, 2), (11, 9), (11, 10), (8, 9)]

    To compute a topological sort for graph ``(V, E)`` issue::

        >>> from sympy.utilities.iterables import topological_sort

        >>> topological_sort((V, E))
        [3, 5, 7, 8, 11, 2, 9, 10]

    If specific tie breaking approach is needed, use ``key`` parameter::

        >>> topological_sort((V, E), key=lambda v: -v)
        [7, 5, 11, 3, 10, 8, 9, 2]

    Only acyclic graphs can be sorted. If the input graph has a cycle,
    then ``ValueError`` will be raised::

        >>> topological_sort((V, E + [(10, 7)]))
        Traceback (most recent call last):
        ...
        ValueError: cycle detected

    References
    ==========

    .. [1] https://en.wikipedia.org/wiki/Topological_sorting

    """
    V, E = graph

    L = []
    S = set(V)
    E = list(E)

    for v, u in E:
        S.discard(u)

    if key is None:
        key = lambda value: value

    S = sorted(S, key=key, reverse=True)

    while S:
        node = S.pop()
        L.append(node)

        for u, v in list(E):
            if u == node:
                E.remove((u, v))

                for _u, _v in E:
                    if v == _v:
                        break
                else:
                    kv = key(v)

                    for i, s in enumerate(S):
                        ks = key(s)

                        if kv > ks:
                            S.insert(i, v)
                            break
                    else:
                        S.append(v)

    if E:
        raise ValueError("cycle detected")
    else:
        return L


def strongly_connected_components(G):
    r"""
    Strongly connected components of a directed graph in reverse topological
    order.


    Parameters
    ==========

    graph : tuple[list, list[tuple[T, T]]
        A tuple consisting of a list of vertices and a list of edges of
        a graph whose strongly connected components are to be found.


    Examples
    ========

    Consider a directed graph (in dot notation)::

        digraph {
            A -> B
            A -> C
            B -> C
            C -> B
            B -> D
        }

    where vertices are the letters A, B, C and D. This graph can be encoded
    using Python's elementary data structures as follows::

        >>> V = ['A', 'B', 'C', 'D']
        >>> E = [('A', 'B'), ('A', 'C'), ('B', 'C'), ('C', 'B'), ('B', 'D')]

    The strongly connected components of this graph can be computed as

        >>> from sympy.utilities.iterables import strongly_connected_components

        >>> strongly_connected_components((V, E))
        [['D'], ['B', 'C'], ['A']]

    This also gives the components in reverse topological order.

    Since the subgraph containing B and C has a cycle they must be together in
    a strongly connected component. A and D are connected to the rest of the
    graph but not in a cyclic manner so they appear as their own strongly
    connected components.


    Notes
    =====

    The vertices of the graph must be hashable for the data structures used.
    If the vertices are unhashable replace them with integer indices.

    This function uses Tarjan's algorithm to compute the strongly connected
    components in `O(|V|+|E|)` (linear) time.


    References
    ==========

    .. [1] https://en.wikipedia.org/wiki/Strongly_connected_component
    .. [2] https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm


    See Also
    ========

    sympy.utilities.iterables.connected_components

    """
    # Map from a vertex to its neighbours
    V, E = G
    Gmap = {vi: [] for vi in V}
    for v1, v2 in E:
        Gmap[v1].append(v2)

    # Non-recursive Tarjan's algorithm:
    lowlink = {}
    indices = {}
    stack = OrderedDict()
    callstack = []
    components = []
    nomore = object()

    def start(v):
        index = len(stack)
        indices[v] = lowlink[v] = index
        stack[v] = None
        callstack.append((v, iter(Gmap[v])))

    def finish(v1):
        # Finished a component?
        if lowlink[v1] == indices[v1]:
            component = [stack.popitem()[0]]
            while component[-1] is not v1:
                component.append(stack.popitem()[0])
            components.append(component[::-1])
        v2, _ = callstack.pop()
        if callstack:
            v1, _ = callstack[-1]
            lowlink[v1] = min(lowlink[v1], lowlink[v2])

    for v in V:
        if v in indices:
            continue
        start(v)
        while callstack:
            v1, it1 = callstack[-1]
            v2 = next(it1, nomore)
            # Finished children of v1?
            if v2 is nomore:
                finish(v1)
            # Recurse on v2
            elif v2 not in indices:
                start(v2)
            elif v2 in stack:
                lowlink[v1] = min(lowlink[v1], indices[v2])

    # Reverse topological sort order:
    return components


def connected_components(G):
    r"""
    Connected components of an undirected graph or weakly connected components
    of a directed graph.


    Parameters
    ==========

    graph : tuple[list, list[tuple[T, T]]
        A tuple consisting of a list of vertices and a list of edges of
        a graph whose connected components are to be found.


    Examples
    ========


    Given an undirected graph::

        graph {
            A -- B
            C -- D
        }

    We can find the connected components using this function if we include
    each edge in both directions::

        >>> from sympy.utilities.iterables import connected_components

        >>> V = ['A', 'B', 'C', 'D']
        >>> E = [('A', 'B'), ('B', 'A'), ('C', 'D'), ('D', 'C')]
        >>> connected_components((V, E))
        [['A', 'B'], ['C', 'D']]

    The weakly connected components of a directed graph can found the same
    way.


    Notes
    =====

    The vertices of the graph must be hashable for the data structures used.
    If the vertices are unhashable replace them with integer indices.

    This function uses Tarjan's algorithm to compute the connected components
    in `O(|V|+|E|)` (linear) time.


    References
    ==========

    .. [1] https://en.wikipedia.org/wiki/Connected_component_(graph_theory)
    .. [2] https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm


    See Also
    ========

    sympy.utilities.iterables.strongly_connected_components

    """
    # Duplicate edges both ways so that the graph is effectively undirected
    # and return the strongly connected components:
    V, E = G
    E_undirected = []
    for v1, v2 in E:
        E_undirected.extend([(v1, v2), (v2, v1)])
    return strongly_connected_components((V, E_undirected))


def rotate_left(x, y):
    """
    Left rotates a list x by the number of steps specified
    in y.

    Examples
    ========

    >>> from sympy.utilities.iterables import rotate_left
    >>> a = [0, 1, 2]
    >>> rotate_left(a, 1)
    [1, 2, 0]
    """
    if len(x) == 0:
        return []
    y = y % len(x)
    return x[y:] + x[:y]


def rotate_right(x, y):
    """
    Right rotates a list x by the number of steps specified
    in y.

    Examples
    ========

    >>> from sympy.utilities.iterables import rotate_right
    >>> a = [0, 1, 2]
    >>> rotate_right(a, 1)
    [2, 0, 1]
    """
    if len(x) == 0:
        return []
    y = len(x) - y % len(x)
    return x[y:] + x[:y]


def least_rotation(x, key=None):
    '''
    Returns the number of steps of left rotation required to
    obtain lexicographically minimal string/list/tuple, etc.

    Examples
    ========

    >>> from sympy.utilities.iterables import least_rotation, rotate_left
    >>> a = [3, 1, 5, 1, 2]
    >>> least_rotation(a)
    3
    >>> rotate_left(a, _)
    [1, 2, 3, 1, 5]

    References
    ==========

    .. [1] https://en.wikipedia.org/wiki/Lexicographically_minimal_string_rotation

    '''
    if key is None: key = sympy.Id
    S = x + x      # Concatenate string to it self to avoid modular arithmetic
    f = [-1] * len(S)     # Failure function
    k = 0       # Least rotation of string found so far
    for j in range(1,len(S)):
        sj = S[j]
        i = f[j-k-1]
        while i != -1 and sj != S[k+i+1]:
            if key(sj) < key(S[k+i+1]):
                k = j-i-1
            i = f[i]
        if sj != S[k+i+1]:
            if key(sj) < key(S[k]):
                k = j
            f[j-k] = -1
        else:
            f[j-k] = i+1
    return k


def multiset_combinations(m, n, g=None):
    """
    Return the unique combinations of size ``n`` from multiset ``m``.

    Examples
    ========

    >>> from sympy.utilities.iterables import multiset_combinations
    >>> from itertools import combinations
    >>> [''.join(i) for i in  multiset_combinations('baby', 3)]
    ['abb', 'aby', 'bby']

    >>> def count(f, s): return len(list(f(s, 3)))

    The number of combinations depends on the number of letters; the
    number of unique combinations depends on how the letters are
    repeated.

    >>> s1 = 'abracadabra'
    >>> s2 = 'banana tree'
    >>> count(combinations, s1), count(multiset_combinations, s1)
    (165, 23)
    >>> count(combinations, s2), count(multiset_combinations, s2)
    (165, 54)

    """
    if g is None:
        if type(m) is dict:
            if n > sum(m.values()):
                return
            g = [[k, m[k]] for k in ordered(m)]
        else:
            m = list(m)
            if n > len(m):
                return
            try:
                m = multiset(m)
                g = [(k, m[k]) for k in ordered(m)]
            except TypeError:
                m = list(ordered(m))
                g = [list(i) for i in group(m, multiple=False)]
        del m
    if sum(v for k, v in g) < n or not n:
        yield []
    else:
        for i, (k, v) in enumerate(g):
            if v >= n:
                yield [k]*n
                v = n - 1
            for v in range(min(n, v), 0, -1):
                for j in multiset_combinations(None, n - v, g[i + 1:]):
                    rv = [k]*v + j
                    if len(rv) == n:
                        yield rv


def multiset_permutations(m, size=None, g=None):
    """
    Return the unique permutations of multiset ``m``.

    Examples
    ========

    >>> from sympy.utilities.iterables import multiset_permutations
    >>> from sympy import factorial
    >>> [''.join(i) for i in multiset_permutations('aab')]
    ['aab', 'aba', 'baa']
    >>> factorial(len('banana'))
    720
    >>> len(list(multiset_permutations('banana')))
    60
    """
    if g is None:
        if type(m) is dict:
            g = [[k, m[k]] for k in ordered(m)]
        else:
            m = list(ordered(m))
            g = [list(i) for i in group(m, multiple=False)]
        del m
    do = [gi for gi in g if gi[1] > 0]
    SUM = sum([gi[1] for gi in do])
    if not do or size is not None and (size > SUM or size < 1):
        if size < 1:
            yield []
        return
    elif size == 1:
        for k, v in do:
            yield [k]
    elif len(do) == 1:
        k, v = do[0]
        v = v if size is None else (size if size <= v else 0)
        yield [k for i in range(v)]
    elif all(v == 1 for k, v in do):
        for p in permutations([k for k, v in do], size):
            yield list(p)
    else:
        size = size if size is not None else SUM
        for i, (k, v) in enumerate(do):
            do[i][1] -= 1
            for j in multiset_permutations(None, size - 1, do):
                if j:
                    yield [k] + j
            do[i][1] += 1


def _partition(seq, vector, m=None):
    """
    Return the partition of seq as specified by the partition vector.

    Examples
    ========

    >>> from sympy.utilities.iterables import _partition
    >>> _partition('abcde', [1, 0, 1, 2, 0])
    [['b', 'e'], ['a', 'c'], ['d']]

    Specifying the number of bins in the partition is optional:

    >>> _partition('abcde', [1, 0, 1, 2, 0], 3)
    [['b', 'e'], ['a', 'c'], ['d']]

    The output of _set_partitions can be passed as follows:

    >>> output = (3, [1, 0, 1, 2, 0])
    >>> _partition('abcde', *output)
    [['b', 'e'], ['a', 'c'], ['d']]

    See Also
    ========

    combinatorics.partitions.Partition.from_rgs

    """
    if m is None:
        m = max(vector) + 1
    elif type(vector) is int:  # entered as m, vector
        vector, m = m, vector
    p = [[] for i in range(m)]
    for i, v in enumerate(vector):
        p[v].append(seq[i])
    return p


def _set_partitions(n):
    """Cycle through all partions of n elements, yielding the
    current number of partitions, ``m``, and a mutable list, ``q``
    such that element[i] is in part q[i] of the partition.

    NOTE: ``q`` is modified in place and generally should not be changed
    between function calls.

    Examples
    ========

    >>> from sympy.utilities.iterables import _set_partitions, _partition
    >>> for m, q in _set_partitions(3):
    ...     print('%s %s %s' % (m, q, _partition('abc', q, m)))
    1 [0, 0, 0] [['a', 'b', 'c']]
    2 [0, 0, 1] [['a', 'b'], ['c']]
    2 [0, 1, 0] [['a', 'c'], ['b']]
    2 [0, 1, 1] [['a'], ['b', 'c']]
    3 [0, 1, 2] [['a'], ['b'], ['c']]

    Notes
    =====

    This algorithm is similar to, and solves the same problem as,
    Algorithm 7.2.1.5H, from volume 4A of Knuth's The Art of Computer
    Programming.  Knuth uses the term "restricted growth string" where
    this code refers to a "partition vector". In each case, the meaning is
    the same: the value in the ith element of the vector specifies to
    which part the ith set element is to be assigned.

    At the lowest level, this code implements an n-digit big-endian
    counter (stored in the array q) which is incremented (with carries) to
    get the next partition in the sequence.  A special twist is that a
    digit is constrained to be at most one greater than the maximum of all
    the digits to the left of it.  The array p maintains this maximum, so
    that the code can efficiently decide when a digit can be incremented
    in place or whether it needs to be reset to 0 and trigger a carry to
    the next digit.  The enumeration starts with all the digits 0 (which
    corresponds to all the set elements being assigned to the same 0th
    part), and ends with 0123...n, which corresponds to each set element
    being assigned to a different, singleton, part.

    This routine was rewritten to use 0-based lists while trying to
    preserve the beauty and efficiency of the original algorithm.

    References
    ==========

    .. [1] Nijenhuis, Albert and Wilf, Herbert. (1978) Combinatorial Algorithms,
        2nd Ed, p 91, algorithm "nexequ". Available online from
        https://www.math.upenn.edu/~wilf/website/CombAlgDownld.html (viewed
        November 17, 2012).

    """
    p = [0]*n
    q = [0]*n
    nc = 1
    yield nc, q
    while nc != n:
        m = n
        while 1:
            m -= 1
            i = q[m]
            if p[i] != 1:
                break
            q[m] = 0
        i += 1
        q[m] = i
        m += 1
        nc += m - n
        p[0] += n - m
        if i == nc:
            p[nc] = 0
            nc += 1
        p[i - 1] -= 1
        p[i] += 1
        yield nc, q


def multiset_partitions(multiset, m=None):
    """
    Return unique partitions of the given multiset (in list form).
    If ``m`` is None, all multisets will be returned, otherwise only
    partitions with ``m`` parts will be returned.

    If ``multiset`` is an integer, a range [0, 1, ..., multiset - 1]
    will be supplied.

    Examples
    ========

    >>> from sympy.utilities.iterables import multiset_partitions
    >>> list(multiset_partitions([1, 2, 3, 4], 2))
    [[[1, 2, 3], [4]], [[1, 2, 4], [3]], [[1, 2], [3, 4]],
    [[1, 3, 4], [2]], [[1, 3], [2, 4]], [[1, 4], [2, 3]],
    [[1], [2, 3, 4]]]
    >>> list(multiset_partitions([1, 2, 3, 4], 1))
    [[[1, 2, 3, 4]]]

    Only unique partitions are returned and these will be returned in a
    canonical order regardless of the order of the input:

    >>> a = [1, 2, 2, 1]
    >>> ans = list(multiset_partitions(a, 2))
    >>> a.sort()
    >>> list(multiset_partitions(a, 2)) == ans
    True
    >>> a = range(3, 1, -1)
    >>> (list(multiset_partitions(a)) ==
    ...  list(multiset_partitions(sorted(a))))
    True

    If m is omitted then all partitions will be returned:

    >>> list(multiset_partitions([1, 1, 2]))
    [[[1, 1, 2]], [[1, 1], [2]], [[1, 2], [1]], [[1], [1], [2]]]
    >>> list(multiset_partitions([1]*3))
    [[[1, 1, 1]], [[1], [1, 1]], [[1], [1], [1]]]

    Counting
    ========

    The number of partitions of a set is given by the bell number:

    >>> from sympy import bell
    >>> len(list(multiset_partitions(5))) == bell(5) == 52
    True

    The number of partitions of length k from a set of size n is given by the
    Stirling Number of the 2nd kind:

    >>> from sympy.functions.combinatorial.numbers import stirling
    >>> stirling(5, 2) == len(list(multiset_partitions(5, 2))) == 15
    True

    These comments on counting apply to *sets*, not multisets.

    Notes
    =====

    When all the elements are the same in the multiset, the order
    of the returned partitions is determined by the ``partitions``
    routine. If one is counting partitions then it is better to use
    the ``nT`` function.

    See Also
    ========

    partitions
    sympy.combinatorics.partitions.Partition
    sympy.combinatorics.partitions.IntegerPartition
    sympy.functions.combinatorial.numbers.nT

    """
    # This function looks at the supplied input and dispatches to
    # several special-case routines as they apply.
    if type(multiset) is int:
        n = multiset
        if m and m > n:
            return
        multiset = list(range(n))
        if m == 1:
            yield [multiset[:]]
            return

        # If m is not None, it can sometimes be faster to use
        # MultisetPartitionTraverser.enum_range() even for inputs
        # which are sets.  Since the _set_partitions code is quite
        # fast, this is only advantageous when the overall set
        # partitions outnumber those with the desired number of parts
        # by a large factor.  (At least 60.)  Such a switch is not
        # currently implemented.
        for nc, q in _set_partitions(n):
            if m is None or nc == m:
                rv = [[] for i in range(nc)]
                for i in range(n):
                    rv[q[i]].append(multiset[i])
                yield rv
        return

    if len(multiset) == 1 and isinstance(multiset, str):
        multiset = [multiset]

    if not has_variety(multiset):
        # Only one component, repeated n times.  The resulting
        # partitions correspond to partitions of integer n.
        n = len(multiset)
        if m and m > n:
            return
        if m == 1:
            yield [multiset[:]]
            return
        x = multiset[:1]
        for size, p in partitions(n, m, size=True):
            if m is None or size == m:
                rv = []
                for k in sorted(p):
                    rv.extend([x*k]*p[k])
                yield rv
    else:
        multiset = list(ordered(multiset))
        n = len(multiset)
        if m and m > n:
            return
        if m == 1:
            yield [multiset[:]]
            return

        # Split the information of the multiset into two lists -
        # one of the elements themselves, and one (of the same length)
        # giving the number of repeats for the corresponding element.
        elements, multiplicities = zip(*group(multiset, False))

        if len(elements) < len(multiset):
            # General case - multiset with more than one distinct element
            # and at least one element repeated more than once.
            if m:
                mpt = MultisetPartitionTraverser()
                for state in mpt.enum_range(multiplicities, m-1, m):
                    yield list_visitor(state, elements)
            else:
                for state in multiset_partitions_taocp(multiplicities):
                    yield list_visitor(state, elements)
        else:
            # Set partitions case - no repeated elements. Pretty much
            # same as int argument case above, with same possible, but
            # currently unimplemented optimization for some cases when
            # m is not None
            for nc, q in _set_partitions(n):
                if m is None or nc == m:
                    rv = [[] for i in range(nc)]
                    for i in range(n):
                        rv[q[i]].append(i)
                    yield [[multiset[j] for j in i] for i in rv]


def partitions(n, m=None, k=None, size=False):
    """Generate all partitions of positive integer, n.

    Parameters
    ==========

    m : integer (default gives partitions of all sizes)
        limits number of parts in partition (mnemonic: m, maximum parts)
    k : integer (default gives partitions number from 1 through n)
        limits the numbers that are kept in the partition (mnemonic: k, keys)
    size : bool (default False, only partition is returned)
        when ``True`` then (M, P) is returned where M is the sum of the
        multiplicities and P is the generated partition.

    Each partition is represented as a dictionary, mapping an integer
    to the number of copies of that integer in the partition.  For example,
    the first partition of 4 returned is {4: 1}, "4: one of them".

    Examples
    ========

    >>> from sympy.utilities.iterables import partitions

    The numbers appearing in the partition (the key of the returned dict)
    are limited with k:

    >>> for p in partitions(6, k=2):  # doctest: +SKIP
    ...     print(p)
    {2: 3}
    {1: 2, 2: 2}
    {1: 4, 2: 1}
    {1: 6}

    The maximum number of parts in the partition (the sum of the values in
    the returned dict) are limited with m (default value, None, gives
    partitions from 1 through n):

    >>> for p in partitions(6, m=2):  # doctest: +SKIP
    ...     print(p)
    ...
    {6: 1}
    {1: 1, 5: 1}
    {2: 1, 4: 1}
    {3: 2}

    References
    ==========

    .. [1] modified from Tim Peter's version to allow for k and m values:
           http://code.activestate.com/recipes/218332-generator-for-integer-partitions/

    See Also
    ========

    sympy.combinatorics.partitions.Partition
    sympy.combinatorics.partitions.IntegerPartition

    """
    if (n <= 0 or
        m is not None and m < 1 or
        k is not None and k < 1 or
        m and k and m*k < n):
        # the empty set is the only way to handle these inputs
        # and returning {} to represent it is consistent with
        # the counting convention, e.g. nT(0) == 1.
        if size:
            yield 0, {}
        else:
            yield {}
        return

    if m is None:
        m = n
    else:
        m = min(m, n)

    if n == 0:
        if size:
            yield 1, {0: 1}
        else:
            yield {0: 1}
        return

    k = min(k or n, n)

    n, m, k = as_int(n), as_int(m), as_int(k)
    q, r = divmod(n, k)
    ms = {k: q}
    keys = [k]  # ms.keys(), from largest to smallest
    if r:
        ms[r] = 1
        keys.append(r)
    room = m - q - bool(r)
    if size:
        yield sum(ms.values()), ms.copy()
    else:
        yield ms.copy()

    while keys != [1]:
        # Reuse any 1's.
        if keys[-1] == 1:
            del keys[-1]
            reuse = ms.pop(1)
            room += reuse
        else:
            reuse = 0

        while 1:
            # Let i be the smallest key larger than 1.  Reuse one
            # instance of i.
            i = keys[-1]
            newcount = ms[i] = ms[i] - 1
            reuse += i
            if newcount == 0:
                del keys[-1], ms[i]
            room += 1

            # Break the remainder into pieces of size i-1.
            i -= 1
            q, r = divmod(reuse, i)
            need = q + bool(r)
            if need > room:
                if not keys:
                    return
                continue

            ms[i] = q
            keys.append(i)
            if r:
                ms[r] = 1
                keys.append(r)
            break
        room -= need
        if size:
            yield sum(ms.values()), ms.copy()
        else:
            yield ms.copy()


def ordered_partitions(n, m=None, sort=True):
    """Generates ordered partitions of integer ``n``.

    Parameters
    ==========

    m : integer (default None)
        The default value gives partitions of all sizes else only
        those with size m. In addition, if ``m`` is not None then
        partitions are generated *in place* (see examples).
    sort : bool (default True)
        Controls whether partitions are
        returned in sorted order when ``m`` is not None; when False,
        the partitions are returned as fast as possible with elements
        sorted, but when m|n the partitions will not be in
        ascending lexicographical order.

    Examples
    ========

    >>> from sympy.utilities.iterables import ordered_partitions

    All partitions of 5 in ascending lexicographical:

    >>> for p in ordered_partitions(5):
    ...     print(p)
    [1, 1, 1, 1, 1]
    [1, 1, 1, 2]
    [1, 1, 3]
    [1, 2, 2]
    [1, 4]
    [2, 3]
    [5]

    Only partitions of 5 with two parts:

    >>> for p in ordered_partitions(5, 2):
    ...     print(p)
    [1, 4]
    [2, 3]

    When ``m`` is given, a given list objects will be used more than
    once for speed reasons so you will not see the correct partitions
    unless you make a copy of each as it is generated:

    >>> [p for p in ordered_partitions(7, 3)]
    [[1, 1, 1], [1, 1, 1], [1, 1, 1], [2, 2, 2]]
    >>> [list(p) for p in ordered_partitions(7, 3)]
    [[1, 1, 5], [1, 2, 4], [1, 3, 3], [2, 2, 3]]

    When ``n`` is a multiple of ``m``, the elements are still sorted
    but the partitions themselves will be *unordered* if sort is False;
    the default is to return them in ascending lexicographical order.

    >>> for p in ordered_partitions(6, 2):
    ...     print(p)
    [1, 5]
    [2, 4]
    [3, 3]

    But if speed is more important than ordering, sort can be set to
    False:

    >>> for p in ordered_partitions(6, 2, sort=False):
    ...     print(p)
    [1, 5]
    [3, 3]
    [2, 4]

    References
    ==========

    .. [1] Generating Integer Partitions, [online],
        Available: https://jeromekelleher.net/generating-integer-partitions.html
    .. [2] Jerome Kelleher and Barry O'Sullivan, "Generating All
        Partitions: A Comparison Of Two Encodings", [online],
        Available: https://arxiv.org/pdf/0909.2331v2.pdf
    """
    if n < 1 or m is not None and m < 1:
        # the empty set is the only way to handle these inputs
        # and returning {} to represent it is consistent with
        # the counting convention, e.g. nT(0) == 1.
        yield []
        return

    if m is None:
        # The list `a`'s leading elements contain the partition in which
        # y is the biggest element and x is either the same as y or the
        # 2nd largest element; v and w are adjacent element indices
        # to which x and y are being assigned, respectively.
        a = [1]*n
        y = -1
        v = n
        while v > 0:
            v -= 1
            x = a[v] + 1
            while y >= 2 * x:
                a[v] = x
                y -= x
                v += 1
            w = v + 1
            while x <= y:
                a[v] = x
                a[w] = y
                yield a[:w + 1]
                x += 1
                y -= 1
            a[v] = x + y
            y = a[v] - 1
            yield a[:w]
    elif m == 1:
        yield [n]
    elif n == m:
        yield [1]*n
    else:
        # recursively generate partitions of size m
        for b in range(1, n//m + 1):
            a = [b]*m
            x = n - b*m
            if not x:
                if sort:
                    yield a
            elif not sort and x <= m:
                for ax in ordered_partitions(x, sort=False):
                    mi = len(ax)
                    a[-mi:] = [i + b for i in ax]
                    yield a
                    a[-mi:] = [b]*mi
            else:
                for mi in range(1, m):
                    for ax in ordered_partitions(x, mi, sort=True):
                        a[-mi:] = [i + b for i in ax]
                        yield a
                        a[-mi:] = [b]*mi


def binary_partitions(n):
    """
    Generates the binary partition of n.

    A binary partition consists only of numbers that are
    powers of two. Each step reduces a `2^{k+1}` to `2^k` and
    `2^k`. Thus 16 is converted to 8 and 8.

    Examples
    ========

    >>> from sympy.utilities.iterables import binary_partitions
    >>> for i in binary_partitions(5):
    ...     print(i)
    ...
    [4, 1]
    [2, 2, 1]
    [2, 1, 1, 1]
    [1, 1, 1, 1, 1]

    References
    ==========

    .. [1] TAOCP 4, section 7.2.1.5, problem 64

    """
    from math import ceil, log
    pow = int(2**(ceil(log(n, 2))))
    sum = 0
    partition = []
    while pow:
        if sum + pow <= n:
            partition.append(pow)
            sum += pow
        pow >>= 1

    last_num = len(partition) - 1 - (n & 1)
    while last_num >= 0:
        yield partition
        if partition[last_num] == 2:
            partition[last_num] = 1
            partition.append(1)
            last_num -= 1
            continue
        partition.append(1)
        partition[last_num] >>= 1
        x = partition[last_num + 1] = partition[last_num]
        last_num += 1
        while x > 1:
            if x <= len(partition) - last_num - 1:
                del partition[-x + 1:]
                last_num += 1
                partition[last_num] = x
            else:
                x >>= 1
    yield [1]*n


def has_dups(seq):
    """Return True if there are any duplicate elements in ``seq``.

    Examples
    ========

    >>> from sympy.utilities.iterables import has_dups
    >>> from sympy import Dict, Set

    >>> has_dups((1, 2, 1))
    True
    >>> has_dups(range(3))
    False
    >>> all(has_dups(c) is False for c in (set(), Set(), dict(), Dict()))
    True
    """
    from sympy.core.containers import Dict
    from sympy.sets.sets import Set
    if isinstance(seq, (dict, set, Dict, Set)):
        return False
    uniq = set()
    return any(True for s in seq if s in uniq or uniq.add(s))


def has_variety(seq):
    """Return True if there are any different elements in ``seq``.

    Examples
    ========

    >>> from sympy.utilities.iterables import has_variety

    >>> has_variety((1, 2, 1))
    True
    >>> has_variety((1, 1, 1))
    False
    """
    for i, s in enumerate(seq):
        if i == 0:
            sentinel = s
        else:
            if s != sentinel:
                return True
    return False


def uniq(seq, result=None):
    """
    Yield unique elements from ``seq`` as an iterator. The second
    parameter ``result``  is used internally; it is not necessary
    to pass anything for this.

    Note: changing the sequence during iteration will raise a
    RuntimeError if the size of the sequence is known; if you pass
    an iterator and advance the iterator you will change the
    output of this routine but there will be no warning.

    Examples
    ========

    >>> from sympy.utilities.iterables import uniq
    >>> dat = [1, 4, 1, 5, 4, 2, 1, 2]
    >>> type(uniq(dat)) in (list, tuple)
    False

    >>> list(uniq(dat))
    [1, 4, 5, 2]
    >>> list(uniq(x for x in dat))
    [1, 4, 5, 2]
    >>> list(uniq([[1], [2, 1], [1]]))
    [[1], [2, 1]]
    """
    try:
        n = len(seq)
    except TypeError:
        n = None
    def check():
        # check that size of seq did not change during iteration;
        # if n == None the object won't support size changing, e.g.
        # an iterator can't be changed
        if n is not None and len(seq) != n:
            raise RuntimeError('sequence changed size during iteration')
    try:
        seen = set()
        result = result or []
        for i, s in enumerate(seq):
            if not (s in seen or seen.add(s)):
                yield s
                check()
    except TypeError:
        if s not in result:
            yield s
            check()
            result.append(s)
        if hasattr(seq, '__getitem__'):
            yield from uniq(seq[i + 1:], result)
        else:
            yield from uniq(seq, result)


def generate_bell(n):
    """Return permutations of [0, 1, ..., n - 1] such that each permutation
    differs from the last by the exchange of a single pair of neighbors.
    The ``n!`` permutations are returned as an iterator. In order to obtain
    the next permutation from a random starting permutation, use the
    ``next_trotterjohnson`` method of the Permutation class (which generates
    the same sequence in a different manner).

    Examples
    ========

    >>> from itertools import permutations
    >>> from sympy.utilities.iterables import generate_bell
    >>> from sympy import zeros, Matrix

    This is the sort of permutation used in the ringing of physical bells,
    and does not produce permutations in lexicographical order. Rather, the
    permutations differ from each other by exactly one inversion, and the
    position at which the swapping occurs varies periodically in a simple
    fashion. Consider the first few permutations of 4 elements generated
    by ``permutations`` and ``generate_bell``:

    >>> list(permutations(range(4)))[:5]
    [(0, 1, 2, 3), (0, 1, 3, 2), (0, 2, 1, 3), (0, 2, 3, 1), (0, 3, 1, 2)]
    >>> list(generate_bell(4))[:5]
    [(0, 1, 2, 3), (0, 1, 3, 2), (0, 3, 1, 2), (3, 0, 1, 2), (3, 0, 2, 1)]

    Notice how the 2nd and 3rd lexicographical permutations have 3 elements
    out of place whereas each "bell" permutation always has only two
    elements out of place relative to the previous permutation (and so the
    signature (+/-1) of a permutation is opposite of the signature of the
    previous permutation).

    How the position of inversion varies across the elements can be seen
    by tracing out where the largest number appears in the permutations:

    >>> m = zeros(4, 24)
    >>> for i, p in enumerate(generate_bell(4)):
    ...     m[:, i] = Matrix([j - 3 for j in list(p)])  # make largest zero
    >>> m.print_nonzero('X')
    [XXX  XXXXXX  XXXXXX  XXX]
    [XX XX XXXX XX XXXX XX XX]
    [X XXXX XX XXXX XX XXXX X]
    [ XXXXXX  XXXXXX  XXXXXX ]

    See Also
    ========

    sympy.combinatorics.permutations.Permutation.next_trotterjohnson

    References
    ==========

    .. [1] https://en.wikipedia.org/wiki/Method_ringing

    .. [2] https://stackoverflow.com/questions/4856615/recursive-permutation/4857018

    .. [3] http://programminggeeks.com/bell-algorithm-for-permutation/

    .. [4] https://en.wikipedia.org/wiki/Steinhaus%E2%80%93Johnson%E2%80%93Trotter_algorithm

    .. [5] Generating involutions, derangements, and relatives by ECO
           Vincent Vajnovszki, DMTCS vol 1 issue 12, 2010

    """
    n = as_int(n)
    if n < 1:
        raise ValueError('n must be a positive integer')
    if n == 1:
        yield (0,)
    elif n == 2:
        yield (0, 1)
        yield (1, 0)
    elif n == 3:
        yield from [(0, 1, 2), (0, 2, 1), (2, 0, 1), (2, 1, 0), (1, 2, 0), (1, 0, 2)]
    else:
        m = n - 1
        op = [0] + [-1]*m
        l = list(range(n))
        while True:
            yield tuple(l)
            # find biggest element with op
            big = None, -1  # idx, value
            for i in range(n):
                if op[i] and l[i] > big[1]:
                    big = i, l[i]
            i, _ = big
            if i is None:
                break  # there are no ops left
            # swap it with neighbor in the indicated direction
            j = i + op[i]
            l[i], l[j] = l[j], l[i]
            op[i], op[j] = op[j], op[i]
            # if it landed at the end or if the neighbor in the same
            # direction is bigger then turn off op
            if j == 0 or j == m or l[j + op[j]] > l[j]:
                op[j] = 0
            # any element bigger to the left gets +1 op
            for i in range(j):
                if l[i] > l[j]:
                    op[i] = 1
            # any element bigger to the right gets -1 op
            for i in range(j + 1, n):
                if l[i] > l[j]:
                    op[i] = -1


def generate_involutions(n):
    """
    Generates involutions.

    An involution is a permutation that when multiplied
    by itself equals the identity permutation. In this
    implementation the involutions are generated using
    Fixed Points.

    Alternatively, an involution can be considered as
    a permutation that does not contain any cycles with
    a length that is greater than two.

    Examples
    ========

    >>> from sympy.utilities.iterables import generate_involutions
    >>> list(generate_involutions(3))
    [(0, 1, 2), (0, 2, 1), (1, 0, 2), (2, 1, 0)]
    >>> len(list(generate_involutions(4)))
    10

    References
    ==========

    .. [1] http://mathworld.wolfram.com/PermutationInvolution.html

    """
    idx = list(range(n))
    for p in permutations(idx):
        for i in idx:
            if p[p[i]] != i:
                break
        else:
            yield p


def generate_derangements(perm):
    """
    Routine to generate unique derangements.

    TODO: This will be rewritten to use the
    ECO operator approach once the permutations
    branch is in master.

    Examples
    ========

    >>> from sympy.utilities.iterables import generate_derangements
    >>> list(generate_derangements([0, 1, 2]))
    [[1, 2, 0], [2, 0, 1]]
    >>> list(generate_derangements([0, 1, 2, 3]))
    [[1, 0, 3, 2], [1, 2, 3, 0], [1, 3, 0, 2], [2, 0, 3, 1], \
    [2, 3, 0, 1], [2, 3, 1, 0], [3, 0, 1, 2], [3, 2, 0, 1], \
    [3, 2, 1, 0]]
    >>> list(generate_derangements([0, 1, 1]))
    []

    See Also
    ========

    sympy.functions.combinatorial.factorials.subfactorial

    """
    for p in multiset_permutations(perm):
        if not any(i == j for i, j in zip(perm, p)):
            yield p


def necklaces(n, k, free=False):
    """
    A routine to generate necklaces that may (free=True) or may not
    (free=False) be turned over to be viewed. The "necklaces" returned
    are comprised of ``n`` integers (beads) with ``k`` different
    values (colors). Only unique necklaces are returned.

    Examples
    ========

    >>> from sympy.utilities.iterables import necklaces, bracelets
    >>> def show(s, i):
    ...     return ''.join(s[j] for j in i)

    The "unrestricted necklace" is sometimes also referred to as a
    "bracelet" (an object that can be turned over, a sequence that can
    be reversed) and the term "necklace" is used to imply a sequence
    that cannot be reversed. So ACB == ABC for a bracelet (rotate and
    reverse) while the two are different for a necklace since rotation
    alone cannot make the two sequences the same.

    (mnemonic: Bracelets can be viewed Backwards, but Not Necklaces.)

    >>> B = [show('ABC', i) for i in bracelets(3, 3)]
    >>> N = [show('ABC', i) for i in necklaces(3, 3)]
    >>> set(N) - set(B)
    {'ACB'}

    >>> list(necklaces(4, 2))
    [(0, 0, 0, 0), (0, 0, 0, 1), (0, 0, 1, 1),
     (0, 1, 0, 1), (0, 1, 1, 1), (1, 1, 1, 1)]

    >>> [show('.o', i) for i in bracelets(4, 2)]
    ['....', '...o', '..oo', '.o.o', '.ooo', 'oooo']

    References
    ==========

    .. [1] http://mathworld.wolfram.com/Necklace.html

    """
    return uniq(minlex(i, directed=not free) for i in
        variations(list(range(k)), n, repetition=True))


def bracelets(n, k):
    """Wrapper to necklaces to return a free (unrestricted) necklace."""
    return necklaces(n, k, free=True)


def generate_oriented_forest(n):
    """
    This algorithm generates oriented forests.

    An oriented graph is a directed graph having no symmetric pair of directed
    edges. A forest is an acyclic graph, i.e., it has no cycles. A forest can
    also be described as a disjoint union of trees, which are graphs in which
    any two vertices are connected by exactly one simple path.

    Examples
    ========

    >>> from sympy.utilities.iterables import generate_oriented_forest
    >>> list(generate_oriented_forest(4))
    [[0, 1, 2, 3], [0, 1, 2, 2], [0, 1, 2, 1], [0, 1, 2, 0], \
    [0, 1, 1, 1], [0, 1, 1, 0], [0, 1, 0, 1], [0, 1, 0, 0], [0, 0, 0, 0]]

    References
    ==========

    .. [1] T. Beyer and S.M. Hedetniemi: constant time generation of
           rooted trees, SIAM J. Computing Vol. 9, No. 4, November 1980

    .. [2] https://stackoverflow.com/questions/1633833/oriented-forest-taocp-algorithm-in-python

    """
    P = list(range(-1, n))
    while True:
        yield P[1:]
        if P[n] > 0:
            P[n] = P[P[n]]
        else:
            for p in range(n - 1, 0, -1):
                if P[p] != 0:
                    target = P[p] - 1
                    for q in range(p - 1, 0, -1):
                        if P[q] == target:
                            break
                    offset = p - q
                    for i in range(p, n + 1):
                        P[i] = P[i - offset]
                    break
            else:
                break


def minlex(seq, directed=True, key=None):
    """
    Return the rotation of the sequence in which the lexically smallest
    elements appear first, e.g. `cba ->acb`.

    The sequence returned is a tuple, unless the input sequence is a string
    in which case a string is returned.

    If ``directed`` is False then the smaller of the sequence and the
    reversed sequence is returned, e.g. `cba -> abc`.

    If ``key`` is not None then it is used to extract a comparison key from each element in iterable.

    Examples
    ========

    >>> from sympy.combinatorics.polyhedron import minlex
    >>> minlex((1, 2, 0))
    (0, 1, 2)
    >>> minlex((1, 0, 2))
    (0, 2, 1)
    >>> minlex((1, 0, 2), directed=False)
    (0, 1, 2)

    >>> minlex('11010011000', directed=True)
    '00011010011'
    >>> minlex('11010011000', directed=False)
    '00011001011'

    >>> minlex(('bb', 'aaa', 'c', 'a'))
    ('a', 'bb', 'aaa', 'c')
    >>> minlex(('bb', 'aaa', 'c', 'a'), key=len)
    ('c', 'a', 'bb', 'aaa')

    """

    if key is None: key = sympy.Id
    best = rotate_left(seq, least_rotation(seq, key=key))
    if not directed:
        rseq = seq[::-1]
        rbest = rotate_left(rseq, least_rotation(rseq, key=key))
        best = min(best, rbest, key=key)

    # Convert to tuple, unless we started with a string.
    return tuple(best) if not isinstance(seq, str) else best


def runs(seq, op=gt):
    """Group the sequence into lists in which successive elements
    all compare the same with the comparison operator, ``op``:
    op(seq[i + 1], seq[i]) is True from all elements in a run.

    Examples
    ========

    >>> from sympy.utilities.iterables import runs
    >>> from operator import ge
    >>> runs([0, 1, 2, 2, 1, 4, 3, 2, 2])
    [[0, 1, 2], [2], [1, 4], [3], [2], [2]]
    >>> runs([0, 1, 2, 2, 1, 4, 3, 2, 2], op=ge)
    [[0, 1, 2, 2], [1, 4], [3], [2, 2]]
    """
    cycles = []
    seq = iter(seq)
    try:
        run = [next(seq)]
    except StopIteration:
        return []
    while True:
        try:
            ei = next(seq)
        except StopIteration:
            break
        if op(ei, run[-1]):
            run.append(ei)
            continue
        else:
            cycles.append(run)
            run = [ei]
    if run:
        cycles.append(run)
    return cycles


def kbins(l, k, ordered=None):
    """
    Return sequence ``l`` partitioned into ``k`` bins.

    Examples
    ========

    >>> from __future__ import print_function

    The default is to give the items in the same order, but grouped
    into k partitions without any reordering:

    >>> from sympy.utilities.iterables import kbins
    >>> for p in kbins(list(range(5)), 2):
    ...     print(p)
    ...
    [[0], [1, 2, 3, 4]]
    [[0, 1], [2, 3, 4]]
    [[0, 1, 2], [3, 4]]
    [[0, 1, 2, 3], [4]]

    The ``ordered`` flag is either None (to give the simple partition
    of the elements) or is a 2 digit integer indicating whether the order of
    the bins and the order of the items in the bins matters. Given::

        A = [[0], [1, 2]]
        B = [[1, 2], [0]]
        C = [[2, 1], [0]]
        D = [[0], [2, 1]]

    the following values for ``ordered`` have the shown meanings::

        00 means A == B == C == D
        01 means A == B
        10 means A == D
        11 means A == A

    >>> for ordered_flag in [None, 0, 1, 10, 11]:
    ...     print('ordered = %s' % ordered_flag)
    ...     for p in kbins(list(range(3)), 2, ordered=ordered_flag):
    ...         print('     %s' % p)
    ...
    ordered = None
         [[0], [1, 2]]
         [[0, 1], [2]]
    ordered = 0
         [[0, 1], [2]]
         [[0, 2], [1]]
         [[0], [1, 2]]
    ordered = 1
         [[0], [1, 2]]
         [[0], [2, 1]]
         [[1], [0, 2]]
         [[1], [2, 0]]
         [[2], [0, 1]]
         [[2], [1, 0]]
    ordered = 10
         [[0, 1], [2]]
         [[2], [0, 1]]
         [[0, 2], [1]]
         [[1], [0, 2]]
         [[0], [1, 2]]
         [[1, 2], [0]]
    ordered = 11
         [[0], [1, 2]]
         [[0, 1], [2]]
         [[0], [2, 1]]
         [[0, 2], [1]]
         [[1], [0, 2]]
         [[1, 0], [2]]
         [[1], [2, 0]]
         [[1, 2], [0]]
         [[2], [0, 1]]
         [[2, 0], [1]]
         [[2], [1, 0]]
         [[2, 1], [0]]

    See Also
    ========

    partitions, multiset_partitions

    """
    def partition(lista, bins):
        #  EnricoGiampieri's partition generator from
        #  https://stackoverflow.com/questions/13131491/
        #  partition-n-items-into-k-bins-in-python-lazily
        if len(lista) == 1 or bins == 1:
            yield [lista]
        elif len(lista) > 1 and bins > 1:
            for i in range(1, len(lista)):
                for part in partition(lista[i:], bins - 1):
                    if len([lista[:i]] + part) == bins:
                        yield [lista[:i]] + part

    if ordered is None:
        yield from partition(l, k)
    elif ordered == 11:
        for pl in multiset_permutations(l):
            pl = list(pl)
            yield from partition(pl, k)
    elif ordered == 00:
        yield from multiset_partitions(l, k)
    elif ordered == 10:
        for p in multiset_partitions(l, k):
            for perm in permutations(p):
                yield list(perm)
    elif ordered == 1:
        for kgot, p in partitions(len(l), k, size=True):
            if kgot != k:
                continue
            for li in multiset_permutations(l):
                rv = []
                i = j = 0
                li = list(li)
                for size, multiplicity in sorted(p.items()):
                    for m in range(multiplicity):
                        j = i + size
                        rv.append(li[i: j])
                        i = j
                yield rv
    else:
        raise ValueError(
            'ordered must be one of 00, 01, 10 or 11, not %s' % ordered)


def permute_signs(t):
    """Return iterator in which the signs of non-zero elements
    of t are permuted.

    Examples
    ========

    >>> from sympy.utilities.iterables import permute_signs
    >>> list(permute_signs((0, 1, 2)))
    [(0, 1, 2), (0, -1, 2), (0, 1, -2), (0, -1, -2)]
    """
    for signs in cartes(*[(1, -1)]*(len(t) - t.count(0))):
        signs = list(signs)
        yield type(t)([i*signs.pop() if i else i for i in t])


def signed_permutations(t):
    """Return iterator in which the signs of non-zero elements
    of t and the order of the elements are permuted.

    Examples
    ========

    >>> from sympy.utilities.iterables import signed_permutations
    >>> list(signed_permutations((0, 1, 2)))
    [(0, 1, 2), (0, -1, 2), (0, 1, -2), (0, -1, -2), (0, 2, 1),
    (0, -2, 1), (0, 2, -1), (0, -2, -1), (1, 0, 2), (-1, 0, 2),
    (1, 0, -2), (-1, 0, -2), (1, 2, 0), (-1, 2, 0), (1, -2, 0),
    (-1, -2, 0), (2, 0, 1), (-2, 0, 1), (2, 0, -1), (-2, 0, -1),
    (2, 1, 0), (-2, 1, 0), (2, -1, 0), (-2, -1, 0)]
    """
    return (type(t)(i) for j in permutations(t)
        for i in permute_signs(j))


def rotations(s, dir=1):
    """Return a generator giving the items in s as list where
    each subsequent list has the items rotated to the left (default)
    or right (dir=-1) relative to the previous list.

    Examples
    ========

    >>> from sympy.utilities.iterables import rotations
    >>> list(rotations([1,2,3]))
    [[1, 2, 3], [2, 3, 1], [3, 1, 2]]
    >>> list(rotations([1,2,3], -1))
    [[1, 2, 3], [3, 1, 2], [2, 3, 1]]
    """
    seq = list(s)
    for i in range(len(seq)):
        yield seq
        seq = rotate_left(seq, dir)


def roundrobin(*iterables):
    """roundrobin recipe taken from itertools documentation:
    https://docs.python.org/2/library/itertools.html#recipes

    roundrobin('ABC', 'D', 'EF') --> A D E B F C

    Recipe credited to George Sakkis
    """
    import itertools

    nexts = itertools.cycle(iter(it).__next__ for it in iterables)

    pending = len(iterables)
    while pending:
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            pending -= 1
            nexts = itertools.cycle(itertools.islice(nexts, pending))
