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name : structs.py
 import itertools

from .compat import collections_abc


class DirectedGraph(object):
    """A graph structure with directed edges."""

    def __init__(self):
        self._vertices = set()
        self._forwards = {}  # <key> -> Set[<key>]
        self._backwards = {}  # <key> -> Set[<key>]

    def __iter__(self):
        return iter(self._vertices)

    def __len__(self):
        return len(self._vertices)

    def __contains__(self, key):
        return key in self._vertices

    def copy(self):
        """Return a shallow copy of this graph."""
        other = DirectedGraph()
        other._vertices = set(self._vertices)
        other._forwards = {k: set(v) for k, v in self._forwards.items()}
        other._backwards = {k: set(v) for k, v in self._backwards.items()}
        return other

    def add(self, key):
        """Add a new vertex to the graph."""
        if key in self._vertices:
            raise ValueError("vertex exists")
        self._vertices.add(key)
        self._forwards[key] = set()
        self._backwards[key] = set()

    def remove(self, key):
        """Remove a vertex from the graph, disconnecting all edges from/to it."""
        self._vertices.remove(key)
        for f in self._forwards.pop(key):
            self._backwards[f].remove(key)
        for t in self._backwards.pop(key):
            self._forwards[t].remove(key)

    def connected(self, f, t):
        return f in self._backwards[t] and t in self._forwards[f]

    def connect(self, f, t):
        """Connect two existing vertices.

        Nothing happens if the vertices are already connected.
        """
        if t not in self._vertices:
            raise KeyError(t)
        self._forwards[f].add(t)
        self._backwards[t].add(f)

    def iter_edges(self):
        for f, children in self._forwards.items():
            for t in children:
                yield f, t

    def iter_children(self, key):
        return iter(self._forwards[key])

    def iter_parents(self, key):
        return iter(self._backwards[key])


class IteratorMapping(collections_abc.Mapping):
    def __init__(self, mapping, accessor, appends=None):
        self._mapping = mapping
        self._accessor = accessor
        self._appends = appends or {}

    def __repr__(self):
        return "IteratorMapping({!r}, {!r}, {!r})".format(
            self._mapping,
            self._accessor,
            self._appends,
        )

    def __bool__(self):
        return bool(self._mapping or self._appends)

    __nonzero__ = __bool__  # XXX: Python 2.

    def __contains__(self, key):
        return key in self._mapping or key in self._appends

    def __getitem__(self, k):
        try:
            v = self._mapping[k]
        except KeyError:
            return iter(self._appends[k])
        return itertools.chain(self._accessor(v), self._appends.get(k, ()))

    def __iter__(self):
        more = (k for k in self._appends if k not in self._mapping)
        return itertools.chain(self._mapping, more)

    def __len__(self):
        more = sum(1 for k in self._appends if k not in self._mapping)
        return len(self._mapping) + more


class _FactoryIterableView(object):
    """Wrap an iterator factory returned by `find_matches()`.

    Calling `iter()` on this class would invoke the underlying iterator
    factory, making it a "collection with ordering" that can be iterated
    through multiple times, but lacks random access methods presented in
    built-in Python sequence types.
    """

    def __init__(self, factory):
        self._factory = factory
        self._iterable = None

    def __repr__(self):
        return "{}({})".format(type(self).__name__, list(self))

    def __bool__(self):
        try:
            next(iter(self))
        except StopIteration:
            return False
        return True

    __nonzero__ = __bool__  # XXX: Python 2.

    def __iter__(self):
        iterable = (
            self._factory() if self._iterable is None else self._iterable
        )
        self._iterable, current = itertools.tee(iterable)
        return current


class _SequenceIterableView(object):
    """Wrap an iterable returned by find_matches().

    This is essentially just a proxy to the underlying sequence that provides
    the same interface as `_FactoryIterableView`.
    """

    def __init__(self, sequence):
        self._sequence = sequence

    def __repr__(self):
        return "{}({})".format(type(self).__name__, self._sequence)

    def __bool__(self):
        return bool(self._sequence)

    __nonzero__ = __bool__  # XXX: Python 2.

    def __iter__(self):
        return iter(self._sequence)


def build_iter_view(matches):
    """Build an iterable view from the value returned by `find_matches()`."""
    if callable(matches):
        return _FactoryIterableView(matches)
    if not isinstance(matches, collections_abc.Sequence):
        matches = list(matches)
    return _SequenceIterableView(matches)
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