--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/env/lib/python3.7/site-packages/networkx/utils/heaps.py	Thu May 14 14:56:58 2020 -0400
@@ -0,0 +1,369 @@
+"""
+Min-heaps.
+"""
+
+__author__ = """ysitu <ysitu@users.noreply.github.com>"""
+# Copyright (C) 2014 ysitu <ysitu@users.noreply.github.com>
+# All rights reserved.
+# BSD license.
+
+from heapq import heappop, heappush
+from itertools import count
+import networkx as nx
+
+__all__ = ['MinHeap', 'PairingHeap', 'BinaryHeap']
+
+
+class MinHeap(object):
+    """Base class for min-heaps.
+
+    A MinHeap stores a collection of key-value pairs ordered by their values.
+    It supports querying the minimum pair, inserting a new pair, decreasing the
+    value in an existing pair and deleting the minimum pair.
+    """
+
+    class _Item(object):
+        """Used by subclassess to represent a key-value pair.
+        """
+        __slots__ = ('key', 'value')
+
+        def __init__(self, key, value):
+            self.key = key
+            self.value = value
+
+        def __repr__(self):
+            return repr((self.key, self.value))
+
+    def __init__(self):
+        """Initialize a new min-heap.
+        """
+        self._dict = {}
+
+    def min(self):
+        """Query the minimum key-value pair.
+
+        Returns
+        -------
+        key, value : tuple
+            The key-value pair with the minimum value in the heap.
+
+        Raises
+        ------
+        NetworkXError
+            If the heap is empty.
+        """
+        raise NotImplementedError
+
+    def pop(self):
+        """Delete the minimum pair in the heap.
+
+        Returns
+        -------
+        key, value : tuple
+            The key-value pair with the minimum value in the heap.
+
+        Raises
+        ------
+        NetworkXError
+            If the heap is empty.
+        """
+        raise NotImplementedError
+
+    def get(self, key, default=None):
+        """Returns the value associated with a key.
+
+        Parameters
+        ----------
+        key : hashable object
+            The key to be looked up.
+
+        default : object
+            Default value to return if the key is not present in the heap.
+            Default value: None.
+
+        Returns
+        -------
+        value : object.
+            The value associated with the key.
+        """
+        raise NotImplementedError
+
+    def insert(self, key, value, allow_increase=False):
+        """Insert a new key-value pair or modify the value in an existing
+        pair.
+
+        Parameters
+        ----------
+        key : hashable object
+            The key.
+
+        value : object comparable with existing values.
+            The value.
+
+        allow_increase : bool
+            Whether the value is allowed to increase. If False, attempts to
+            increase an existing value have no effect. Default value: False.
+
+        Returns
+        -------
+        decreased : bool
+            True if a pair is inserted or the existing value is decreased.
+        """
+        raise NotImplementedError
+
+    def __nonzero__(self):
+        """Returns whether the heap if empty.
+        """
+        return bool(self._dict)
+
+    def __bool__(self):
+        """Returns whether the heap if empty.
+        """
+        return bool(self._dict)
+
+    def __len__(self):
+        """Returns the number of key-value pairs in the heap.
+        """
+        return len(self._dict)
+
+    def __contains__(self, key):
+        """Returns whether a key exists in the heap.
+
+        Parameters
+        ----------
+        key : any hashable object.
+            The key to be looked up.
+        """
+        return key in self._dict
+
+
+def _inherit_doc(cls):
+    """Decorator for inheriting docstrings from base classes.
+    """
+    def func(fn):
+        fn.__doc__ = cls.__dict__[fn.__name__].__doc__
+        return fn
+    return func
+
+
+class PairingHeap(MinHeap):
+    """A pairing heap.
+    """
+
+    class _Node(MinHeap._Item):
+        """A node in a pairing heap.
+
+        A tree in a pairing heap is stored using the left-child, right-sibling
+        representation.
+        """
+        __slots__ = ('left', 'next', 'prev', 'parent')
+
+        def __init__(self, key, value):
+            super(PairingHeap._Node, self).__init__(key, value)
+            # The leftmost child.
+            self.left = None
+            # The next sibling.
+            self.next = None
+            # The previous sibling.
+            self.prev = None
+            # The parent.
+            self.parent = None
+
+    def __init__(self):
+        """Initialize a pairing heap.
+        """
+        super(PairingHeap, self).__init__()
+        self._root = None
+
+    @_inherit_doc(MinHeap)
+    def min(self):
+        if self._root is None:
+            raise nx.NetworkXError('heap is empty.')
+        return (self._root.key, self._root.value)
+
+    @_inherit_doc(MinHeap)
+    def pop(self):
+        if self._root is None:
+            raise nx.NetworkXError('heap is empty.')
+        min_node = self._root
+        self._root = self._merge_children(self._root)
+        del self._dict[min_node.key]
+        return (min_node.key, min_node.value)
+
+    @_inherit_doc(MinHeap)
+    def get(self, key, default=None):
+        node = self._dict.get(key)
+        return node.value if node is not None else default
+
+    @_inherit_doc(MinHeap)
+    def insert(self, key, value, allow_increase=False):
+        node = self._dict.get(key)
+        root = self._root
+        if node is not None:
+            if value < node.value:
+                node.value = value
+                if node is not root and value < node.parent.value:
+                    self._cut(node)
+                    self._root = self._link(root, node)
+                return True
+            elif allow_increase and value > node.value:
+                node.value = value
+                child = self._merge_children(node)
+                # Nonstandard step: Link the merged subtree with the root. See
+                # below for the standard step.
+                if child is not None:
+                    self._root = self._link(self._root, child)
+                # Standard step: Perform a decrease followed by a pop as if the
+                # value were the smallest in the heap. Then insert the new
+                # value into the heap.
+                # if node is not root:
+                #     self._cut(node)
+                #     if child is not None:
+                #         root = self._link(root, child)
+                #     self._root = self._link(root, node)
+                # else:
+                #     self._root = (self._link(node, child)
+                #                   if child is not None else node)
+            return False
+        else:
+            # Insert a new key.
+            node = self._Node(key, value)
+            self._dict[key] = node
+            self._root = self._link(root, node) if root is not None else node
+            return True
+
+    def _link(self, root, other):
+        """Link two nodes, making the one with the smaller value the parent of
+        the other.
+        """
+        if other.value < root.value:
+            root, other = other, root
+        next = root.left
+        other.next = next
+        if next is not None:
+            next.prev = other
+        other.prev = None
+        root.left = other
+        other.parent = root
+        return root
+
+    def _merge_children(self, root):
+        """Merge the subtrees of the root using the standard two-pass method.
+        The resulting subtree is detached from the root.
+        """
+        node = root.left
+        root.left = None
+        if node is not None:
+            link = self._link
+            # Pass 1: Merge pairs of consecutive subtrees from left to right.
+            # At the end of the pass, only the prev pointers of the resulting
+            # subtrees have meaningful values. The other pointers will be fixed
+            # in pass 2.
+            prev = None
+            while True:
+                next = node.next
+                if next is None:
+                    node.prev = prev
+                    break
+                next_next = next.next
+                node = link(node, next)
+                node.prev = prev
+                prev = node
+                if next_next is None:
+                    break
+                node = next_next
+            # Pass 2: Successively merge the subtrees produced by pass 1 from
+            # right to left with the rightmost one.
+            prev = node.prev
+            while prev is not None:
+                prev_prev = prev.prev
+                node = link(prev, node)
+                prev = prev_prev
+            # Now node can become the new root. Its has no parent nor siblings.
+            node.prev = None
+            node.next = None
+            node.parent = None
+        return node
+
+    def _cut(self, node):
+        """Cut a node from its parent.
+        """
+        prev = node.prev
+        next = node.next
+        if prev is not None:
+            prev.next = next
+        else:
+            node.parent.left = next
+        node.prev = None
+        if next is not None:
+            next.prev = prev
+            node.next = None
+        node.parent = None
+
+
+class BinaryHeap(MinHeap):
+    """A binary heap.
+    """
+
+    def __init__(self):
+        """Initialize a binary heap.
+        """
+        super(BinaryHeap, self).__init__()
+        self._heap = []
+        self._count = count()
+
+    @_inherit_doc(MinHeap)
+    def min(self):
+        dict = self._dict
+        if not dict:
+            raise nx.NetworkXError('heap is empty')
+        heap = self._heap
+        pop = heappop
+        # Repeatedly remove stale key-value pairs until a up-to-date one is
+        # met.
+        while True:
+            value, _, key = heap[0]
+            if key in dict and value == dict[key]:
+                break
+            pop(heap)
+        return (key, value)
+
+    @_inherit_doc(MinHeap)
+    def pop(self):
+        dict = self._dict
+        if not dict:
+            raise nx.NetworkXError('heap is empty')
+        heap = self._heap
+        pop = heappop
+        # Repeatedly remove stale key-value pairs until a up-to-date one is
+        # met.
+        while True:
+            value, _, key = heap[0]
+            pop(heap)
+            if key in dict and value == dict[key]:
+                break
+        del dict[key]
+        return (key, value)
+
+    @_inherit_doc(MinHeap)
+    def get(self, key, default=None):
+        return self._dict.get(key, default)
+
+    @_inherit_doc(MinHeap)
+    def insert(self, key, value, allow_increase=False):
+        dict = self._dict
+        if key in dict:
+            old_value = dict[key]
+            if value < old_value or (allow_increase and value > old_value):
+                # Since there is no way to efficiently obtain the location of a
+                # key-value pair in the heap, insert a new pair even if ones
+                # with the same key may already be present. Deem the old ones
+                # as stale and skip them when the minimum pair is queried.
+                dict[key] = value
+                heappush(self._heap, (value, next(self._count), key))
+                return value < old_value
+            return False
+        else:
+            dict[key] = value
+            heappush(self._heap, (value, next(self._count), key))
+            return True