def test_cut__leaves_bothTreesIntact(self):
# arrange
tree = LinkCutTree()
a1 = tree.makeTree('a1')
a2 = tree.makeTree('a2')
a3 = tree.makeTree('a3')
a4 = tree.makeTree('a4')
a5 = tree.makeTree('a5')
a6 = tree.makeTree('a6')
tree.link(a1,a2)
tree.link(a2,a3)
tree.link(a3,a4)
tree.link(a4,a5)
tree.link(a5,a6)
c1 = tree.makeTree('c1')
c2 = tree.makeTree('c2')
c3 = tree.makeTree('c3')
tree.link(c1,c2)
tree.link(c2,c3)
tree.link(a6,c1);
#act
tree.cut(c2)
#assert
self.assertEqual(a1, tree.getRoot(c1))
self.assertEqual(c2, tree.getRoot(c3))
def test_cut__leaves_bothTreesIntact(self):
# arrange
tree = LinkCutTree()
a1 = tree.makeTree('a1')
a2 = tree.makeTree('a2')
a3 = tree.makeTree('a3')
a4 = tree.makeTree('a4')
a5 = tree.makeTree('a5')
a6 = tree.makeTree('a6')
tree.link(a1, a2)
tree.link(a2, a3)
tree.link(a3, a4)
tree.link(a4, a5)
tree.link(a5, a6)
c1 = tree.makeTree('c1')
c2 = tree.makeTree('c2')
c3 = tree.makeTree('c3')
tree.link(c1, c2)
tree.link(c2, c3)
tree.link(a6, c1)
#act
tree.cut(c2)
#assert
self.assertEqual(a1, tree.getRoot(c1))
self.assertEqual(c2, tree.getRoot(c3))
class RetroactiveUnionFind(object):
"""Fully retroactive union find implemented using link-cut trees to represented disjoint forests"""
def __init__(self):
self.forest = LinkCutTree()
self.time = 0
# unionAgo(a,b) links nodes a and b in the LinkCutTree if they weren't already linked.
# If they were it cuts the latest edge on the path between a and b (if it is later than the new link time).
# The new subtree will contain exactly one of a and b. We make that node the root of the subtree and link it to
# the other node still in the main tree.
# This preserves old unions because any edge below the cut edge will now follow a path through the a-b edge up to
# the lca, which is guaranteed not to have an edge value greater than the cut edge. Any edge above the cut \
# edge will not be affected.
def unionAgo(self, a_data, b_data, tdelta=0):
# if the sets are already connected at the specified time return
if self.sameSetAgo(a_data, b_data, tdelta):
return
#get node objects to work with
a = self.forest.getNode(a_data)
b = self.forest.getNode(b_data)
union_time = self.time + tdelta
if a is None:
a = self.forest.makeTree(a_data)
if b is None:
b = self.forest.makeTree(b_data)
# if the nodes are not connected at all, connect them. If they are connected at a later time,
# cut the oldest edge on the path between the two nodes, make the union'ed node the root of that tree
# and attach it to the other union'ed node.
if self.forest.getRoot(a) != self.forest.getRoot(b):
self.forest.makeRoot(b)
self.forest.link(a, b, union_time)
else:
lca = self.forest.lca(a, b)
max_time = float("-inf")
max_time_node = None
for next in [a, b]:
while next is not lca:
if next.parent_edge_weight > max_time:
max_time = next.parent_edge_weight
max_time_node = next
next = next.represented_parent
self.forest.cut(max_time_node)
if self.forest.getRoot(a) == next:
self.forest.makeRoot(a)
self.forest.link(b, a, union_time)
else:
self.forest.makeRoot(b)
self.forest.link(a, b, union_time)
self.time += 1
# sameSetAgo(a,b,t) will find the lca of a and b and traverse the path from both to the lca,
# finding the largest edge on the path between a and b. If any edge is larger than time + tdelta then a, and b
# were not connected at (time + tdelta)
def sameSetAgo(self, a_data, b_data, tdelta=0):
# sameset is reflexive
if a_data == b_data:
return True
a = self.forest.getNode(a_data)
b = self.forest.getNode(b_data)
query_time = self.time + tdelta
if a is None or b is None:
return False
lca = self.forest.lca(a, b)
if lca is None:
return False
for next in [a, b]:
while next is not lca:
if next.parent_edge_weight > query_time:
return False
next = next.represented_parent
return True
# sameSetWhen(a,b) traverses the path between a and b and return the largest edge,
# which is the time at which a and b were connected.
def sameSetWhen(self, a, b):
lca = self.forest.lca(a, b)
if lca is None:
return float("-inf")
max_time = float("-inf")
for next in [a, b]:
while next is not lca:
if next.parent_edge_weight > max_time:
max_time = next.parent_edge_weight
next = next.represented_parent
return max_time
