[英]Finding a set of edges to add to a set of graphs to satisfy connectivity constraints
我有一组 N 个无向、断开连接的图,它们共享相同的 M 个顶点,但具有不同的边。 对于“V 0连接到 V 1 ”或“V 3未连接到 V 5 ”等形式的每个图形,我也有一组约束。
我想找到一组边,以便将这些边添加到每个图中会导致每个图都满足其约束。
举一个简单的例子,考虑两个顶点为 V 0 , V 1 , V 2 , V 3 , V 4的图:
使用这些给定的参数,可以通过将边 {(V 1 , V 2 ), (V 3 , V 4 )} 添加到两个图中来解决问题。
在未能用脚本解决问题后,我寻求 z3 提供帮助,但我在尝试表达连接时遇到了麻烦。 我目前的解决方案包括一个只有布尔项的未量化公式:
和断言:
然而,最后一个子句似乎没有按预期工作以表达连接性,并且 z3 正在返回sat
,但在结果模型中显然必要的 s i,j是错误的。
这是我能做到的最短的问题示例:
from itertools import combinations
from z3 import *
vertices = [0, 1, 2, 3, 4]
edges = [tuple(sorted(edge)) for edge in list(combinations(vertices, 2))]
graphs = {
"G": [(0, 1)],
"H": [(1, 3)],
}
facts = Solver()
connected_in_graph = {}
for graph in graphs:
for edge in edges:
connected_in_graph[(graph, edge)] = Bool(f"{edge}_conn_in_{graph}")
solution_edges = {}
graph_given_edges = {}
for edge in edges:
edge_in_solution = Bool(f"{edge}_in_solution")
solution_edges[edge] = edge_in_solution
for graph in graphs:
given = Bool(f"{graph}_{edge}_given")
graph_given_edges[(graph, edge)] = given
if edge in graphs[graph]:
facts.append(given)
else:
facts.append(Not(given))
facts.append(
connected_in_graph[("G", (0, 2))],
connected_in_graph[("H", (2, 4))],
Not(connected_in_graph[("H", (0, 2))]),
)
for edge in edges:
for graph in graphs:
ors = [
solution_edges[edge],
graph_given_edges[(graph, edge)],
]
for vertex in vertices:
if vertex in edge:
continue
l_edge = tuple(sorted((edge[0], vertex)))
r_edge = tuple(sorted((edge[1], vertex)))
ors.append(
And(
Or(
solution_edges[l_edge],
graph_given_edges[(graph, l_edge)],
),
connected_in_graph[(graph, r_edge)],
)
)
facts.append(connected_in_graph[(graph, edge)] == Or(*ors))
print(facts.check())
model = facts.model()
for edge in edges:
c = solution_edges[edge]
if model[c]:
print(c)
我想我实际上需要表达的,而不是最后一个约束,是关系:
c2( i , j , k , through ) = s i,j ∨ g i,j,k ∨ (∃z.z ∉ (忽略 ∪ {i, j}) ∧ (s i,z ∨ g i,z, k ) ∧ c(z, j, k, 忽略 ∪ {i}))
c( i , j , k ) = c2( i , j , k , {})
但是,将其简化为未量化的布尔项显然需要 M 的数量级! 时间和空间。
有没有更好的方法来表达 SAT/SMT 图形中路径的存在,或者完全解决这个问题的更好方法?
Alias 建议使用传递闭包似乎是解决此问题的方法,但我似乎无法正确使用它。 我修改后的代码:
from itertools import combinations
from z3 import *
vertices = [0, 1, 2, 3, 4]
edges = [tuple(sorted(edge)) for edge in list(combinations(vertices, 2))]
graphs = {
"G": [(0, 1)],
"H": [(1, 3)],
}
facts = Solver()
vs = {}
VertexSort = DeclareSort("VertexSort")
for vertex in vertices:
vs[vertex] = Const(f"V{vertex}", VertexSort)
facts.add(Distinct(*vs.values()))
given = {}
directly_connected = {}
tc_connected = {}
for graph in graphs:
given[graph] = Function(
f"{graph}_given", VertexSort, VertexSort, BoolSort()
)
directly_connected[graph] = Function(
f"directly_connected_{graph}", VertexSort, VertexSort, BoolSort()
)
tc_connected[graph] = TransitiveClosure(directly_connected[graph])
in_solution = Function("in_solution", VertexSort, VertexSort, BoolSort())
for edge in edges:
# commutativity
facts.add(
in_solution(vs[edge[0]], vs[edge[1]])
== in_solution(vs[edge[1]], vs[edge[0]])
)
for graph in graphs:
# commutativity
facts.add(
given[graph](vs[edge[0]], vs[edge[1]])
== given[graph](vs[edge[1]], vs[edge[0]])
)
facts.add(
directly_connected[graph](vs[edge[0]], vs[edge[1]])
== directly_connected[graph](vs[edge[1]], vs[edge[0]])
)
# definition of direct connection
facts.add(
directly_connected[graph](vs[edge[0]], vs[edge[1]])
== Or(
in_solution(vs[edge[0]], vs[edge[1]]),
given[graph](vs[edge[0]], vs[edge[1]]),
),
)
if edge in graphs[graph]:
facts.add(given[graph](vs[edge[0]], vs[edge[1]]))
else:
facts.add(Not(given[graph](vs[edge[0]], vs[edge[1]])))
facts.append(
tc_connected["G"](vs[0], vs[2]),
tc_connected["H"](vs[2], vs[4]),
Not(tc_connected["H"](vs[0], vs[2])),
)
print(facts.check())
model = facts.model()
print(model)
print(f"solution: {model[in_solution]}")
打印sat
但找到定义in_solution = [else -> False]
而不是我正在寻找的解决方案。 我究竟做错了什么?
正如@alias在此评论中所建议的那样,通过使用传递闭包来解决问题成为可能。
经过:
我让求解器为我的所有测试用例返回正确的解决方案。
修改后的代码:
from itertools import combinations
from z3 import *
vertices = [0, 1, 2, 3, 4]
edges = [tuple(sorted(edge)) for edge in list(combinations(vertices, 2))]
graphs = {
"G": [(0, 1)],
"H": [(1, 3)],
}
facts = Solver()
VertexSort = Datatype("VertexSort")
for vertex in vertices:
VertexSort.declare(str(vertex))
VertexSort = VertexSort.create()
vs = {}
for vertex in vertices:
vs[vertex] = getattr(VertexSort, str(vertex))
given = {}
directly_connected = {}
tc_connected = {}
for graph in graphs:
given[graph] = Function(
f"{graph}_given", VertexSort, VertexSort, BoolSort()
)
directly_connected[graph] = Function(
f"directly_connected_{graph}", VertexSort, VertexSort, BoolSort()
)
tc_connected[graph] = TransitiveClosure(directly_connected[graph])
in_solution = Function("in_solution", VertexSort, VertexSort, BoolSort())
for edge in edges:
# commutativity
facts.add(
in_solution(vs[edge[0]], vs[edge[1]])
== in_solution(vs[edge[1]], vs[edge[0]])
)
for graph in graphs:
# commutativity
facts.add(
given[graph](vs[edge[0]], vs[edge[1]])
== given[graph](vs[edge[1]], vs[edge[0]])
)
facts.add(
directly_connected[graph](vs[edge[0]], vs[edge[1]])
== directly_connected[graph](vs[edge[1]], vs[edge[0]])
)
# definition of direct connection
facts.add(
directly_connected[graph](vs[edge[0]], vs[edge[1]])
== Or(
in_solution(vs[edge[0]], vs[edge[1]]),
given[graph](vs[edge[0]], vs[edge[1]]),
),
)
if edge in graphs[graph]:
facts.add(given[graph](vs[edge[0]], vs[edge[1]]))
else:
facts.add(Not(given[graph](vs[edge[0]], vs[edge[1]])))
facts.append(
tc_connected["G"](vs[0], vs[2]),
tc_connected["H"](vs[2], vs[4]),
Not(tc_connected["H"](vs[0], vs[2])),
)
print(facts.check())
model = facts.model()
print(f"solution: {model[in_solution]}")
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