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# Copyright (c) 2008-2009 Pedro Matiello <pmatiello@gmail.com>
#
# Permission is hereby granted, free of charge, to any person
# obtaining a copy of this software and associated documentation
# files (the "Software"), to deal in the Software without
# restriction, including without limitation the rights to use,
# copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following
# conditions:
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
# OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
# WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
# OTHER DEALINGS IN THE SOFTWARE.
"""
Cycle detection algorithms.
@sort: find_cycle
"""
# Imports
from pygraph.classes.exceptions import InvalidGraphType
from pygraph.classes.digraph import digraph as digraph_class
from pygraph.classes.graph import graph as graph_class
def find_cycle(graph):
"""
Find a cycle in the given graph.
This function will return a list of nodes which form a cycle in the graph or an empty list if
no cycle exists.
@type graph: graph, digraph
@param graph: Graph.
@rtype: list
@return: List of nodes.
"""
if (type(graph) == graph_class):
directed = False
elif (type(graph) == digraph_class):
directed = True
else:
raise InvalidGraphType
def find_cycle_to_ancestor(node, ancestor):
"""
Find a cycle containing both node and ancestor.
"""
path = []
orignode = node
while (node != ancestor):
if (node is None):
return []
path.append(node)
node = spanning_tree[node]
path.append(node)
path.reverse()
return path
def dfs(node):
"""
Depht-first search subfunction.
"""
visited[node] = 1
# Explore recursively the connected component
for each in graph[node]:
if (cycle):
return
if (each not in visited):
spanning_tree[each] = node
dfs(each)
else:
if (directed or spanning_tree[node] is not each):
cycle.extend(find_cycle_to_ancestor(node, each))
visited = {} # List for marking visited and non-visited nodes
spanning_tree = {} # Spanning tree
cycle = []
# Algorithm outer-loop
for each in graph:
# Select a non-visited node
if (each not in visited):
spanning_tree[each] = None
root = each
# Explore node's connected component
dfs(each)
if (cycle):
return cycle
return []