106 lines
3.4 KiB
Python
106 lines
3.4 KiB
Python
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#!/usr/bin/python
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# tsort.py
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# Topological sorting.
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#
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# Copyright (C) 2010 Red Hat, Inc.
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#
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# This copyrighted material is made available to anyone wishing to use,
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# modify, copy, or redistribute it subject to the terms and conditions of
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# the GNU General Public License v.2, or (at your option) any later version.
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# This program is distributed in the hope that it will be useful, but WITHOUT
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# ANY WARRANTY expressed or implied, including the implied warranties of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
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# Public License for more details. You should have received a copy of the
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# GNU General Public License along with this program; if not, write to the
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# Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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# 02110-1301, USA. Any Red Hat trademarks that are incorporated in the
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# source code or documentation are not subject to the GNU General Public
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# License and may only be used or replicated with the express permission of
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# Red Hat, Inc.
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#
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# Red Hat Author(s): Dave Lehman <dlehman@redhat.com>
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#
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class CyclicGraphError(Exception):
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pass
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def tsort(graph):
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order = [] # sorted list of items
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if not graph or not graph['items']:
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return order
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# determine which nodes have no incoming edges
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roots = [n for n in graph['items'] if graph['incoming'][n] == 0]
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if not roots:
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raise CyclicGraphError("no root nodes")
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visited = [] # list of nodes visited, for cycle detection
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while roots:
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# remove a root, add it to the order
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root = roots.pop()
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if root in visited:
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raise CyclicGraphError("graph contains cycles")
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visited.append(root)
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i = graph['items'].index(root)
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order.append(root)
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# remove each edge from the root to another node
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for (parent, child) in [e for e in graph['edges'] if e[0] == root]:
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graph['incoming'][child] -= 1
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graph['edges'].remove((parent, child))
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# if destination node is now a root, add it to roots
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if graph['incoming'][child] == 0:
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roots.append(child)
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if len(graph['items']) != len(visited):
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raise CyclicGraphError("graph contains cycles")
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return order
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def create_graph(items, edges):
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""" Create a graph based on a list of items and a list of edges.
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Arguments:
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items - an iterable containing (hashable) items to sort
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edges - an iterable containing (parent, child) edge pair tuples
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Return Value:
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The return value is a dictionary representing the directed graph.
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It has three keys:
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items is the same as the input argument of the same name
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edges is the same as the input argument of the same name
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incoming is a dict of incoming edge count hashed by item
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"""
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graph = {'items': [], # the items to sort
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'edges': [], # partial order info: (parent, child) pairs
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'incoming': {}} # incoming edge count for each item
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graph['items'] = items
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graph['edges'] = edges
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for item in items:
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graph['incoming'][item] = 0
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for (parent, child) in edges:
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graph['incoming'][child] += 1
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return graph
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if __name__ == "__main__":
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items = [5, 2, 3, 4, 1]
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edges = [(1, 2), (2, 4), (4, 5), (3, 2)]
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print items
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print edges
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graph = create_graph(items, edges)
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print tsort(graph)
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