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# (C) 2005 Canonical
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
import pdb
def topo_sort(nodes, pairs):
"""Topological sort a graph.
nodes -- list of all nodes in the graph
pairs -- list of (a, b) pairs, meaning a is a predecessor of b.
both a and b must occur in the node list.
node identifiers can be any hashable object, and are typically strings.
"""
parents = {} # node -> list of parents
children = {} # node -> list of children
for n in nodes:
parents[n] = set()
children[n] = set()
for p, c in pairs:
parents[c].add(p)
children[p].add(c)
result = []
while parents:
# find nodes with no parents, and take them now
ready = [n for n in parents if len(parents[n]) == 0]
if not ready:
raise AssertionError('cycle in graph?')
for n in ready:
result.append(n)
for child in children[n]:
assert n in parents[child]
parents[child].remove(n)
del children[n]
del parents[n]
return result
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