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