37 lines
1.2 KiB
Python
37 lines
1.2 KiB
Python
class CyclicDependencyError(ValueError):
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pass
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def topological_sort_as_sets(dependency_graph):
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"""
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Variation of Kahn's algorithm (1962) that returns sets.
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Take a dependency graph as a dictionary of node => dependencies.
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Yield sets of items in topological order, where the first set contains
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all nodes without dependencies, and each following set contains all
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nodes that may depend on the nodes only in the previously yielded sets.
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"""
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todo = dependency_graph.copy()
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while todo:
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current = {node for node, deps in todo.items() if not deps}
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if not current:
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raise CyclicDependencyError('Cyclic dependency in graph: {}'.format(
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', '.join(repr(x) for x in todo.items())))
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yield current
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# remove current from todo's nodes & dependencies
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todo = {node: (dependencies - current) for node, dependencies in
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todo.items() if node not in current}
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def stable_topological_sort(nodes, dependency_graph):
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result = []
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for layer in topological_sort_as_sets(dependency_graph):
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for node in nodes:
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if node in layer:
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result.append(node)
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return result
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