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# Copyright (C) 2005, 2006, 2008 Canonical Ltd
#
# 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
# This code originated in bzrlib. In order to make it suitable for use in
# Germinate, Colin Watson removed its bzrlib.errors dependency and stripped
# out the implementation of specialised merge-graph sorting.
"""Topological sorting routines."""
__all__ = ["topo_sort", "TopoSorter"]
class GraphCycleError(StandardError):
_fmt = "Cycle in graph %(graph)r"
def __init__(self, graph):
StandardError.__init__(self)
self.graph = graph
def __str__(self):
d = dict(self.__dict__)
# special case: python2.5 puts the 'message' attribute in a
# slot, so it isn't seen in __dict__
d['message'] = getattr(self, 'message', 'no message')
s = self._fmt % d
# __str__() should always return a 'str' object
# never a 'unicode' object.
if isinstance(s, unicode):
return s.encode('utf8')
return s
def topo_sort(graph):
"""Topological sort a graph.
graph -- sequence of pairs of node->parents_list.
The result is a list of node names, such that all parents come before
their children.
node identifiers can be any hashable object, and are typically strings.
"""
return TopoSorter(graph).sorted()
class TopoSorter(object):
def __init__(self, graph):
"""Topological sorting of a graph.
:param graph: sequence of pairs of node_name->parent_names_list.
i.e. [('C', ['B']), ('B', ['A']), ('A', [])]
For this input the output from the sort or
iter_topo_order routines will be:
'A', 'B', 'C'
node identifiers can be any hashable object, and are typically strings.
If you have a graph like [('a', ['b']), ('a', ['c'])] this will only use
one of the two values for 'a'.
The graph is sorted lazily: until you iterate or sort the input is
not processed other than to create an internal representation.
iteration or sorting may raise GraphCycleError if a cycle is present
in the graph.
"""
# a dict of the graph.
self._graph = dict(graph)
self._visitable = set(self._graph)
### if debugging:
# self._original_graph = dict(graph)
# this is a stack storing the depth first search into the graph.
self._node_name_stack = []
# at each level of 'recursion' we have to check each parent. This
# stack stores the parents we have not yet checked for the node at the
# matching depth in _node_name_stack
self._pending_parents_stack = []
# this is a set of the completed nodes for fast checking whether a
# parent in a node we are processing on the stack has already been
# emitted and thus can be skipped.
self._completed_node_names = set()
def sorted(self):
"""Sort the graph and return as a list.
After calling this the sorter is empty and you must create a new one.
"""
return list(self.iter_topo_order())
### Useful if fiddling with this code.
### # cross check
### sorted_names = list(self.iter_topo_order())
### for index in range(len(sorted_names)):
### rev = sorted_names[index]
### for left_index in range(index):
### if rev in self.original_graph[sorted_names[left_index]]:
### print "revision in parent list of earlier revision"
### import pdb;pdb.set_trace()
def iter_topo_order(self):
"""Yield the nodes of the graph in a topological order.
After finishing iteration the sorter is empty and you cannot continue
iteration.
"""
while self._graph:
# now pick a random node in the source graph, and transfer it to the
# top of the depth first search stack.
node_name, parents = self._graph.popitem()
self._push_node(node_name, parents)
while self._node_name_stack:
# loop until this call completes.
parents_to_visit = self._pending_parents_stack[-1]
# if all parents are done, the revision is done
if not parents_to_visit:
# append the revision to the topo sorted list
# all the nodes parents have been added to the output, now
# we can add it to the output.
yield self._pop_node()
else:
while self._pending_parents_stack[-1]:
# recurse depth first into a single parent
next_node_name = self._pending_parents_stack[-1].pop()
if next_node_name in self._completed_node_names:
# this parent was completed by a child on the
# call stack. skip it.
continue
if next_node_name not in self._visitable:
continue
# otherwise transfer it from the source graph into the
# top of the current depth first search stack.
try:
parents = self._graph.pop(next_node_name)
except KeyError:
# if the next node is not in the source graph it has
# already been popped from it and placed into the
# current search stack (but not completed or we would
# have hit the continue 4 lines up.
# this indicates a cycle.
raise GraphCycleError(self._node_name_stack)
self._push_node(next_node_name, parents)
# and do not continue processing parents until this 'call'
# has recursed.
break
def _push_node(self, node_name, parents):
"""Add node_name to the pending node stack.
Names in this stack will get emitted into the output as they are popped
off the stack.
"""
self._node_name_stack.append(node_name)
self._pending_parents_stack.append(list(parents))
def _pop_node(self):
"""Pop the top node off the stack
The node is appended to the sorted output.
"""
# we are returning from the flattened call frame:
# pop off the local variables
node_name = self._node_name_stack.pop()
self._pending_parents_stack.pop()
self._completed_node_names.add(node_name)
return node_name
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