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tsort.py

# 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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