def topological_sort(self):
"""
Orders nodes (permutes their topo_index's) such that no node is
before any of its parents. Node with lowest topo_index number is a
root node. So this could also be called a chronological or birthday
sort, youngest nodes first. Exception is raised if graph has cycles
and cannot be ordered topologically.
Returns
-------
None
"""
self.detect_two_node_cycle()
sorted_set = set()
i = 0
num_unsorted_nds = len(self.nodes)
while len(self.nodes) > 0:
if num_unsorted_nds <= 0:
raise BadGraphStructure("Graph must be acyclic")
num_unsorted_nds -= 1
for node in self.nodes:
if sorted_set >= node.parents:
sorted_set.add(node)
self.nodes.remove(node)
node.topo_index = i
i += 1
break
self.nodes = sorted_set
def detect_two_node_cycle(self):
"""
Detects a 2 node cycle. That is, when 2 nodes are both parents and
children of each other.
Returns
-------
None
"""
for node in self.nodes:
overlap = node.parents & node.children
if overlap:
raise BadGraphStructure("two node cycle detected")