/usr/lib/python3/dist-packages/pygraph/algorithms/heuristics/chow.py is in python3-pygraph 1.8.2-6.
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# Salim Fadhley <sal@stodge.org>
#
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"""
Edmond Chow's heuristic for A*.
"""
# Imports
from pygraph.algorithms.minmax import shortest_path
class chow(object):
"""
An implementation of the graph searching heuristic proposed by Edmond Chow.
Remember to call the C{optimize()} method before the heuristic search.
For details, check: U{http://www.edmondchow.com/pubs/levdiff-aaai.pdf}.
"""
def __init__(self, *centers):
"""
Initialize a Chow heuristic object.
"""
self.centers = centers
self.nodes = {}
def optimize(self, graph):
"""
Build a dictionary mapping each pair of nodes to a number (the distance between them).
@type graph: graph
@param graph: Graph.
"""
for center in self.centers:
shortest_routes = shortest_path(graph, center)[1]
for node, weight in list(shortest_routes.items()):
self.nodes.setdefault(node, []).append(weight)
def __call__(self, start, end):
"""
Estimate how far start is from end.
@type start: node
@param start: Start node.
@type end: node
@param end: End node.
"""
assert len( list(self.nodes.keys()) ) > 0, "You need to optimize this heuristic for your graph before it can be used to estimate."
cmp_sequence = list(zip( self.nodes[start], self.nodes[end] ))
chow_number = max( abs( a-b ) for a,b in cmp_sequence )
return chow_number
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