from collections import deque
class Graph:
# example of adjacency list (or rather map)
# adjacency_list = {
# 'A': [('B', 1), ('C', 3), ('D', 7)],
# 'B': [('D', 5)],
# 'C': [('D', 12)]
# }
def __init__(self, adjacency_list):
self.adjacency_list = adjacency_list
def get_neighbors(self, v):
return self.adjacency_list[v]
# heuristic function with equal values for all nodes
def h(self, n):
H = {
'A': 1,
'B': 1,
'C': 1,
'D': 1
}
return H[n]
def a_star_algorithm(self, start_node, stop_node):
# open_list is a list of nodes which have been visited, but who's neighbors
# haven't all been inspected, starts off with the start node
# closed_list is a list of nodes which have been visited
# and who's neighbors have been inspected
open_list = set([start_node])
closed_list = set([])
# g contains current distances from start_node to all other nodes
# the default value (if it's not found in the map) is +infinity
g = {}
g[start_node] = 0
# parents contains an adjacency map of all nodes
parents = {}
parents[start_node] = start_node
while len(open_list) > 0:
n = None
# find a node with the lowest value of f() - evaluation function
for v in open_list:
if n == None or g[v] + self.h(v) < g[n] + self.h(n):
n = v;
if n == None:
print('Path does not exist!')
return None
# if the current node is the stop_node
# then we begin reconstructin the path from it to the start_node
if n == stop_node:
reconst_path = []
while parents[n] != n:
reconst_path.append(n)
n = parents[n]
reconst_path.append(start_node)
reconst_path.reverse()
print('Path found: {}'.format(reconst_path))
return reconst_path
# for all neighbors of the current node do
for (m, weight) in self.get_neighbors(n):
# if the current node isn't in both open_list and closed_list
# add it to open_list and note n as it's parent
if m not in open_list and m not in closed_list:
open_list.add(m)
parents[m] = n
g[m] = g[n] + weight
# otherwise, check if it's quicker to first visit n, then m
# and if it is, update parent data and g data
# and if the node was in the closed_list, move it to open_list
else:
if g[m] > g[n] + weight:
g[m] = g[n] + weight
parents[m] = n
if m in closed_list:
closed_list.remove(m)
open_list.add(m)
# remove n from the open_list, and add it to closed_list
# because all of his neighbors were inspected
open_list.remove(n)
closed_list.add(n)
print('Path does not exist!')
return None