bidirectional a star Algorithm
The bidirectional A* algorithm is an extension of the popular A* search algorithm, which is known for its efficiency in finding the shortest path between two points in a graph or grid. This algorithm operates by exploring the graph from both the start and the goal nodes simultaneously, meeting in the middle, effectively reducing the search space and improving the performance. The bidirectional A* algorithm maintains two sets of open nodes, one for the forward search (from the start node) and one for the backward search (from the goal node). During each iteration, the algorithm selects the node with the lowest combined cost (g-cost and h-cost) from both sets and expands its neighbors, updating their costs and maintaining their parent pointers. The search continues until the two searches meet, and the optimal path is reconstructed by traversing the parent pointers from the meeting point back to the start and goal nodes.
The key advantage of the bidirectional A* algorithm is its improved performance compared to the unidirectional A* search, particularly in large graphs or complex environments. By searching from both the start and the goal nodes, the algorithm can significantly reduce the number of nodes that need to be expanded, leading to a faster solution. This is especially useful in situations where there is a clear and direct path between the start and goal nodes, or when the graph has a high branching factor. Furthermore, the bidirectional A* algorithm inherits the optimality and admissibility properties of the original A* search, guaranteeing that the solution found is indeed the shortest path, provided that the heuristic function used is consistent (or monotonic). Overall, the bidirectional A* algorithm provides an effective and efficient method for solving the shortest path problem in various applications, such as robotics, navigation, and artificial intelligence.
"""
https://en.wikipedia.org/wiki/Bidirectional_search
"""
import time
from math import sqrt
from typing import List, Tuple
# 1 for manhattan, 0 for euclidean
HEURISTIC = 0
grid = [
[0, 0, 0, 0, 0, 0, 0],
[0, 1, 0, 0, 0, 0, 0], # 0 are free path whereas 1's are obstacles
[0, 0, 0, 0, 0, 0, 0],
[0, 0, 1, 0, 0, 0, 0],
[1, 0, 1, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 1, 0, 0],
]
delta = [[-1, 0], [0, -1], [1, 0], [0, 1]] # up, left, down, right
class Node:
"""
>>> k = Node(0, 0, 4, 3, 0, None)
>>> k.calculate_heuristic()
5.0
>>> n = Node(1, 4, 3, 4, 2, None)
>>> n.calculate_heuristic()
2.0
>>> l = [k, n]
>>> n == l[0]
False
>>> l.sort()
>>> n == l[0]
True
"""
def __init__(self, pos_x, pos_y, goal_x, goal_y, g_cost, parent):
self.pos_x = pos_x
self.pos_y = pos_y
self.pos = (pos_y, pos_x)
self.goal_x = goal_x
self.goal_y = goal_y
self.g_cost = g_cost
self.parent = parent
self.h_cost = self.calculate_heuristic()
self.f_cost = self.g_cost + self.h_cost
def calculate_heuristic(self) -> float:
"""
Heuristic for the A*
"""
dy = self.pos_x - self.goal_x
dx = self.pos_y - self.goal_y
if HEURISTIC == 1:
return abs(dx) + abs(dy)
else:
return sqrt(dy ** 2 + dx ** 2)
def __lt__(self, other) -> bool:
return self.f_cost < other.f_cost
class AStar:
"""
>>> astar = AStar((0, 0), (len(grid) - 1, len(grid[0]) - 1))
>>> (astar.start.pos_y + delta[3][0], astar.start.pos_x + delta[3][1])
(0, 1)
>>> [x.pos for x in astar.get_successors(astar.start)]
[(1, 0), (0, 1)]
>>> (astar.start.pos_y + delta[2][0], astar.start.pos_x + delta[2][1])
(1, 0)
>>> astar.retrace_path(astar.start)
[(0, 0)]
>>> astar.search() # doctest: +NORMALIZE_WHITESPACE
[(0, 0), (1, 0), (2, 0), (2, 1), (2, 2), (2, 3), (3, 3),
(4, 3), (4, 4), (5, 4), (5, 5), (6, 5), (6, 6)]
"""
def __init__(self, start, goal):
self.start = Node(start[1], start[0], goal[1], goal[0], 0, None)
self.target = Node(goal[1], goal[0], goal[1], goal[0], 99999, None)
self.open_nodes = [self.start]
self.closed_nodes = []
self.reached = False
def search(self) -> List[Tuple[int]]:
while self.open_nodes:
# Open Nodes are sorted using __lt__
self.open_nodes.sort()
current_node = self.open_nodes.pop(0)
if current_node.pos == self.target.pos:
self.reached = True
return self.retrace_path(current_node)
self.closed_nodes.append(current_node)
successors = self.get_successors(current_node)
for child_node in successors:
if child_node in self.closed_nodes:
continue
if child_node not in self.open_nodes:
self.open_nodes.append(child_node)
else:
# retrieve the best current path
better_node = self.open_nodes.pop(self.open_nodes.index(child_node))
if child_node.g_cost < better_node.g_cost:
self.open_nodes.append(child_node)
else:
self.open_nodes.append(better_node)
if not (self.reached):
return [(self.start.pos)]
def get_successors(self, parent: Node) -> List[Node]:
"""
Returns a list of successors (both in the grid and free spaces)
"""
successors = []
for action in delta:
pos_x = parent.pos_x + action[1]
pos_y = parent.pos_y + action[0]
if not (0 <= pos_x <= len(grid[0]) - 1 and 0 <= pos_y <= len(grid) - 1):
continue
if grid[pos_y][pos_x] != 0:
continue
successors.append(
Node(
pos_x,
pos_y,
self.target.pos_y,
self.target.pos_x,
parent.g_cost + 1,
parent,
)
)
return successors
def retrace_path(self, node: Node) -> List[Tuple[int]]:
"""
Retrace the path from parents to parents until start node
"""
current_node = node
path = []
while current_node is not None:
path.append((current_node.pos_y, current_node.pos_x))
current_node = current_node.parent
path.reverse()
return path
class BidirectionalAStar:
"""
>>> bd_astar = BidirectionalAStar((0, 0), (len(grid) - 1, len(grid[0]) - 1))
>>> bd_astar.fwd_astar.start.pos == bd_astar.bwd_astar.target.pos
True
>>> bd_astar.retrace_bidirectional_path(bd_astar.fwd_astar.start,
... bd_astar.bwd_astar.start)
[(0, 0)]
>>> bd_astar.search() # doctest: +NORMALIZE_WHITESPACE
[(0, 0), (0, 1), (0, 2), (1, 2), (1, 3), (2, 3), (2, 4),
(2, 5), (3, 5), (4, 5), (5, 5), (5, 6), (6, 6)]
"""
def __init__(self, start, goal):
self.fwd_astar = AStar(start, goal)
self.bwd_astar = AStar(goal, start)
self.reached = False
def search(self) -> List[Tuple[int]]:
while self.fwd_astar.open_nodes or self.bwd_astar.open_nodes:
self.fwd_astar.open_nodes.sort()
self.bwd_astar.open_nodes.sort()
current_fwd_node = self.fwd_astar.open_nodes.pop(0)
current_bwd_node = self.bwd_astar.open_nodes.pop(0)
if current_bwd_node.pos == current_fwd_node.pos:
self.reached = True
return self.retrace_bidirectional_path(
current_fwd_node, current_bwd_node
)
self.fwd_astar.closed_nodes.append(current_fwd_node)
self.bwd_astar.closed_nodes.append(current_bwd_node)
self.fwd_astar.target = current_bwd_node
self.bwd_astar.target = current_fwd_node
successors = {
self.fwd_astar: self.fwd_astar.get_successors(current_fwd_node),
self.bwd_astar: self.bwd_astar.get_successors(current_bwd_node),
}
for astar in [self.fwd_astar, self.bwd_astar]:
for child_node in successors[astar]:
if child_node in astar.closed_nodes:
continue
if child_node not in astar.open_nodes:
astar.open_nodes.append(child_node)
else:
# retrieve the best current path
better_node = astar.open_nodes.pop(
astar.open_nodes.index(child_node)
)
if child_node.g_cost < better_node.g_cost:
astar.open_nodes.append(child_node)
else:
astar.open_nodes.append(better_node)
if not self.reached:
return [self.fwd_astar.start.pos]
def retrace_bidirectional_path(
self, fwd_node: Node, bwd_node: Node
) -> List[Tuple[int]]:
fwd_path = self.fwd_astar.retrace_path(fwd_node)
bwd_path = self.bwd_astar.retrace_path(bwd_node)
bwd_path.pop()
bwd_path.reverse()
path = fwd_path + bwd_path
return path
if __name__ == "__main__":
# all coordinates are given in format [y,x]
import doctest
doctest.testmod()
init = (0, 0)
goal = (len(grid) - 1, len(grid[0]) - 1)
for elem in grid:
print(elem)
start_time = time.time()
a_star = AStar(init, goal)
path = a_star.search()
end_time = time.time() - start_time
print(f"AStar execution time = {end_time:f} seconds")
bd_start_time = time.time()
bidir_astar = BidirectionalAStar(init, goal)
path = bidir_astar.search()
bd_end_time = time.time() - bd_start_time
print(f"BidirectionalAStar execution time = {bd_end_time:f} seconds")
