Python/graphs/bidirectional_breadth_first_search.py
Christian Clauss cecf43d648
Pyupgrade to Python 3.9 (#4718)
* Pyupgrade to Python 3.9

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
2021-09-07 13:37:03 +02:00

185 lines
5.7 KiB
Python

"""
https://en.wikipedia.org/wiki/Bidirectional_search
"""
from __future__ import annotations
import time
Path = list[tuple[int, int]]
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:
def __init__(
self, pos_x: int, pos_y: int, goal_x: int, goal_y: int, parent: Node | None
):
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.parent = parent
class BreadthFirstSearch:
"""
>>> bfs = BreadthFirstSearch((0, 0), (len(grid) - 1, len(grid[0]) - 1))
>>> (bfs.start.pos_y + delta[3][0], bfs.start.pos_x + delta[3][1])
(0, 1)
>>> [x.pos for x in bfs.get_successors(bfs.start)]
[(1, 0), (0, 1)]
>>> (bfs.start.pos_y + delta[2][0], bfs.start.pos_x + delta[2][1])
(1, 0)
>>> bfs.retrace_path(bfs.start)
[(0, 0)]
>>> bfs.search() # doctest: +NORMALIZE_WHITESPACE
[(0, 0), (1, 0), (2, 0), (3, 0), (3, 1), (4, 1),
(5, 1), (5, 2), (5, 3), (5, 4), (5, 5), (6, 5), (6, 6)]
"""
def __init__(self, start: tuple[int, int], goal: tuple[int, int]):
self.start = Node(start[1], start[0], goal[1], goal[0], None)
self.target = Node(goal[1], goal[0], goal[1], goal[0], None)
self.node_queue = [self.start]
self.reached = False
def search(self) -> Path | None:
while self.node_queue:
current_node = self.node_queue.pop(0)
if current_node.pos == self.target.pos:
self.reached = True
return self.retrace_path(current_node)
successors = self.get_successors(current_node)
for node in successors:
self.node_queue.append(node)
if not self.reached:
return [self.start.pos]
return None
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)
)
return successors
def retrace_path(self, node: Node | None) -> Path:
"""
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 BidirectionalBreadthFirstSearch:
"""
>>> bd_bfs = BidirectionalBreadthFirstSearch((0, 0), (len(grid) - 1,
... len(grid[0]) - 1))
>>> bd_bfs.fwd_bfs.start.pos == bd_bfs.bwd_bfs.target.pos
True
>>> bd_bfs.retrace_bidirectional_path(bd_bfs.fwd_bfs.start,
... bd_bfs.bwd_bfs.start)
[(0, 0)]
>>> bd_bfs.search() # doctest: +NORMALIZE_WHITESPACE
[(0, 0), (0, 1), (0, 2), (1, 2), (2, 2), (2, 3),
(2, 4), (3, 4), (3, 5), (3, 6), (4, 6), (5, 6), (6, 6)]
"""
def __init__(self, start, goal):
self.fwd_bfs = BreadthFirstSearch(start, goal)
self.bwd_bfs = BreadthFirstSearch(goal, start)
self.reached = False
def search(self) -> Path | None:
while self.fwd_bfs.node_queue or self.bwd_bfs.node_queue:
current_fwd_node = self.fwd_bfs.node_queue.pop(0)
current_bwd_node = self.bwd_bfs.node_queue.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_bfs.target = current_bwd_node
self.bwd_bfs.target = current_fwd_node
successors = {
self.fwd_bfs: self.fwd_bfs.get_successors(current_fwd_node),
self.bwd_bfs: self.bwd_bfs.get_successors(current_bwd_node),
}
for bfs in [self.fwd_bfs, self.bwd_bfs]:
for node in successors[bfs]:
bfs.node_queue.append(node)
if not self.reached:
return [self.fwd_bfs.start.pos]
return None
def retrace_bidirectional_path(self, fwd_node: Node, bwd_node: Node) -> Path:
fwd_path = self.fwd_bfs.retrace_path(fwd_node)
bwd_path = self.bwd_bfs.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_bfs_time = time.time()
bfs = BreadthFirstSearch(init, goal)
path = bfs.search()
bfs_time = time.time() - start_bfs_time
print("Unidirectional BFS computation time : ", bfs_time)
start_bd_bfs_time = time.time()
bd_bfs = BidirectionalBreadthFirstSearch(init, goal)
bd_path = bd_bfs.search()
bd_bfs_time = time.time() - start_bd_bfs_time
print("Bidirectional BFS computation time : ", bd_bfs_time)