Python/graphs/breadth_first_search_zero_one_shortest_path.py

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"""
Finding the shortest path in 0-1-graph in O(E + V) which is faster than dijkstra.
0-1-graph is the weighted graph with the weights equal to 0 or 1.
Link: https://codeforces.com/blog/entry/22276
"""
from __future__ import annotations
from collections import deque
from collections.abc import Iterator
from dataclasses import dataclass
@dataclass
class Edge:
"""Weighted directed graph edge."""
destination_vertex: int
weight: int
class AdjacencyList:
"""Graph adjacency list."""
def __init__(self, size: int):
self._graph: list[list[Edge]] = [[] for _ in range(size)]
self._size = size
def __getitem__(self, vertex: int) -> Iterator[Edge]:
"""Get all the vertices adjacent to the given one."""
return iter(self._graph[vertex])
@property
def size(self):
return self._size
def add_edge(self, from_vertex: int, to_vertex: int, weight: int):
"""
>>> g = AdjacencyList(2)
>>> g.add_edge(0, 1, 0)
>>> g.add_edge(1, 0, 1)
>>> list(g[0])
[Edge(destination_vertex=1, weight=0)]
>>> list(g[1])
[Edge(destination_vertex=0, weight=1)]
>>> g.add_edge(0, 1, 2)
Traceback (most recent call last):
...
ValueError: Edge weight must be either 0 or 1.
>>> g.add_edge(0, 2, 1)
Traceback (most recent call last):
...
ValueError: Vertex indexes must be in [0; size).
"""
if weight not in (0, 1):
raise ValueError("Edge weight must be either 0 or 1.")
if to_vertex < 0 or to_vertex >= self.size:
raise ValueError("Vertex indexes must be in [0; size).")
self._graph[from_vertex].append(Edge(to_vertex, weight))
def get_shortest_path(self, start_vertex: int, finish_vertex: int) -> int | None:
"""
Return the shortest distance from start_vertex to finish_vertex in 0-1-graph.
1 1 1
0--------->3 6--------7>------->8
| ^ ^ ^ |1
| | | |0 v
0| |0 1| 9-------->10
| | | ^ 1
v | | |0
1--------->2<-------4------->5
0 1 1
>>> g = AdjacencyList(11)
>>> g.add_edge(0, 1, 0)
>>> g.add_edge(0, 3, 1)
>>> g.add_edge(1, 2, 0)
>>> g.add_edge(2, 3, 0)
>>> g.add_edge(4, 2, 1)
>>> g.add_edge(4, 5, 1)
>>> g.add_edge(4, 6, 1)
>>> g.add_edge(5, 9, 0)
>>> g.add_edge(6, 7, 1)
>>> g.add_edge(7, 8, 1)
>>> g.add_edge(8, 10, 1)
>>> g.add_edge(9, 7, 0)
>>> g.add_edge(9, 10, 1)
>>> g.add_edge(1, 2, 2)
Traceback (most recent call last):
...
ValueError: Edge weight must be either 0 or 1.
>>> g.get_shortest_path(0, 3)
0
>>> g.get_shortest_path(0, 4)
Traceback (most recent call last):
...
ValueError: No path from start_vertex to finish_vertex.
>>> g.get_shortest_path(4, 10)
2
>>> g.get_shortest_path(4, 8)
2
>>> g.get_shortest_path(0, 1)
0
>>> g.get_shortest_path(1, 0)
Traceback (most recent call last):
...
ValueError: No path from start_vertex to finish_vertex.
"""
queue = deque([start_vertex])
distances: list[int | None] = [None] * self.size
distances[start_vertex] = 0
while queue:
current_vertex = queue.popleft()
current_distance = distances[current_vertex]
if current_distance is None:
continue
for edge in self[current_vertex]:
new_distance = current_distance + edge.weight
dest_vertex_distance = distances[edge.destination_vertex]
if (
isinstance(dest_vertex_distance, int)
and new_distance >= dest_vertex_distance
):
continue
distances[edge.destination_vertex] = new_distance
if edge.weight == 0:
queue.appendleft(edge.destination_vertex)
else:
queue.append(edge.destination_vertex)
if distances[finish_vertex] is None:
raise ValueError("No path from start_vertex to finish_vertex.")
return distances[finish_vertex]
if __name__ == "__main__":
import doctest
doctest.testmod()