grokking-algorithms/chapter9/with_heap/ex171a.py

56 lines
1.4 KiB
Python

import heapq
import logging
import math
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)
graph = {
"start": {"a": 5, "b": 2},
"a": {"b": 8, "c": 4, "d": 2},
"b": {"a": 8, "d": 7},
"c": {"d": 6, "fin": 3},
"d": {"fin": 1},
"fin": {},
}
def dijkstra(graph, node):
costs = {node: math.inf for node in graph}
costs[node] = 0
parents = {node: None for node in graph}
queue = [(0, node)]
while queue:
current_cost, current_node = heapq.heappop(queue)
logger.debug(f"node {current_node} with cost {current_cost} popped from pqueue")
for next_node, weight in graph[current_node].items():
new_cost = current_cost + weight
if new_cost < costs[next_node]:
costs[next_node] = new_cost
parents[next_node] = current_node
heapq.heappush(queue, (new_cost, next_node))
logger.debug(
f"node {next_node} with new cost {new_cost} appended to pqueue"
)
return costs, parents
costs, parents = dijkstra(graph, "start")
print(f"lowest cost from start to fin: {costs['fin']}")
def get_full_route():
route = []
next = "fin"
while next != "start":
route.append(next)
next = parents[next]
route.append("start")
return list(reversed(route))
print(f"full route: {get_full_route()}")