Grokking Algorithms

07-djikstra.py

#!/usr/bin/env python
graph = {
    "start": {
        "a" : 6,
        "b" : 2
    },
    "a" : {
        "fin" : 1
    },
    "b" : {
        "a" : 3,
        "fin" : 5
    },
    "fin" : {}
}

infinity = float("inf")

costs = {
    "a" : 6,
    "b" : 2,
    "fin" : infinity
}

parents = {
    "a" : "start",
    "b" : "start",
    "fin" : None
}

processed = []

def find_lowest_cost_node(costs):
    lowest_cost = float("inf")
    lowest_cost_node = None
    for node in costs:
        cost = costs[node]
        if cost < lowest_cost and node not in processed:
            lowest_cost = cost
            lowest_cost_node = node
    return lowest_cost_node

node = find_lowest_cost_node(costs)
while node is not None:
    cost = costs[node]
    neighbors = graph[node]
    for n in neighbors.keys():
        new_cost = cost + neighbors[n]
        if costs[n] > new_cost:
            costs[n] = new_cost
            parents[n] = node
    processed.append(node)
    node = find_lowest_cost_node(costs)

assert costs["fin"] == 6