I am stuck on day5 of this year’s AOC. I try to do it all in python as to learn it.
In short, the task needs a graph traversal. I chose to do it with DFS.

The following code snipped works:

        nodes = [*self.adj_list.keys()]
        node_states = [TopologicalStates.WHITE for _ in nodes]
        predecessors = []

        def visit_node(node_index, node):
            node_states[node_index] = TopologicalStates.GRAY
            adj_nodes = self.adj_list[node]
            for adj_node in adj_nodes:
                adj_node_index = nodes.index(adj_node)
                if node_states[adj_node_index] == TopologicalStates.WHITE:
                    visit_node(adj_node_index, adj_node)
            node_states[node_index] = TopologicalStates.BLACK
            predecessors.append(node)

        for (node_index, node) in enumerate([predecessor for (predecessor, successors) in self.adj_list.items() if node in successors]):
            if node_states[node_index] == TopologicalStates.WHITE:
                visit_node(node_index, node)

        return predecessors

However, as soon as I try to remove the node_index parameter from visit_node and instead define a new variable at the top of visit_node like below, I get infinite recursion!

        nodes = [*self.adj_list.keys()]
        node_states = [TopologicalStates.WHITE for _ in nodes]
        predecessors = []

        def visit_node(node):
            node_index = nodes.index(p)
            node_states[node_index] = TopologicalStates.GRAY

            adj_nodes = self.adj_list[node]
            for adj_node in adj_nodes:
                adj_node_index = nodes.index(adj_node)
                if node_states[adj_node_index] == TopologicalStates.WHITE:
                    visit_node(adj_node)

            node_states[node_index] = TopologicalStates.BLACK
            predecessors.append(node)

        for (i, p) in enumerate([predecessor for (predecessor, successors) in self.adj_list.items() if node in successors]):
            if node_states[i] == TopologicalStates.WHITE:
                visit_node(p)

        return predecessors

I started a debug session to only find out that node_states has an initial value, or rather, does not seem to persist node_states[node_index] = TopologicalState.GRAY calls between recursion calls.

How is this possible? What am I doing wrong? Is it a “my computer” issue?

Thanks, any help is appreciated