Pausing a recursive depth-first search

Question

I have a basic recursive depth first search program that searches unexplored paths in a large tree.
Is there a way to pause and save the tree so I can resume searching later rather than restarting the search every time I end my program?
I am using python and a python IDE called pycharm.

Answer 1

You would have to use an iterative version of DFS by keeping a stack object in place of the inbuilt recursion stack.

Then we can listen to a Ctrl+C button press event to signal that the DFS should be paused.
When its paused, we can pickle(serialize) the dfs state to a file.
When the script is restarted, it will detect the presence of this pickle file and will resume the DFS.

import pickle
import os
import multiprocessing
import signal
import time


ctrl_c_pressed_event = multiprocessing.Event()
ctrl_c_pressed_event.clear()

def ctrl_c_handler(*args):
    ctrl_c_pressed_event.set()

signal.signal(signal.SIGINT, ctrl_c_handler)

GRAPH_PICKLE_PATH = 'graph_pickle.pkl'


class DfsState:
    def __init__(self, visited=set(), path=[], dfs_q=[]):
        self.visited = visited
        self.path = path
        self.dfs_q = dfs_q
    def __str__(self):
        return 'visited={}\npath={}\nq={}'.format(self.visited, self.path, self.dfs_q)


class Graph:
    def __init__(self):
        # Graph is stored in a dictionary of <vertex> vs <set of vertices>
        # Each vertex is just an int
        self.neighbours = {}
        self.vertices = set()
        self.dfs_state = DfsState()

    def add_edge(self, src, dst):
        """
        Add an edge from `src` to `dst`.
        """
        self.vertices.add(src)
        self.vertices.add(dst)
        src_neighbours = self.neighbours.get(src, set())
        src_neighbours.add(dst)
        self.neighbours[src] = src_neighbours

    def get_all_edges(self):
        return self.neighbours

    def get_neighbours(self, src):
        return self.neighbours.get(src, set())
    
    def add_vertex(self, src):
        if src not in self.neighbours:
            self.neighbours[src] = set()
        self.vertices.add(src)


def dfs_iterative(graph):
    visited = graph.dfs_state.visited
    path = graph.dfs_state.path
    dfs_q = graph.dfs_state.dfs_q
    while dfs_q:
        if ctrl_c_pressed_event.is_set():
            return 'PAUSED'
        current = dfs_q.pop()
        print(current)
        if current not in visited:
            visited.add(current)
            path.append(current)
            neighbours = graph.get_neighbours(current)
            for neighbour in neighbours:
                if neighbour not in visited:
                    dfs_q.append(neighbour)
            # Just for testing
            time.sleep(1)
    return 'DONE'


def init_graph():
    graph = Graph()
    graph.add_edge(1, 2)
    graph.add_edge(1, 3)
    graph.add_edge(2, 3)
    graph.add_edge(2, 5)
    graph.add_edge(3, 2)
    graph.add_edge(3, 4)
    graph.add_edge(4, 5)
    graph.add_edge(5, 6)
    graph.add_edge(5, 7)
    graph.add_edge(7, 8)
    start_vertex = 1
    graph.dfs_state.dfs_q = [start_vertex]
    return graph


def pickle_graph(graph):
    pickle.dump(graph, open(GRAPH_PICKLE_PATH, 'wb'))


def load_pickle_graph():
    return pickle.load(open(GRAPH_PICKLE_PATH, "rb" ))


def main():
    if os.path.exists(GRAPH_PICKLE_PATH):
        graph = load_pickle_graph()
        print('Resuming DFS')
        print(graph.dfs_state)
    else:
        graph = init_graph()
    status = dfs_iterative(graph)
    pickle_graph(graph)
    print('\n' + str(graph.dfs_state))
    if status == 'PAUSED':
        print('DFS paused!')
    else:
        print('DFS complete!')


if __name__ == "__main__":
    main()

Output:

>>> ajaggi-mn1:~/Documents/personal/practice/python$ python a.py
1
3
4
^C
visited=set([1, 3, 4])
path=[1, 3, 4]
q=[2, 2, 5]
DFS paused!


>>> ajaggi-mn1:~/Documents/personal/practice/python$ python a.py
Resuming DFS
visited=set([1, 3, 4])
path=[1, 3, 4]
q=[2, 2, 5]
5
7
8
^C
visited=set([1, 3, 4, 5, 7, 8])
path=[1, 3, 4, 5, 7, 8]
q=[2, 2, 6]
DFS paused!


>>> ajaggi-mn1:~/Documents/personal/practice/python$ python a.py
Resuming DFS
visited=set([1, 3, 4, 5, 7, 8])
path=[1, 3, 4, 5, 7, 8]
q=[2, 2, 6]
6
2
2

visited=set([1, 2, 3, 4, 5, 6, 7, 8])
path=[1, 3, 4, 5, 7, 8, 6, 2]
q=[]
DFS complete!


>>> ajaggi-mn1:~/Documents/personal/practice/python$ python a.py
Resuming DFS
visited=set([1, 2, 3, 4, 5, 6, 7, 8])
path=[1, 3, 4, 5, 7, 8, 6, 2]
q=[]

visited=set([1, 2, 3, 4, 5, 6, 7, 8])
path=[1, 3, 4, 5, 7, 8, 6, 2]
q=[]
DFS complete!