Create Kosaraju-algorithm.py

graph/Kosaraju-algorithm.py

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+def build_graph(filename):
+	graph = dict()
+	with open(filename, 'r') as f:
+		for line in f:
+			s,t = line.split()
+			s,t = int(s), int(t)
+			dest_nodes = graph.get(s, set())
+			dest_nodes.add(t)
+			graph[s] = dest_nodes
+	return graph
+
+def kosaraju_strongly_connected_components(graph):
+
+	t = 0
+	s = 0
+	explored_nodes = None
+	finishing_times = None
+	leaders = None
+
+	def dfs(graph, node):
+		nonlocal t,s,finishing_times,leaders
+		explored_nodes.add(node)
+		leaders[node] = s
+		if node in graph:
+			for dest_node in graph[node]:
+				if dest_node not in explored_nodes:
+					dfs(graph, dest_node)
+		t += 1
+		finishing_times[node] = t
+
+	def dfs_loop(graph):
+		nonlocal t,s,finishing_times,leaders, explored_nodes
+		finishing_times, leaders, explored_nodes = dict(), dict(), set()
+		nb_nodes = len(graph.keys())
+		for i in reversed(range(1,nb_nodes+1)):
+			if i not in explored_nodes:
+				s = i
+				dfs(graph, i)
+
+	def reverse_graph(graph):
+		edges = set()
+		new_graph = dict()
+		for node,dest_nodes in graph.items():
+			for dest_node in dest_nodes:
+				edges.add((node, dest_node))
+		for edge in edges:
+			dest_nodes = new_graph.get(edge[1],set())
+			dest_nodes.add(edge[0])
+			new_graph[edge[1]] = dest_nodes
+		return new_graph
+
+	rev_graph = reverse_graph(graph)
+	#1st dsf_loop pass: we are interested in the finishing_times to create the times_graph
+	dfs_loop(graph)
+	rev_times_graph = dict()
+	for node,dest_nodes in graph.items():
+		time_dest_nodes = set()
+		for dest_node in dest_nodes:
+			time_dest_nodes.add(finishing_times[dest_node])
+		rev_times_graph[finishing_times[node]] = time_dest_nodes
+	times_graph = reverse_graph(rev_times_graph)
+	#2nd dsf_loop pass: we are interested in getting the leader for every node to form the sccs
+	dfs_loop(times_graph)
+	rev_leaders = dict()
+	for node,leader in leaders.items():
+		rev_leaders[leader] = rev_leaders.get(leader,[])
+		rev_leaders[leader].append(node)
+	return list(rev_leaders.values())
+
+def main():
+	print('Building graph from file...')
+	graph = build_graph('test.txt')
+	print('Computing strongly connected components...')
+	print(kosaraju_strongly_connected_components(graph))
+
+
+if (__name__ == '__main__'):
+	main()