# Copyright (c) 2007-2008 Pedro Matiello <pmatiello@gmail.com>
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"""
Accessibility algorithms for python-graph.

@sort: accessibility, connected_components, cut_edges, cut_nodes, mutual_accessibility
"""


# Transitive-closure

def accessibility(graph):
	"""
	Accessibility matrix (transitive closure).

	@type  graph: graph
	@param graph: Graph.

	@rtype:  dictionary
	@return: Accessibility information for each node.
	"""
	accessibility = {}		# Accessibility matrix

	# For each node i, mark each node j if that exists a path from i to j.
	for each in graph:
		access = {}
		# Perform DFS to explore all reachable nodes
		_dfs(graph, access, 1, each)
		accessibility[each] = access.keys()
	return accessibility


# Strongly connected components

def mutual_accessibility(graph):
	"""
	Mutual-accessibility matrix (strongly connected components).

	@type  graph: graph
	@param graph: Graph.

	@rtype:  dictionary
	@return: Mutual-accessibility information for each node.
	"""
	mutual_access = {}
	access = graph.accessibility()

	for i in graph:
		mutual_access[i] = []
		for j in graph:
			if (i in access[j] and j in access[i]):
				mutual_access[i].append(j)

	return mutual_access


# Connected components

def connected_components(graph):
	"""
	Connected components.

	@attention: Indentification of connected components is meaningful only for non-directed graphs.

	@type  graph: graph
	@param graph: Graph.

	@rtype:  dictionary
	@return: Pairing that associates each node to its connected component.
	"""
	visited = {}
	count = 1

	# For 'each' node not found to belong to a connected component, find its connected component.
	for each in graph:
		if (each not in visited):
			_dfs(graph, visited, count, each)
			count = count + 1
	
	return visited


# Limited DFS implementations used by algorithms here

def _dfs(graph, visited, count, node):
	"""
	Depht-first search subfunction adapted for accessibility algorithms.
	
	@type  graph: graph
	@param graph: Graph.

	@type  visited: dictionary
	@param visited: List of nodes (visited nodes are marked non-zero).

	@type  count: number
	@param count: Counter of connected components.

	@type  node: number
	@param node: Node to be explored by DFS.
	"""
	visited[node] = count
	# Explore recursively the connected component
	for each in graph[node]:
		if (each not in visited):
			_dfs(graph, visited, count, each)


# Cut-Edge and Cut-Vertex identification

def cut_edges(graph):
	"""
	Return the cut-edges of the given graph.
	
	@rtype:  list
	@return: List of cut-edges.
	"""
	pre = {}
	low = {}
	spanning_tree = {}
	reply = []
	pre[None] = 0

	for each in graph:
		if (not pre.has_key(each)):
			spanning_tree[each] = None
			_cut_dfs(graph, spanning_tree, pre, low, reply, each)
	return reply


def cut_nodes(graph):
	"""
	Return the cut-nodes of the given graph.
	
	@rtype:  list
	@return: List of cut-nodes.
	"""
	pre = {}	# Pre-ordering
	low = {}	# Lowest pre[] reachable from this node going down the spanning tree + one backedge
	reply = {}
	spanning_tree = {}
	pre[None] = 0
	
	# Create spanning trees, calculate pre[], low[]
	for each in graph:
		if (not pre.has_key(each)):
			spanning_tree[each] = None
			_cut_dfs(graph, spanning_tree, pre, low, [], each)

	# Find cuts
	for each in graph:
		# If node is not a root
		if (spanning_tree[each] is not None):
			for other in graph[each]:
				# If there is no back-edge from descendent to a ancestral of each
				if (low[other] >= pre[each] and spanning_tree[other] == each):
					reply[each] = 1
		# If node is a root
		else:
			children = 0
			for other in graph:
				if (spanning_tree[other] == each):
					children = children + 1
			# root is cut-vertex iff it has two or more children
			if (children >= 2):
				reply[each] = 1

	return reply.keys()


def _cut_dfs(graph, spanning_tree, pre, low, reply, node):
	"""
	Depth first search adapted for identification of cut-edges and cut-nodes.
	
	@type  graph: graph
	@param graph: Graph
	
	@type  spanning_tree: dictionary
	@param spanning_tree: Spanning tree being built for the graph by DFS.

	@type  pre: dictionary
	@param pre: Graph's preordering.
	
	@type  low: dictionary
	@param low: Associates to each node, the preordering index of the node of lowest preordering
	accessible from the given node.

	@type  reply: list
	@param reply: List of cut-edges.
	
	@type  node: node
	@param node: Node to be explored by DFS.
	"""
	pre[node] = pre[None]
	low[node] = pre[None]
	pre[None] = pre[None] + 1
	
	for each in graph[node]:
		if (not pre.has_key(each)):
			spanning_tree[each] = node
			_cut_dfs(graph, spanning_tree, pre, low, reply, each)
			if (low[node] > low[each]):
				low[node] = low[each]
			if (low[each] == pre[each]):
				reply.append((node, each))
		elif (low[node] > pre[each] and spanning_tree[node] != each):
			low[node] = pre[each]
