/**
* Computer minimal weight for every node. read-only array.
*
* @param <E> type of element in node
* @param graph graph
* @return minimal weight array for every node.
*/
private static <E> Map<Node<E>, Double> computeMinWeight(Graph<E> graph) {
// minWeight record the minimal weight for every node
Map<Node<E>, Double> minWeight = new ConcurrentHashMap<Node<E>, Double>();
// linked nodes of target node
Collection<AdjacentNode<E>> linkedNodes;
for (Node<E> node : graph.getAllNodes()) {
// get linked nodes of target node
linkedNodes = graph.getLinkedNodes(node);
// recored the min weight of target node if exist, otherwise put
// infinity
if (!linkedNodes.isEmpty()) {
minWeight.put(node, Collections.min(linkedNodes, nodeComparator).getWeight());
} else {
minWeight.put(node, Double.POSITIVE_INFINITY);
}
}
return minWeight;
}
/**
* Initial tentative array. a tentative array which store the internal value of final shortest
* path array
*
* @param <E> type of element in the node
* @param graph graph
* @param source source node
* @return initial a tentative array which store the internal value of final shortest path array
*/
private static <E> Map<Node<E>, Double> computeTentative(Graph<E> graph, Node<E> source) {
// tentative array return
Map<Node<E>, Double> tentative = new ConcurrentHashMap<Node<E>, Double>();
// put the initial value
for (Node<E> node : graph.getAllNodes()) {
tentative.put(node, Double.POSITIVE_INFINITY);
}
// set length of source - source zero
tentative.put(source, 0.0);
return tentative;
}
/**
* Initial predecessor array. A predecessor array stores the information to recover shortest path
* from tentative array.
*
* @param <E> type of element in the node
* @param graph graph
* @param source source node
* @return initial a predecessor array stores the information to recover shortest path from
* tentative array.
*/
@SuppressWarnings("unchecked")
private static <E> ConcurrentHashMap<Node<E>, AdjacentNode<E>> computePredecessor(
Graph<E> graph, Node<E> source) {
// predecessor array stores the information to recover shortest path
// from tentative array
ConcurrentHashMap<Node<E>, AdjacentNode<E>> predecessor =
new ConcurrentHashMap<Node<E>, AdjacentNode<E>>();
// put the initial value
for (Node<E> node : graph.getAllNodes()) {
predecessor.put(node, new AdjacentNode<E>(DUMMY_NODE, 0.0));
}
return predecessor;
}
/**
* parallel MST algorithm based on Boruvka's algorithm.
*
* @param <E> value type of the node in the target graph
* @param graph the graph to compute minimum spanning tree
* @param pool external thread pool
* @return a minimum spanning tree in the form of undirected graph
* @throws InterruptedException thrown exception if be interrupted
* @throws ExecutionException Exception thrown when attempting to retrieve the result of a task
* that aborted by throwing an exception
*/
public static <E> Graph<E> getMST(final UndirectedGraph graph, ExecutorService pool)
throws InterruptedException, ExecutionException {
if (!(graph instanceof UndirectedGraph)) {
throw new IllegalArgumentException("graph is not an undirected graph");
}
final Graph<E> mst = new UndirectedGraph<E>();
Collection<Node<E>> nodes = graph.getAllNodes();
final Collection<Node<E>> graphNodes = graph.getAllNodes();
int size = nodes.size();
Runnable[] tasks = new Runnable[size];
int index = 0;
final ConcurrentHashMap<Node<E>, Collection<Edge<E>>> edges =
new ConcurrentHashMap<Node<E>, Collection<Edge<E>>>();
// step 0: init , get all the edge list and store them in map edges and
// add all node into mst
Iterator<Node<E>> it = nodes.iterator();
while (it.hasNext()) {
final Node<E> n = it.next();
tasks[index++] =
new Runnable() {
public void run() {
Collection<Edge<E>> edge = graph.getLinkedEdges(n);
edges.put(n, edge);
mst.addNode(n.getValue());
}
};
}
runTasks(tasks, 0, index, pool);
final AtomicBoolean finish = new AtomicBoolean(false);
final ConcurrentHashMap<Node<E>, Node<E>> roots = new ConcurrentHashMap<Node<E>, Node<E>>();
while ((size = (nodes = edges.keySet()).size()) > 1) {
// System.out.println("222 " + size);
Iterator<Node<E>> iter = nodes.iterator();
index = 0;
// final CountDownLatch wait = new CountDownLatch(size);
while (iter.hasNext()) {
final Node<E> cur = iter.next();
// step 1:find lightest edge to each vertex , add it to
// the new graph and remove it
tasks[index++] =
new Runnable() {
public void run() {
Edge<E> e = getMinimumEdgeWith(edges, cur);
if (e == null) {
finish.set(true);
return;
}
Node<E> end = e.getEnd();
Node<E> start = e.getStart();
Node<E> n = roots.get(end);
Node<E> c = roots.get(start);
if (n == null) n = end;
if (c == null) c = start;
// merge the two trees. we vote the node with small
// compare as root
int cmp = n.compareTo(c);
// if (cmp > 0) {
// Node<E> v = roots.putIfAbsent(n, c);
// } else if (cmp < 0) {
// Node<E> v = roots.putIfAbsent(c, n);
// }
if (cmp != 0) mst.addEdge(start, end, e.getWeight());
}
};
}
runTasks(tasks, 0, index, pool);
if (finish.get()) throw new IllegalArgumentException("graph is not a connected graph");
// step 2: find parent
Enumeration<Node<E>> enu = roots.keys();
index = 0;
while (enu.hasMoreElements()) {
// System.out.println("444");
final Node<E> cur = enu.nextElement();
tasks[index++] =
new Runnable() {
public void run() {
Node<E> p = cur;
Node<E> tmp = cur;
do {
p = tmp;
tmp = roots.get(tmp);
} while (tmp != null); // TODO GZ:sometime infinite
// loop here
if (cur != p) {
roots.put(cur, p);
}
}
};
}
runTasks(tasks, 0, index, pool);
// step 3: rename vertex in original graph and remove the edges;
index = 0;
iter = graphNodes.iterator();
while (iter.hasNext()) {
// System.out.println("555");
final Node<E> cur = iter.next();
tasks[index++] =
new Runnable() {
public void run() {
Node<E> p = roots.get(cur);
// for every parent, add all children's edgelist and
// remove all the inner edge
if (p == null) {
Collection<Edge<E>> coll = edges.get(cur);
HashSet<Node<E>> children = new HashSet<Node<E>>();
Iterator<Node<E>> it = graphNodes.iterator();
while (it.hasNext()) {
Node<E> n = it.next();
if (roots.get(n) == cur) {
children.add(n);
if (edges.containsKey(n)) {
coll.addAll(edges.get(n));
edges.remove(n);
}
}
}
Iterator<Edge<E>> iterator = coll.iterator();
while (iterator.hasNext()) {
Edge<E> e = iterator.next();
Node<E> end = e.getEnd();
if (end.equals(cur) || children.contains(end)) {
iterator.remove();
}
}
}
}
};
}
runTasks(tasks, 0, index, pool);
}
// remove loop edge in MST
nodes = mst.getAllNodes();
index = 0;
Iterator<Node<E>> iter = nodes.iterator();
while (iter.hasNext()) {
final Node<E> cur = iter.next();
tasks[index++] =
new Runnable() {
public void run() {
// use a hashset to find the same node, pay space for time
HashSet<Node<E>> sets = new HashSet<Node<E>>();
Collection<AdjacentNode<E>> lns = mst.getLinkedNodes(cur);
Iterator<AdjacentNode<E>> iterAdj = lns.iterator();
AdjacentNode<E> adj;
while (iterAdj.hasNext()) {
adj = iterAdj.next();
Node<E> tar = adj.getNode();
if (!sets.add(tar)) {
iterAdj.remove();
}
}
}
};
}
runTasks(tasks, 0, index, pool);
return mst;
}
/**
* This is an implementation of a parallelization of Dijkstra's shortest path algorithm described
* by Crauser, Mehlhorn, Meyer and Sanders in their paper "A Parallelization of Dijkstra's
* Shortest Path Algorithm" in 23rd Symposium on Mathematical Foundations of Computer Science,
* 1998. To gain a complete understanding of this data structure, please first read this paper,
* available at: http://citeseer.ist.psu.edu/crauser98parallelization.html
*
* <p>This paper propose simple criteria which divide Dijkstra's sequential SSSP algorithm into a
* number of phases,such that the operations within a phase can be done in parallel.
*
* <p>In the first variant (OUT-version) we compute a threshold defined via the weights of the
* outgoing edges:let L=min{tent(u) + c(u,z) : u is queued and (u,z) belongs E} and remove all
* nodes v from the queue then dist(v) = tent(v). The threshold for the OUT-criterion can either
* be computed via a second priority queue for o(v) = tent(v) + min(c(u,v) : (u,v) belongs to E}
* or even on the fly while while removing nodes.
*
* <p>The second variant, the IN-version, is defined via the incoming edges: let M = min{tent(u) :
* u is queued} and i(v) = tent(v) - min{c(u,v) : (u,v) belongs to E} for any queued vertex v.
* Then v can be safely removed from the queue if i(v) <= M. Removable nodes of the IN-type can be
* found efficiently by using an additional priority queue for i(.).
*
* <p>Finally, the INOUT-applies both criteria in conjunction.
*
* <p>Sample performance results here.
*
* @param <E> type of element on node
* @param graph Graph
* @param exec Thread pool for executing task in parallel.
* @param source source node
* @param end end node
* @return the length of shortest path from source node to end node
*/
public static <E> double getShortestPath(
Graph<E> graph, ExecutorService exec, Node<E> source, Node<E> end) {
// if source is the same as end, return 0
if (end.equals(source)) {
return 0;
}
// executor for processing node over threshold
final CompletionService<Boolean> completionService =
new ExecutorCompletionService<Boolean>(exec);
// size of nodes in the graph
final int nodeSize = graph.getAllNodes().size();
// min weight on the edges in the graph. precompute once and for all
// upon initialization
final Map<Node<E>, Double> minWeight = computeMinWeight(graph);
// tentative length of path from source node to end nodes
final Map<Node<E>, Double> tentative = computeTentative(graph, source);
// auxiliary array to record predecessor of shortest path. use to
// recover the shortest path
final Map<Node<E>, AdjacentNode<E>> predecessor = computePredecessor(graph, source);
// nodes queue waiting for being processed
final PriorityQueue<AdjacentNode<E>> queued =
new PriorityQueue<AdjacentNode<E>>(nodeSize, nodeComparator);
// enqueue the source node as initiation
queued.offer(new AdjacentNode<E>(source, (tentative.get(source))));
/*
* thresholds decide which nodes should be choose to be processed
* parallely
*/
final PriorityQueue<AdjacentNode<E>> thresholds =
new PriorityQueue<AdjacentNode<E>>(nodeSize, nodeComparator);
// enqueue the source node as initiation
thresholds.offer(new AdjacentNode<E>(source, (tentative.get(source) + minWeight.get(source))));
// min threshold node from threshold queue
AdjacentNode<E> thrsNode;
// value of min threshold
double threshold;
// settled list all nodes has be processed
final List<Node<E>> settled = new ArrayList<Node<E>>();
while (!queued.isEmpty()) {
// there is still nodes to be processed
// find the first node not be settled from thresholds
do {
thrsNode = thresholds.poll();
} while (settled.contains(thrsNode.getNode()));
// get value of threshold
threshold = thrsNode.getWeight();
while (!queued.isEmpty()) {
// there is still nodes to be processed
if (queued.peek().getWeight() <= threshold) {
// weight of the first unprocessed node is less than
// threshold. it should be dequed for processing.
final Node<E> v = queued.poll().getNode();
// mark it as processed
settled.add(v);
// get the size of linked nodes
int size = graph.getLinkedNodes(v).size();
for (AdjacentNode<E> wAdj : graph.getLinkedNodes(v)) {
// process linked node
final Node<E> w = wAdj.getNode();
final double weight = wAdj.getWeight();
// submit the linked node to executor to parallelly
// process
completionService.submit(
new Callable<Boolean>() {
public Boolean call() {
double x = tentative.get(v) + weight;
// find a shorter path from source node v to
// end node w
if (x < tentative.get(w)) {
// update tentative array and predecessor
// array
tentative.put(w, x);
predecessor.put(w, new AdjacentNode<E>(v, weight));
// update the priority queue queued and
// threshold to reflect the new shorter path
decreasKey(queued, new AdjacentNode<E>(w, x));
decreasKey(thresholds, new AdjacentNode<E>(w, x + minWeight.get(w)));
}
return true;
}
});
}
// synchronization bar.continue after all linked node has
// been processed
try {
for (int i = 0; i < size; i++) {
completionService.take();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
} else {
break;
}
}
}
// recover path using predecessor array
double length = 0;
boolean reachable = false;
AdjacentNode<E> start;
while ((start = predecessor.get(end)).getNode() != DUMMY_NODE) {
reachable = true;
length += start.getWeight();
end = start.getNode();
}
// return the length if reachable, otherwise return infinity
if (reachable) {
return length;
} else {
return Double.POSITIVE_INFINITY;
}
}
