/**
* Constructor.
*
* @param graph Target directed graph.
* @param thread_num number of threads to compute strong components if thread_num == 1, then it
* is a sequential execution of this algorithm, but not a serial version
*/
public ConnectedComponents(UndirectedGraph graph, ExecutorService threadPool) {
this.graph = (UndirectedGraph) graph;
this.ccs = new Vector<Collection<Node<E>>>(INIT_SIZE_FOR_CCS);
this.pool = threadPool;
this.numThread = Runtime.getRuntime().availableProcessors(); // should
// be
// better
// determined
this.numNodes = graph.size();
this.average = this.numNodes / numThread;
this.nodesArray.ensureCapacity(numNodes);
this.pvalues = new int[numNodes];
this.map = new ConcurrentHashMap<Node<E>, Integer>(numNodes);
// make it parallel
this.nodesArray.addAll(graph.getAllNodes());
}
// for performance comparison, compute connected components serially
public Collection<Collection<Node<E>>> getCCSerially(UndirectedGraph<E> graph) {
// node is one of the elements of nodes, nodes is a subset of dg's
// nodes
Queue<Node<E>> q = new LinkedList<Node<E>>();
Collection<Node<E>> nodes = graph.getAllNodes();
// hashmap to store the nodes, whose children have been visited
HashMap<Node<E>, Integer> hashmap = new HashMap<Node<E>, Integer>(nodes.size());
Iterator<Node<E>> itr = nodes.iterator();
Collection<Node<E>> cc;
Node<E> temp;
Node<E> currentRoot;
while (itr.hasNext()) {
cc = new Vector<Node<E>>(INIT_SIZE_FOR_CC);
currentRoot = itr.next();
if (hashmap.get(currentRoot) != null) // the children already
// been
// visited
continue;
q.add(currentRoot);
while (!q.isEmpty()) {
temp = q.poll(); // temp' children definitely not visited
cc.add(temp);
hashmap.put(temp, 1); // temp's childrent been visited now
Collection<AdjacentNode<E>> collection = graph.getLinkedNodes(temp);
if (collection == null) continue;
Iterator<AdjacentNode<E>> innerIter = collection.iterator();
while (innerIter.hasNext()) {
Node<E> toEnqueue = innerIter.next().getNode();
if (hashmap.get(toEnqueue) == null) // toEnqueue's
// children
// not benn visited yet
q.add(toEnqueue);
}
}
if (cc.size() > 1) ccs.add(cc);
else cc = null;
}
return ccs;
}
/**
* 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;
}
/**
* Compute non-trivial connected components and return them.
*
* @return the result collection.
* @throws java.lang.Exception
*/
public Collection<Collection<Node<E>>> getConnectedComponents() throws Exception {
int i;
if (graph.size() < 2) return this.ccs;
// inittask1
Runnable[] tasks = new Runnable[this.numThread];
for (i = 0; i < this.numThread; i++) {
final int index = i;
tasks[i] =
new Runnable() {
public void run() {
int begin = average * index;
int end = average * (index + 1);
if (index == numThread - 1) end = numNodes;
for (int j = begin; j < end; j++) {
map.put(nodesArray.get(j), j);
pvalues[j] = j;
}
}
};
}
this.runTasks(tasks, 0, numThread);
// barrier
// inittask2
for (i = 0; i < this.numThread; i++) {
final int index = i;
tasks[i] =
new Runnable() {
public void run() {
int begin = average * index;
int end = average * (index + 1);
if (index == numThread - 1) end = numNodes;
Iterator<AdjacentNode<E>> itr;
for (int j = begin; j < end; j++) {
itr = graph.getLinkedNodes(nodesArray.get(j)).iterator();
while (itr.hasNext()) {
pvalues[j] = Math.min(pvalues[j], pvalues[map.get(itr.next().getNode())]);
}
}
}
};
}
this.runTasks(tasks, 0, this.numThread);
// barrier
// main loop
this.updates = 1; // pretend there is an update
while (this.updates != 0) {
this.updates = 0;
Callable<Integer>[] callables = new Callable[this.numThread];
for (i = 0; i < this.numThread; i++) {
final int index = i;
callables[i] =
new Callable<Integer>() {
public Integer call() {
int begin = average * index;
int end = average * (index + 1);
if (index == numThread - 1) end = numNodes;
int localUpdates = 0;
Iterator<AdjacentNode<E>> itr;
// opportunistic pointer jumping
for (int j = begin; j < end; j++) {
int minVertexValue = Integer.MAX_VALUE;
itr = graph.getLinkedNodes(nodesArray.get(j)).iterator();
while (itr.hasNext())
minVertexValue =
Math.min(minVertexValue, pvalues[map.get(itr.next().getNode())]);
if (minVertexValue < pvalues[j]) {
localUpdates++;
}
pvalues[j] = Math.min(pvalues[j], minVertexValue);
}
return localUpdates;
}
};
}
this.runTasks2(callables, 0, numThread); // also acts as an
// implicit barrier
for (i = 0; i < this.numThread; i++) {
final int index = i;
callables[i] =
new Callable<Integer>() {
public Integer call() {
int begin = average * index;
int end = average * (index + 1);
if (index == numThread - 1) end = numNodes;
int localUpdates = 0;
// normal pointer jumping
for (int j = begin; j < end; j++) {
int pvalue = pvalues[j];
if (pvalues[pvalue] < pvalue) {
localUpdates++;
} else if (pvalues[pvalue] > pvalue) throw new RuntimeException("fatal error");
pvalues[j] = pvalues[pvalue];
}
return localUpdates;
}
};
}
this.runTasks2(callables, 0, numThread); // also acts as an
// implicit barrier
// System.out.println("updates:" + this.updates);
}
this.pool.shutdown();
return this.ccs;
}
