/**
* Find the maximum weight matching of a path using dynamic programming.
*
* @param path a list of edges. The code assumes that the list of edges is a valid simple path,
* and that is not a cycle.
* @return a maximum weight matching of the path
*/
public Pair<Double, Set<E>> getMaximumWeightMatching(Graph<V, E> g, LinkedList<E> path) {
int pathLength = path.size();
// special cases
switch (pathLength) {
case 0:
// special case, empty path
return Pair.of(Double.valueOf(0d), Collections.emptySet());
case 1:
// special case, one edge
E e = path.getFirst();
double eWeight = g.getEdgeWeight(e);
if (comparator.compare(eWeight, 0d) > 0) {
return Pair.of(eWeight, Collections.singleton(e));
} else {
return Pair.of(Double.valueOf(0d), Collections.emptySet());
}
}
// make sure work array has enough space
if (a.length < pathLength + 1) {
a = new double[pathLength + 1];
}
// first pass to find solution
Iterator<E> it = path.iterator();
E e = it.next();
double eWeight = g.getEdgeWeight(e);
a[0] = 0d;
a[1] = (comparator.compare(eWeight, 0d) > 0) ? eWeight : 0d;
for (int i = 2; i <= pathLength; i++) {
e = it.next();
eWeight = g.getEdgeWeight(e);
if (comparator.compare(a[i - 1], a[i - 2] + eWeight) > 0) {
a[i] = a[i - 1];
} else {
a[i] = a[i - 2] + eWeight;
}
}
// reverse second pass to build solution
Set<E> matching = new HashSet<>();
it = path.descendingIterator();
int i = pathLength;
while (i >= 1) {
e = it.next();
if (comparator.compare(a[i], a[i - 1]) > 0) {
matching.add(e);
// skip next edge
if (i > 1) {
e = it.next();
}
i--;
}
i--;
}
// return solution
return Pair.of(a[pathLength], matching);
}
// the algorithm (improved with additional heuristics)
private Pair<Double, Set<E>> runWithHeuristics(Graph<V, E> graph) {
// lookup all relevant vertices
Set<V> visibleVertex = initVisibleVertices(graph);
// create solver for paths
DynamicProgrammingPathSolver pathSolver = new DynamicProgrammingPathSolver();
Set<E> matching = new HashSet<>();
double matchingWeight = 0d;
Set<V> matchedVertices = new HashSet<>();
// run algorithm
while (!visibleVertex.isEmpty()) {
// find vertex arbitrarily
V x = visibleVertex.stream().findAny().get();
// grow path from x
LinkedList<E> path = new LinkedList<>();
while (x != null) {
// first heaviest edge incident to vertex x (among visible neighbors)
double maxWeight = 0d;
E maxWeightedEdge = null;
V maxWeightedNeighbor = null;
for (E e : graph.edgesOf(x)) {
V other = Graphs.getOppositeVertex(graph, e, x);
if (visibleVertex.contains(other) && !other.equals(x)) {
double curWeight = graph.getEdgeWeight(e);
if (comparator.compare(curWeight, 0d) > 0
&& (maxWeightedEdge == null || comparator.compare(curWeight, maxWeight) > 0)) {
maxWeight = curWeight;
maxWeightedEdge = e;
maxWeightedNeighbor = other;
}
}
}
// add edge to path and remove x
if (maxWeightedEdge != null) {
path.add(maxWeightedEdge);
}
visibleVertex.remove(x);
// go to next vertex
x = maxWeightedNeighbor;
}
// find maximum weight matching of path using dynamic programming
Pair<Double, Set<E>> pathMatching = pathSolver.getMaximumWeightMatching(graph, path);
// add it to result while keeping track of matched vertices
matchingWeight += pathMatching.getFirst();
for (E e : pathMatching.getSecond()) {
V s = graph.getEdgeSource(e);
V t = graph.getEdgeTarget(e);
if (!matchedVertices.add(s)) {
throw new RuntimeException("Set is not a valid matching, please submit a bug report");
}
if (!matchedVertices.add(t)) {
throw new RuntimeException("Set is not a valid matching, please submit a bug report");
}
matching.add(e);
}
}
// extend matching to maximal cardinality (out of edges with positive weight)
for (E e : graph.edgeSet()) {
double edgeWeight = graph.getEdgeWeight(e);
if (comparator.compare(edgeWeight, 0d) <= 0) {
// ignore zero or negative weight
continue;
}
V s = graph.getEdgeSource(e);
if (matchedVertices.contains(s)) {
// matched vertex, ignore
continue;
}
V t = graph.getEdgeTarget(e);
if (matchedVertices.contains(t)) {
// matched vertex, ignore
continue;
}
// add edge to matching
matching.add(e);
matchingWeight += edgeWeight;
}
// return extended matching
return Pair.of(matchingWeight, matching);
}
// the algorithm (no heuristics)
private Pair<Double, Set<E>> run(Graph<V, E> graph) {
// lookup all relevant vertices
Set<V> visibleVertex = initVisibleVertices(graph);
// run algorithm
Set<E> m1 = new HashSet<>();
Set<E> m2 = new HashSet<>();
double m1Weight = 0d, m2Weight = 0d;
int i = 1;
while (!visibleVertex.isEmpty()) {
// find vertex arbitrarily
V x = visibleVertex.stream().findAny().get();
// grow path from x
while (x != null) {
// first heaviest edge incident to vertex x (among visible neighbors)
double maxWeight = 0d;
E maxWeightedEdge = null;
V maxWeightedNeighbor = null;
for (E e : graph.edgesOf(x)) {
V other = Graphs.getOppositeVertex(graph, e, x);
if (visibleVertex.contains(other) && !other.equals(x)) {
double curWeight = graph.getEdgeWeight(e);
if (comparator.compare(curWeight, 0d) > 0
&& (maxWeightedEdge == null || comparator.compare(curWeight, maxWeight) > 0)) {
maxWeight = curWeight;
maxWeightedEdge = e;
maxWeightedNeighbor = other;
}
}
}
// add it to either m1 or m2, alternating between them
if (maxWeightedEdge != null) {
switch (i) {
case 1:
m1.add(maxWeightedEdge);
m1Weight += maxWeight;
break;
case 2:
m2.add(maxWeightedEdge);
m2Weight += maxWeight;
break;
default:
throw new RuntimeException("Failed to figure out matching, seems to be a bug");
}
i = 3 - i;
}
// remove x and incident edges
visibleVertex.remove(x);
// go to next vertex
x = maxWeightedNeighbor;
}
}
// return best matching
if (comparator.compare(m1Weight, m2Weight) > 0) {
return Pair.of(m1Weight, m1);
} else {
return Pair.of(m2Weight, m2);
}
}