added mfdTree implementation

src/main/java/gov/usgs/earthquake/nshmp/model/ClusterRuptureSet.java

@@ -9,6 +9,7 @@ import java.util.function.Predicate;
 
 import gov.usgs.earthquake.nshmp.geo.Location;
 import gov.usgs.earthquake.nshmp.mfd.Mfd;
+import gov.usgs.earthquake.nshmp.tree.LogicGroup;
 import gov.usgs.earthquake.nshmp.tree.LogicTree;
 
 /**
@@ -47,15 +48,26 @@ public class ClusterRuptureSet extends AbstractRuptureSet<ClusterSource> {
 
   @Override
   public LogicTree<Mfd> mfdTree() {
-    System.err.println("TODO Cluster MFD tree implementation");
-    // only returning first cluster source
-    throw new UnsupportedOperationException();
-    // return source.mfdTree;
+    /*
+     * A ClusterSource is composed of multiple FaultRuptureSets that rupture
+     * together at some rate. For the purposes of reporting an MFD tree for the
+     * rupture set, we combine the SINGLE MFDs associated with each
+     * FaultRuptureSet and set the rate of each magnitude to its weight. We then
+     * build a logic group where the weight (scale) of each branch is set to the
+     * cluster rate.
+     */
+    ClusterSource clusterSource = source.get(0);
+    double rate = clusterSource.rate();
+
+    LogicGroup.Builder<Mfd> builder = LogicGroup.builder();
+    for (FaultRuptureSet frs : clusterSource.faults()) {
+      Mfd mfd = ModelTrees.reduceMfdTree(frs.mfdTree());
+      builder.addBranch(frs.name(), mfd, rate);
+    }
+    return builder.build();
   }
 
   @Override
-  // this is problemtic as we are not getting the rate weight
-  // is this still a problem?
   public Iterator<ClusterSource> iterator() {
     return source.iterator();
   }