added static gmm weight map to instance logic tree method

src/main/java/gov/usgs/earthquake/nshmp/gmm/GmmUtils.java

@@ -8,9 +8,12 @@ import static gov.usgs.earthquake.nshmp.gmm.Imt.PGA;
 import static gov.usgs.earthquake.nshmp.gmm.Imt.SA0P02;
 import static java.lang.Math.log;
 
+import java.util.Map;
+
 import com.google.common.primitives.Doubles;
 
 import gov.usgs.earthquake.nshmp.gmm.GroundMotionTables.GroundMotionTable;
+import gov.usgs.earthquake.nshmp.tree.LogicTree;
 
 /**
  * Ground motion model (Gmm) utilities.
@@ -18,6 +21,9 @@ import gov.usgs.earthquake.nshmp.gmm.GroundMotionTables.GroundMotionTable;
  */
 public final class GmmUtils {
 
+  /* Name for gmm logic tree; same as in Deserialize. */
+  private static final String TREE_NAME = "gmm-tree";
+
   /*
    * Base-10 to base-e conversion factor commonly used with ground motion lookup
    * tables that supply log10 instead of natural log values. This conversion
@@ -54,6 +60,22 @@ public final class GmmUtils {
         : (rake >= -135 && rake <= -45) ? NORMAL : STRIKE_SLIP;
   }
 
+  /**
+   * Convert a map of GMMs and weights to a logic tree of instances for the
+   * supplied IMT.
+   */
+  static LogicTree<GroundMotionModel> weightMapToInstanceTree(
+      Map<Gmm, Double> gmms,
+      Imt imt) {
+
+    LogicTree.EnumBuilder<Gmm, GroundMotionModel> tree = LogicTree.enumBuilder(TREE_NAME);
+    gmms.entrySet().forEach(e -> tree.addBranch(
+        e.getKey(),
+        e.getKey().instance(imt),
+        e.getValue()));
+    return tree.build();
+  }
+
   /* Derived from z2p5 via mmoschetti's 2018 ngaw2 db regressions */
   static final double BASIN_Z1P0_UPPER = 0.3;
   static final double BASIN_Z1P0_LOWER = 0.5;