Adding a function to print display representation of a join set.

fhirpath/src/main/java/au/csiro/pathling/fhirpath/execution/DataRoot.java

@@ -30,9 +30,8 @@ public interface DataRoot {
   @Nonnull
   ResourceType getParentResourceType();
 
-  default int depth() {
-    return 0;
-  }
+  @Nonnull
+  String toDisplayString();
 
   @Value(staticConstructor = "of")
   class ResourceRoot implements DataRoot {
@@ -51,6 +50,12 @@ public ResourceType getParentResourceType() {
     public String getTag() {
       return resourceType.toCode();
     }
+
+    @Override
+    @Nonnull
+    public String toDisplayString() {
+      return resourceType.toCode();
+    }
   }
 
 
@@ -65,10 +70,6 @@ default ResourceType getParentResourceType() {
     @Nonnull
     DataRoot getMaster();
 
-    default int depth() {
-      return getMaster().depth() + 1;
-    }
-
     @Nonnull
     JoinTag asTag();
 
@@ -101,6 +102,13 @@ public JoinTag asTag() {
       return JoinTag.ReverseResolveTag.of(foreignResourceType, foreignKeyPath);
     }
 
+    @Override
+    @Nonnull
+    public String toDisplayString() {
+      return foreignResourceType.toCode() + "<-" + foreignKeyPath;
+    }
+
+
     public static ReverseResolveRoot ofResource(@Nonnull final ResourceType masterType,
         @Nonnull final ResourceType foreignResourceType,
         @Nonnull final String foreignResourcePath) {
@@ -126,17 +134,19 @@ public ResourceType getResourceType() {
       return foreignResourceType;
     }
 
-    @Override
-    public int depth() {
-      return master.depth() + 1;
-    }
-
     @Nonnull
     @Override
     public JoinTag asTag() {
       return JoinTag.ResolveTag.of(foreignResourceType);
     }
 
+    @Override
+    @Nonnull
+    public String toDisplayString() {
+      return masterResourcePath + "->" + foreignResourceType.toCode();
+    }
+
+
     public static ResolveRoot ofResource(@Nonnull final ResourceType masterType,
         @Nonnull final ResourceType foreignResourceType,
         @Nonnull final String masterResourcePath) {

fhirpath/src/main/java/au/csiro/pathling/fhirpath/execution/JoinSet.java

@@ -14,7 +14,7 @@
 import lombok.AllArgsConstructor;
 import lombok.Value;
 
-@Value()
+@Value
 @AllArgsConstructor(access = AccessLevel.PRIVATE)
 public class JoinSet {
 
@@ -59,7 +59,7 @@ static List<DataRoot> toPath(@Nonnull final DataRoot root) {
   }
 
   @Nonnull
-  public static List<JoinSet> mergeRoots(@Nonnull final List<List<DataRoot>> paths) {
+  private static List<JoinSet> mergeRoots(@Nonnull final List<List<DataRoot>> paths) {
     // convert to paths and then group by recurively by common prefixes
     // we have got it already somwhere else
     final Map<DataRoot, List<List<DataRoot>>> suffixesByHeads = paths.stream()
@@ -74,10 +74,47 @@ public static List<JoinSet> mergeRoots(@Nonnull final List<List<DataRoot>> paths
         .toList();
   }
 
+  /**
+   * Merges a set of roots into a tree of join sets by recursively grouping by common prefixes.
+   *
+   * @param roots the roots to merge
+   * @return the merged join sets
+   */
   @Nonnull
   public static List<JoinSet> mergeRoots(@Nonnull final Set<DataRoot> roots) {
-    // convert to paths and then group by recurively by common prefixes
-    // we have got it already somwhere else
+    // convert to paths and then group by recursively by common prefixes
     return mergeRoots(roots.stream().map(JoinSet::toPath).toList());
   }
+
+  /**
+   * Returns a string representation of the join set as a tree.
+   */
+  @Nonnull
+  public String toTreeString() {
+    return toTreeString(0);
+  }
+
+  /**
+   * Prints a string representation of the join set as a tree.
+   */
+  public void printTree() {
+    System.out.println(toTreeString());
+  }
+
+  @Nonnull
+  private String toTreeString(final int depth) {
+    final StringBuilder sb = new StringBuilder();
+    sb.append("  ".repeat(depth > 0
+                          ? depth - 1
+                          : 0));
+    sb.append(depth > 0
+              ? "+-"
+              : "");
+    sb.append(master.toDisplayString());
+    sb.append("\n");
+    children.forEach(child -> sb.append(child.toTreeString(depth + 1)));
+    return sb.toString();
+  }
+
+
 }

fhirpath/src/main/java/au/csiro/pathling/fhirpath/execution/MultiFhirPathEvaluator.java

@@ -102,8 +102,10 @@ public Dataset<Row> createInitialDataset() {
     //         createInitialDataset(), null)
     //     : createInitialDataset();
 
+    final JoinSet subjectJoinSet = JoinSet.mergeRoots(joinRoots).iterator().next();
+    System.out.println("Subject join set: \n" + subjectJoinSet.toTreeString());
     Dataset<Row> resolvedDataset = resolveJoins(
-        JoinSet.mergeRoots(joinRoots).iterator().next(),
+        subjectJoinSet,
         createInitialDataset());
 
     System.out.println("Resolved dataset:");
@@ -419,7 +421,6 @@ private Dataset<Row> computeReverseJoin(@Nonnull final Dataset<Row> parentDatase
   @Nonnull
   private Dataset<Row> resolveJoins(@Nonnull final JoinSet joinSet,
       @Nonnull final Dataset<Row> parentDataset) {
-
     // now just reduce current children
     return joinSet.getChildren().stream()
         .reduce(parentDataset, (dataset, subset) ->