improve shortest path by traversing from less direction

hugegraph-core/src/main/java/com/baidu/hugegraph/traversal/algorithm/ShortestPathTraverser.java

@@ -120,16 +120,10 @@ public PathSet allShortestPaths(Id sourceV, Id targetV, Directions dir,
         Traverser traverser = new Traverser(sourceV, targetV, dir, labelMap,
                                             degree, skipDegree, capacity);
         while (true) {
+            paths = traverser.forward() ?
+                    traverser.forward(true) : traverser.backward(true);
             // Found, reach max depth or reach capacity, stop searching
-            if (!(paths = traverser.forward(true)).isEmpty() ||
-                --depth <= 0) {
-                break;
-            }
-            checkCapacity(traverser.capacity, traverser.accessed(),
-                          "shortest path");
-
-            if (!(paths = traverser.backward(true)).isEmpty() ||
-                --depth <= 0) {
+            if (!paths.isEmpty() || --depth <= 0) {
                 break;
             }
             checkCapacity(traverser.capacity, traverser.accessed(),
@@ -238,6 +232,10 @@ public PathSet backward(boolean all) {
             return results;
         }
 
+        public boolean forward() {
+            return this.record.lessSources();
+        }
+
         private boolean superNode(Id vertex, Directions direction) {
             if (this.skipDegree <= 0L) {
                 return false;

hugegraph-core/src/main/java/com/baidu/hugegraph/traversal/algorithm/records/DoubleWayMultiPathsRecords.java

@@ -112,6 +112,11 @@ public PathSet findPath(Id target, Function<Id, Boolean> filter,
         return results;
     }
 
+    public boolean lessSources() {
+        return this.sourceRecords.peek().size() <=
+               this.targetRecords.peek().size();
+    }
+
     @Override
     public long accessed() {
         return this.accessed;