Removing confusing TripletFields

After additional discussion with rxin, I think having all the possible `TripletField` options is confusing.  This pull request reduces the triplet fields to:

```java
  /**
   * None of the triplet fields are exposed.
   */
  public static final TripletFields None = new TripletFields(false, false, false);

  /**
   * Expose only the edge field and not the source or destination field.
   */
  public static final TripletFields EdgeOnly = new TripletFields(false, false, true);

  /**
   * Expose the source and edge fields but not the destination field. (Same as Src)
   */
  public static final TripletFields Src = new TripletFields(true, false, true);

  /**
   * Expose the destination and edge fields but not the source field. (Same as Dst)
   */
  public static final TripletFields Dst = new TripletFields(false, true, true);

  /**
   * Expose all the fields (source, edge, and destination).
   */
  public static final TripletFields All = new TripletFields(true, true, true);
```

Author: Joseph E. Gonzalez <joseph.e.gonzalez@gmail.com>

Closes #3472 from jegonzal/SimplifyTripletFields and squashes the following commits:

91796b5 [Joseph E. Gonzalez] removing confusing triplet fields

4 files changed, 8 insertions, 33 deletions

diff --git a/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala b/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
index d5150382d5..116d1ea700 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
@@ -129,15 +129,15 @@ class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) extends Seriali
             ctx.sendToSrc(Array((ctx.dstId, ctx.dstAttr)))
             ctx.sendToDst(Array((ctx.srcId, ctx.srcAttr)))
           },
-          (a, b) => a ++ b, TripletFields.SrcDstOnly)
+          (a, b) => a ++ b, TripletFields.All)
       case EdgeDirection.In =>
         graph.aggregateMessages[Array[(VertexId,VD)]](
           ctx => ctx.sendToDst(Array((ctx.srcId, ctx.srcAttr))),
-          (a, b) => a ++ b, TripletFields.SrcOnly)
+          (a, b) => a ++ b, TripletFields.Src)
       case EdgeDirection.Out =>
         graph.aggregateMessages[Array[(VertexId,VD)]](
           ctx => ctx.sendToSrc(Array((ctx.dstId, ctx.dstAttr))),
-          (a, b) => a ++ b, TripletFields.DstOnly)
+          (a, b) => a ++ b, TripletFields.Dst)
       case EdgeDirection.Both =>
         throw new SparkException("collectEdges does not support EdgeDirection.Both. Use" +
           "EdgeDirection.Either instead.")
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/TripletFields.java b/graphx/src/main/scala/org/apache/spark/graphx/TripletFields.java
index 8dfccfe2e2..7eb4ae0f44 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/TripletFields.java
+++ b/graphx/src/main/scala/org/apache/spark/graphx/TripletFields.java
@@ -56,39 +56,14 @@ public class TripletFields implements Serializable {
   public static final TripletFields EdgeOnly = new TripletFields(false, false, true);
 
   /**
-   * Expose only the source field and not the edge or destination field.
-   */
-  public static final TripletFields SrcOnly = new TripletFields(true, false, false);
-
-  /**
-   * Expose only the destination field and not the edge or source field.
-   */
-  public static final TripletFields DstOnly = new TripletFields(false, true, false);
-
-  /**
-   * Expose the source and destination fields but not the edge field.
-   */
-  public static final TripletFields SrcDstOnly = new TripletFields(true, true, false);
-
-  /**
    * Expose the source and edge fields but not the destination field. (Same as Src)
    */
-  public static final TripletFields SrcAndEdge = new TripletFields(true, false, true);
-
-  /**
-   * Expose the source and edge fields but not the destination field. (Same as SrcAndEdge)
-   */
-  public static final TripletFields Src = SrcAndEdge;
+  public static final TripletFields Src = new TripletFields(true, false, true);
 
   /**
    * Expose the destination and edge fields but not the source field. (Same as Dst)
    */
-  public static final TripletFields DstAndEdge = new TripletFields(false, true, true);
-
-  /**
-   * Expose the destination and edge fields but not the source field. (Same as DstAndEdge)
-   */
-  public static final TripletFields Dst = DstAndEdge;
+  public static final TripletFields Dst = new TripletFields(false, true, true);
 
   /**
    * Expose all the fields (source, edge, and destination).
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/lib/PageRank.scala b/graphx/src/main/scala/org/apache/spark/graphx/lib/PageRank.scala
index e40ae0d615..e139959c3f 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/lib/PageRank.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/lib/PageRank.scala
@@ -85,7 +85,7 @@ object PageRank extends Logging {
       // Associate the degree with each vertex
       .outerJoinVertices(graph.outDegrees) { (vid, vdata, deg) => deg.getOrElse(0) }
       // Set the weight on the edges based on the degree
-      .mapTriplets( e => 1.0 / e.srcAttr, TripletFields.SrcOnly )
+      .mapTriplets( e => 1.0 / e.srcAttr, TripletFields.Src )
       // Set the vertex attributes to the initial pagerank values
       .mapVertices( (id, attr) => resetProb )
 
@@ -97,7 +97,7 @@ object PageRank extends Logging {
       // Compute the outgoing rank contributions of each vertex, perform local preaggregation, and
       // do the final aggregation at the receiving vertices. Requires a shuffle for aggregation.
       val rankUpdates = rankGraph.aggregateMessages[Double](
-        ctx => ctx.sendToDst(ctx.srcAttr * ctx.attr), _ + _, TripletFields.SrcAndEdge)
+        ctx => ctx.sendToDst(ctx.srcAttr * ctx.attr), _ + _, TripletFields.Src)
 
       // Apply the final rank updates to get the new ranks, using join to preserve ranks of vertices
       // that didn't receive a message. Requires a shuffle for broadcasting updated ranks to the
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala
index df773db6e4..a05d1ddb21 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala
@@ -328,7 +328,7 @@ class GraphSuite extends FunSuite with LocalSparkContext {
               "expected ctx.dstAttr to be null due to TripletFields, but it was " + ctx.dstAttr)
           }
           ctx.sendToDst(ctx.srcAttr)
-        }, _ + _, TripletFields.SrcOnly)
+        }, _ + _, TripletFields.Src)
       assert(agg.collect().toSet === (1 to n).map(x => (x: VertexId, "v")).toSet)
     }
   }