Make breadth-first iterators consume the successor iterators lazily, and refactor to cut about 150 lines of code.

RELNOTES=Lazier Traverser#breadthFirst().

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=321062203

Author: fluentfuture

android/guava-tests/test/com/google/common/graph/TraverserTest.java

@@ -184,6 +184,13 @@ public void forGraph_breadthFirstIterable_javadocExample_canBeIteratedMultipleTi
     assertEqualCharNodes(result, "bfaecd");
   }
 
+  @Test
+  public void forGraph_breadthFirst_infinite() {
+    Iterable<Integer> result =
+        Traverser.forGraph(fixedSuccessors(Iterables.cycle(1, 2, 3))).breadthFirst(0);
+    assertThat(Iterables.limit(result, 4)).containsExactly(0, 1, 2, 3).inOrder();
+  }
+
   @Test
   public void forGraph_breadthFirst_diamond() {
     Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
@@ -373,6 +380,13 @@ public void forGraph_depthFirstPreOrderIterable_javadocExample_canBeIteratedMult
     assertEqualCharNodes(result, "bacefd");
   }
 
+  @Test
+  public void forGraph_depthFirstPreOrder_infinite() {
+    Iterable<Integer> result =
+        Traverser.forGraph(fixedSuccessors(Iterables.cycle(1, 2, 3))).breadthFirst(0);
+    assertThat(Iterables.limit(result, 2)).containsExactly(0, 1).inOrder();
+  }
+
   @Test
   public void forGraph_depthFirstPreOrder_diamond() {
     Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
@@ -784,6 +798,13 @@ public void forTree_withUndirectedNetwork_throws() throws Exception {
     }
   }
 
+  @Test
+  public void forTree_breadthFirst_infinite() {
+    Iterable<Integer> result =
+        Traverser.forTree(fixedSuccessors(Iterables.cycle(1, 2, 3))).breadthFirst(0);
+    assertThat(Iterables.limit(result, 8)).containsExactly(0, 1, 2, 3, 1, 2, 3, 1).inOrder();
+  }
+
   @Test
   public void forTree_breadthFirst_tree() throws Exception {
     Traverser<Character> traverser = Traverser.forTree(TREE);
@@ -912,6 +933,13 @@ public void forTree_breadthFirstIterable_iterableIsLazy() {
     assertThat(graph.requestedNodes).containsExactly('a', 'a', 'd', 'd', 'd', 'g', 'g', 'g');
   }
 
+  @Test
+  public void forTree_depthFirstPreOrder_infinite() {
+    Iterable<Integer> result =
+        Traverser.forTree(fixedSuccessors(Iterables.cycle(1, 2, 3))).depthFirstPreOrder(0);
+    assertThat(Iterables.limit(result, 3)).containsExactly(0, 1, 1).inOrder();
+  }
+
   @Test
   public void forTree_depthFirstPreOrderIterable_tree() throws Exception {
     Traverser<Character> traverser = Traverser.forTree(TREE);
@@ -1238,4 +1266,13 @@ public Iterable<? extends Character> successors(Character node) {
       return delegate.successors(node);
     }
   }
+
+  private static <N> SuccessorsFunction<N> fixedSuccessors(final Iterable<N> successors) {
+    return new SuccessorsFunction<N>() {
+      @Override
+      public Iterable<N> successors(N n) {
+        return successors;
+      }
+    };
+  }
 }