Move iterators into separate package

Also reduce API exposure and use standard library more - and fix bugs I
previously introduces in mongo.

graph/iterator/linksto_iterator_test.go

@@ -0,0 +1,39 @@
+// Copyright 2014 The Cayley Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iterator
+
+import (
+	"testing"
+
+	"github.com/google/cayley/graph"
+)
+
+func TestLinksTo(t *testing.T) {
+	ts := new(TestTripleStore)
+	tsFixed := newFixed()
+	tsFixed.AddValue(2)
+	ts.On("GetIdFor", "cool").Return(1)
+	ts.On("GetTripleIterator", graph.Object, 1).Return(tsFixed)
+	fixed := newFixed()
+	fixed.AddValue(ts.GetIdFor("cool"))
+	lto := NewLinksTo(ts, fixed, graph.Object)
+	val, ok := lto.Next()
+	if !ok {
+		t.Error("At least one triple matches the fixed object")
+	}
+	if val != 2 {
+		t.Errorf("Triple index 2, such as %s, should match %s", ts.GetTriple(2), ts.GetTriple(val))
+	}
+}