Clean up Linkage definition

A linkage is a direction/value pair, and that's all. A set of these can
have helper functions and the like. Cleans up some assumptions of
functionality that isn't useful (yet).

graph/iterator.go

@@ -38,10 +38,12 @@ type Tagger struct {
 // Linkage is a union type representing a set of values established for a given
 // quad direction.
 type Linkage struct {
-	Dir    quad.Direction
-	Values []Value
+	Dir   quad.Direction
+	Value Value
 }
 
+// TODO(barakmich): Helper functions as needed, eg, ValuesForDirection(quad.Direction) []Value
+
 // Add a tag to the iterator.
 func (t *Tagger) Add(tag string) {
 	t.tags = append(t.tags, tag)

graph/mongo/indexed_linksto.go

@@ -66,15 +66,9 @@ func NewLinksTo(qs *QuadStore, it graph.Iterator, collection string, d quad.Dire
 
 func (it *LinksTo) buildConstraint() bson.M {
 	constraint := bson.M{}
-	for _, set := range it.lset {
-		var s []string
-		for _, v := range set.Values {
-			s = append(s, it.qs.NameOf(v))
-		}
-		constraint[set.Dir.String()] = bson.M{"$in": s}
-		if len(s) == 1 {
-			constraint[set.Dir.String()] = s[0]
-		}
+	// TODO(barakmich): Currently this only works for an individual linkage in any direction.
+	for _, link := range it.lset {
+		constraint[link.Dir.String()] = link.Value
 	}
 	return constraint
 }
@@ -201,16 +195,9 @@ func (it *LinksTo) Contains(val graph.Value) bool {
 	graph.ContainsLogIn(it, val)
 	it.runstats.Contains += 1
 
-	for _, set := range it.lset {
-		dval := it.qs.QuadDirection(val, set.Dir)
-		good := false
-		for _, val := range set.Values {
-			if val == dval {
-				good = true
-				break
-			}
-		}
-		if !good {
+	for _, link := range it.lset {
+		dval := it.qs.QuadDirection(val, link.Dir)
+		if dval != link.Value {
 			return graph.ContainsLogOut(it, val, false)
 		}
 	}

graph/mongo/quadstore_iterator_optimize.go

@@ -68,7 +68,7 @@ func (qs *QuadStore) optimizeAndIterator(it *iterator.And) (graph.Iterator, bool
 	lset := []graph.Linkage{
 		{
 			Dir:    mongoIt.dir,
-			Values: []graph.Value{qs.ValueOf(mongoIt.name)},
+			Value: qs.ValueOf(mongoIt.name),
 		},
 	}