graph/path: Add LabelContext to the path query language.

data/testdata.nq

@@ -11,3 +11,5 @@
 <greg> <status> "cool_person" .
 <predicates> <are> <follows> .
 <predicates> <are> <status> .
+<emily> <status> "smart_person" "smart_graph" .
+<greg> <status> "smart_person" "smart_graph" .

graph/path/morphism_apply_functions.go

@@ -25,11 +25,14 @@ import (
 // join puts two iterators together by intersecting their result sets with an AND
 // Since we're using an and iterator, it's a good idea to put the smallest result
 // set first so that Next() produces fewer values to check Contains().
-func join(qs graph.QuadStore, itL, itR graph.Iterator) graph.Iterator {
+func join(qs graph.QuadStore, its ...graph.Iterator) graph.Iterator {
 	and := iterator.NewAnd(qs)
-	and.AddSubIterator(itL)
-	and.AddSubIterator(itR)
-
+	for _, it := range its {
+		if it == nil {
+			continue
+		}
+		and.AddSubIterator(it)
+	}
 	return and
 }
 
@@ -122,8 +125,9 @@ func outMorphism(tags []string, via ...interface{}) morphism {
 		Reversal: func(ctx *context) (morphism, *context) { return inMorphism(tags, via...), ctx },
 		Apply: func(qs graph.QuadStore, in graph.Iterator, ctx *context) (graph.Iterator, *context) {
 			path := buildViaPath(qs, via...)
-			return inOutIterator(path, in, false, tags), ctx
+			return inOutIterator(path, in, false, tags, ctx), ctx
 		},
+		tags: tags,
 	}
 }
 
@@ -134,8 +138,9 @@ func inMorphism(tags []string, via ...interface{}) morphism {
 		Reversal: func(ctx *context) (morphism, *context) { return outMorphism(tags, via...), ctx },
 		Apply: func(qs graph.QuadStore, in graph.Iterator, ctx *context) (graph.Iterator, *context) {
 			path := buildViaPath(qs, via...)
-			return inOutIterator(path, in, true, tags), ctx
+			return inOutIterator(path, in, true, tags, ctx), ctx
 		},
+		tags: tags,
 	}
 }
 
@@ -145,13 +150,36 @@ func bothMorphism(tags []string, via ...interface{}) morphism {
 		Reversal: func(ctx *context) (morphism, *context) { return bothMorphism(tags, via...), ctx },
 		Apply: func(qs graph.QuadStore, in graph.Iterator, ctx *context) (graph.Iterator, *context) {
 			path := buildViaPath(qs, via...)
-			inSide := inOutIterator(path, in, true, tags)
-			outSide := inOutIterator(path, in.Clone(), false, tags)
+			inSide := inOutIterator(path, in, true, tags, ctx)
+			outSide := inOutIterator(path, in.Clone(), false, tags, ctx)
 			or := iterator.NewOr()
 			or.AddSubIterator(inSide)
 			or.AddSubIterator(outSide)
 			return or, ctx
 		},
+		tags: tags,
+	}
+}
+
+func labelContextMorphism(via ...interface{}) morphism {
+	var path *Path
+	if len(via) == 0 {
+		path = nil
+	} else {
+		path = buildViaPath(nil, via...)
+	}
+	return morphism{
+		Name: "label_context",
+		Reversal: func(ctx *context) (morphism, *context) {
+			out := ctx.copy()
+			ctx.labelSet = path
+			return labelContextMorphism(via...), &out
+		},
+		Apply: func(qs graph.QuadStore, in graph.Iterator, ctx *context) (graph.Iterator, *context) {
+			out := ctx.copy()
+			out.labelSet = path
+			return in, &out
+		},
 	}
 }
 
@@ -290,7 +318,7 @@ func buildSave(
 	return join(qs, from, save)
 }
 
-func inOutIterator(viaPath *Path, from graph.Iterator, inIterator bool, tags []string) graph.Iterator {
+func inOutIterator(viaPath *Path, from graph.Iterator, inIterator bool, tags []string, ctx *context) graph.Iterator {
 	start, goal := quad.Subject, quad.Object
 	if inIterator {
 		start, goal = goal, start
@@ -303,8 +331,14 @@ func inOutIterator(viaPath *Path, from graph.Iterator, inIterator bool, tags []s
 
 	source := iterator.NewLinksTo(viaPath.qs, from, start)
 	trail := iterator.NewLinksTo(viaPath.qs, viaIter, quad.Predicate)
-
-	route := join(viaPath.qs, source, trail)
+	var label graph.Iterator
+	if ctx != nil {
+		if ctx.labelSet != nil {
+			labeliter := ctx.labelSet.BuildIteratorOn(viaPath.qs)
+			label = iterator.NewLinksTo(viaPath.qs, labeliter, quad.Label)
+		}
+	}
+	route := join(viaPath.qs, source, trail, label)
 
 	return iterator.NewHasA(viaPath.qs, route, goal)
 }

graph/path/path.go

@@ -23,7 +23,6 @@ type morphism struct {
 	Reversal func(*context) (morphism, *context)
 	Apply    applyMorphism
 	tags     []string
-	context  context
 }
 
 // context allows a high-level change to the way paths are constructed. Some
@@ -51,6 +50,12 @@ type context struct {
 	labelSet *Path
 }
 
+func (c context) copy() context {
+	return context{
+		labelSet: c.labelSet,
+	}
+}
+
 // Path represents either a morphism (a pre-defined path stored for later use),
 // or a concrete path, consisting of a morphism and an underlying QuadStore.
 type Path struct {
@@ -266,6 +271,13 @@ func (p *Path) Has(via interface{}, nodes ...string) *Path {
 	return p
 }
 
+// LabelContext restricts the following operations (such as In, Out) to only
+// traverse edges that match the given set of labels.
+func (p *Path) LabelContext(via ...interface{}) *Path {
+	p.stack = append(p.stack, labelContextMorphism(via...))
+	return p
+}
+
 // Back returns to a previously tagged place in the path. Any constraints applied after the Tag will remain in effect, but traversal continues from the tagged point instead, not from the end of the chain.
 //
 // For example:

graph/path/path_test.go

@@ -146,7 +146,7 @@ func testSet(qs graph.QuadStore) []test {
 		{
 			message: "implicit All",
 			path:    StartPath(qs),
-			expect:  []string{"alice", "bob", "charlie", "dani", "emily", "fred", "greg", "follows", "status", "cool_person", "predicates", "are"},
+			expect:  []string{"alice", "bob", "charlie", "dani", "emily", "fred", "greg", "follows", "status", "cool_person", "predicates", "are", "smart_graph", "smart_person"},
 		},
 		{
 			message: "follow",
@@ -178,7 +178,7 @@ func testSet(qs graph.QuadStore) []test {
 			message: "show a simple save",
 			path:    StartPath(qs).Save("status", "somecool"),
 			tag:     "somecool",
-			expect:  []string{"cool_person", "cool_person", "cool_person"},
+			expect:  []string{"cool_person", "cool_person", "cool_person", "smart_person", "smart_person"},
 		},
 		{
 			message: "show a simple saveR",
@@ -228,6 +228,17 @@ func testSet(qs graph.QuadStore) []test {
 			path:    StartPath(qs, "fred").FollowReverse(grandfollows),
 			expect:  []string{"alice", "charlie", "dani"},
 		},
+		// Context tests
+		{
+			message: "query without label limitation",
+			path:    StartPath(qs, "greg").Out("status"),
+			expect:  []string{"smart_person", "cool_person"},
+		},
+		{
+			message: "query with label limitation",
+			path:    StartPath(qs, "greg").LabelContext("smart_graph").Out("status"),
+			expect:  []string{"smart_person"},
+		},
 	}
 }