iterator: returns correct subiterator slice for empty And; fixes #659

graph/iterator/and.go

@@ -96,9 +96,11 @@ func (it *And) Clone() graph.Iterator {
 
 // Returns a slice of the subiterators, in order (primary iterator first).
 func (it *And) SubIterators() []graph.Iterator {
-	iters := make([]graph.Iterator, len(it.internalIterators)+1)
-	iters[0] = it.primaryIt
-	copy(iters[1:], it.internalIterators)
+	iters := make([]graph.Iterator, 0, len(it.internalIterators)+1)
+	if it.primaryIt != nil {
+		iters = append(iters, it.primaryIt)
+	}
+	iters = append(iters, it.internalIterators...)
 	return iters
 }
 

graph/iterator/and_optimize.go

@@ -58,8 +58,7 @@ func (it *And) Optimize() (graph.Iterator, bool) {
 
 	// If we can find only one subiterator which is equivalent to this whole and,
 	// we can replace the And...
-	out := it.optimizeReplacement(its)
-	if out != nil {
+	if out := it.optimizeReplacement(its); out != nil {
 		// ...Move the tags to the replacement...
 		moveTagsTo(out, it)
 		// ...Close everyone except `out`, our replacement...
@@ -157,7 +156,7 @@ func (it *And) optimizeOrder(its []graph.Iterator) []graph.Iterator {
 	var (
 		// bad contains iterators that can't be (efficiently) nexted, such as
 		// graph.Optional or graph.Not. Separate them out and tack them on at the end.
-		out, bad []graph.Iterator
+		bad      []graph.Iterator
 		best     graph.Iterator
 		bestCost = int64(1 << 62)
 	)
@@ -200,6 +199,7 @@ func (it *And) optimizeOrder(its []graph.Iterator) []graph.Iterator {
 	// Contains() order based on probability of getting a false Contains() first is
 	// useful (fail faster).
 
+	var out []graph.Iterator
 	// Put the best iterator (the one we wish to Next()) at the front...
 	out = append(out, best)