traversal: track seen links, and revisit only if configured to do so.

traversal/common.go

@@ -34,6 +34,9 @@ func (prog *Progress) init() {
 		prog.Cfg = &Config{}
 	}
 	prog.Cfg.init()
+	if prog.Cfg.LinkVisitOnlyOnce {
+		prog.SeenLinks = make(map[datamodel.Link]struct{})
+	}
 }
 
 // asPathSegment figures out how to coerce a node into a PathSegment.

traversal/fns.go

@@ -37,13 +37,15 @@ type Progress struct {
 		Path datamodel.Path
 		Link datamodel.Link
 	}
-	Budget *Budget // If present, tracks "budgets" for how many more steps we're willing to take before we should halt.
+	Budget    *Budget                     // If present, tracks "budgets" for how many more steps we're willing to take before we should halt.
+	SeenLinks map[datamodel.Link]struct{} // Set used to remember which links have been visited before, if Cfg.LinkVisitOnlyOnce is true.
 }
 
 type Config struct {
 	Ctx                            context.Context                // Context carried through a traversal.  Optional; use it if you need cancellation.
 	LinkSystem                     linking.LinkSystem             // LinkSystem used for automatic link loading, and also any storing if mutation features (e.g. traversal.Transform) are used.
 	LinkTargetNodePrototypeChooser LinkTargetNodePrototypeChooser // Chooser for Node implementations to produce during automatic link traversal.
+	LinkVisitOnlyOnce              bool                           // By default, we visit across links wherever we see them again, even if we've visited them before, because the reason for visiting might be different than it was before since we got to it via a different path.  If set to true, track links we've seen before in Progress.SeenLinks and do not visit them again.  Note that sufficiently complex selectors may require valid revisiting of some links, so setting this to true can change behavior noticably and should be done with care.
 }
 
 type Budget struct {

traversal/walk.go

@@ -134,6 +134,12 @@ func (prog Progress) walkAdv_iterateAll(n datamodel.Node, s selector.Selector, f
 			progNext.Path = prog.Path.AppendSegment(ps)
 			if v.Kind() == datamodel.Kind_Link {
 				lnk, _ := v.AsLink()
+				if prog.Cfg.LinkVisitOnlyOnce {
+					if _, seen := prog.SeenLinks[lnk]; seen {
+						continue
+					}
+					prog.SeenLinks[lnk] = struct{}{}
+				}
 				progNext.LastBlock.Path = progNext.Path
 				progNext.LastBlock.Link = lnk
 				v, err = progNext.loadLink(v, n)
@@ -169,6 +175,12 @@ func (prog Progress) walkAdv_iterateSelective(n datamodel.Node, attn []datamodel
 			progNext.Path = prog.Path.AppendSegment(ps)
 			if v.Kind() == datamodel.Kind_Link {
 				lnk, _ := v.AsLink()
+				if prog.Cfg.LinkVisitOnlyOnce {
+					if _, seen := prog.SeenLinks[lnk]; seen {
+						continue
+					}
+					prog.SeenLinks[lnk] = struct{}{}
+				}
 				progNext.LastBlock.Path = progNext.Path
 				progNext.LastBlock.Link = lnk
 				v, err = progNext.loadLink(v, n)