.

wcfs/δbtail.go

@@ -244,7 +244,32 @@ type nodeTrack struct {
 //
 // The result B of applying δ to A is:
 //
-//	B = A.Difference(δ.Del).Union(δ.Add)
+//	B = A.xDifference(δ.Del).xUnion(δ.Add)		(*)
+//
+// (*) NOTE δ.Del and δ.Add might have their leafs starting from non-leaf nodes in A/B.
+//     This situation arises when δ represents a change in path to particular
+//     node, but that node itself does not change, for example:
+//
+//            c*             c
+//           / \            /
+//         41*  42         41
+//          |    |         | \
+//         22   43        46  43
+//               |         |   |
+//              44        22  44
+//
+//     Here nodes {c, 41} are changed, node 42 is unlinked, and node 46 is added.
+//     Nodes 43 and 44 stay unchanged.
+//
+//         δ.Del = c-42-43   | c-41-22
+//         δ.Add = c-41-43   | c-41-46-22
+//
+//     The second component with "-22" builds from leaf, but the first
+//     component with "-43" builds from non-leaf node.
+//
+//     Only complete result of applying both δ.Del and δ.Add produce correctly
+//     PP-connected set.
+//
 // XXX place
 type δtrackIndex struct {
 /*