.

wcfs/internal/xbtree/treediff.go

@@ -324,9 +324,13 @@ func (rs rangeSplit) String() string {
 
 // treediff computes δT/δtrack/δtkeycov for tree/trackSet specified by root in between old..new.
 //
+// It is the main entry-point for BTree-diff algorithm. XXX -> overview
+//
 // δtops is set of top nodes for changed subtrees.
 // δZTC is connected(δZ/T) - connected closure for subset of δZ(old..new) that
 // touches tracked nodes of T.
+//
+// Use δZConnectTracked to prepare δtops and δZTC.
 func treediff(ctx context.Context, root zodb.Oid, δtops setOid, δZTC setOid, trackSet blib.PPTreeSubSet, zconnOld, zconnNew *zodb.Connection) (δT map[Key]ΔValue, δtrack *blib.ΔPPTreeSubSet, δtkeycov *blib.RangedKeySet, err error) {
 	defer xerr.Contextf(&err, "treediff %s..%s %s", zconnOld.At(), zconnNew.At(), root)
 
@@ -340,7 +344,7 @@ func treediff(ctx context.Context, root zodb.Oid, δtops setOid, δZTC setOid, t
 
 	δtrackv := []*blib.ΔPPTreeSubSet{}
 
-	for top := range δtops { // XXX -> sorted?
+	for top := range δtops {
 		a, err1 := zgetNodeOrNil(ctx, zconnOld, top)
 		b, err2 := zgetNodeOrNil(ctx, zconnNew, top)
 		err := xerr.Merge(err1, err2)
@@ -353,15 +357,15 @@ func treediff(ctx context.Context, root zodb.Oid, δtops setOid, δZTC setOid, t
 			return nil, nil, nil, err
 		}
 
-		// FIXME -> merge (VDEL vs add)
-		// XXX no - not needed here - keys cannot migrate in between two disconnected subtrees
-		//     -> assert that keys from different δtop do not overlap
-		// DEL k -> Tkextra += k
-		// +k    -> Tkextra -= k
 		debugfDiff("-> δtop: %v\n", δtop)
 		debugfDiff("-> δtrackTop: %v\n", δtrackTop)
 		debugfDiff("-> δtkeycovTop: %v\n", δtkeycovTop)
 		for k,δv := range δtop {
+			// NOTE keys cannot migrate in between two disconnected subtrees
+			δv_, kdup := δT[k]
+			if kdup {
+				panicf("BUG: key %v present in two disconnected subtrees; δv1: %s  δv2: %s", k, δv_, δv)
+			}
 			δT[k] = δv
 		}
 
@@ -369,7 +373,7 @@ func treediff(ctx context.Context, root zodb.Oid, δtops setOid, δZTC setOid, t
 		δtkeycov.UnionInplace(δtkeycovTop)
 	}
 
-	// adjust trackSet by merge(δtrackTops)
+	// trackSet should be adjusted by merge(δtrackTops)
 	for _, δ := range δtrackv {
 		δtrack.Update(δ)
 	}