.

wcfs/δbtail.go

@@ -726,6 +726,12 @@ func diffT(ctx context.Context, a, b *Tree, δZTC SetOid, trackIdx map[zodb.Oid]
 	Adone  := SetKey{} // "processed" keys on A
 	Bdone  := SetKey{} // "processed" keys on B
 
+	if b != nil {
+		// XXX precise range as for a ^^^ ?
+		btop := &nodeInRange{lo: KeyMin, hi_: KeyMax, node: b} // [-∞, ∞)
+		bv   = rangeSplit{btop}
+	}
+
 	// initial phase: expand changed nodes in a till buckets;
 	// XXX changed buckets -> δ-
 	if a != nil {
@@ -772,14 +778,13 @@ func diffT(ctx context.Context, a, b *Tree, δZTC SetOid, trackIdx map[zodb.Oid]
 		}
 	}
 
+	// phase 2: iterate through keys queued for A and B, delve into
+	// corresponding nodes, and merge diff generated from them into δ.
+	// Each delve for A or B, potentially adds new keys to process on the
+	// other side.
 	for len(Aqueue) > 0 || len(Bqueue) > 0 {
 		// expand keys in new δA -> in B till buckets;
 		// process B buckets that cover new keys into δ+
-		if b != nil {
-			// XXX precise range as for a ^^^ ?
-			btop := &nodeInRange{lo: KeyMin, hi_: KeyMax, node: b} // [-∞, ∞)
-			bv   = rangeSplit{btop}
-		}
 		for k := range Bqueue {
 			Bdone.Add(k)
 			bbucket, ok, err := bv.GetToBucket(ctx, k)