Build item policies without repeated accumulated copies

internal/engine/imports.go

@@ -960,7 +960,12 @@ func projectNestedCollectionPolicies(composed *composedDocument, scope acquisiti
 	if composed == nil {
 		return
 	}
-	baseOffsets := make(map[string]int)
+	type itemPolicyProjection struct {
+		destination Path
+		policy      collectionPolicy
+		base        int
+	}
+	projected := make(map[string]*itemPolicyProjection)
 	var prefixLengths map[string]int
 	for sourceIndex, contribution := range scope.contributions {
 		if prefixLengths != nil && sourceIndex > 0 {
@@ -983,17 +988,21 @@ func projectNestedCollectionPolicies(composed *composedDocument, scope acquisiti
 			}
 			itemIndex = prefixLengths[key]
 		}
-		base, initialized := baseOffsets[key]
-		policy, _ := composed.collectionPolicies.at(contribution.destination)
+		working, initialized := projected[key]
 		if !initialized {
-			base = len(policy.recordOrder)
-			baseOffsets[key] = base
+			// Read one owned copy per destination, then add each incoming
+			// record without recopying earlier records and their scopes.
+			policy, _ := composed.collectionPolicies.at(contribution.destination)
+			working = &itemPolicyProjection{destination: contribution.destination, policy: policy, base: len(policy.recordOrder)}
+			projected[key] = working
 		}
-		policy.setRecordPolicyAt(base+itemIndex, collectionRecordPolicy{
+		working.policy.setRecordPolicyAt(working.base+itemIndex, collectionRecordPolicy{
 			owner: newCollectionRecordPolicy().owner,
 			scope: contribution.nested.collectionPolicies,
 		})
-		composed.collectionPolicies.set(contribution.destination, policy)
+	}
+	for _, working := range projected {
+		composed.collectionPolicies.set(working.destination, working.policy)
 	}
 }