Avoid repeated prefix scans when projecting item policies

internal/engine/imports.go

@@ -961,17 +961,28 @@ func projectNestedCollectionPolicies(composed *composedDocument, scope acquisiti
 		return
 	}
 	baseOffsets := make(map[string]int)
+	var prefixLengths map[string]int
 	for sourceIndex, contribution := range scope.contributions {
+		if prefixLengths != nil && sourceIndex > 0 {
+			addSequencePrefixLengths(scope.contributions[sourceIndex-1].document, Path{}, prefixLengths)
+		}
 		if contribution.nested == nil || len(contribution.nested.collectionPolicies.collections) == 0 {
 			continue
 		}
-		itemIndex := priorSequenceLength([]acquisitionScope{{contributions: scope.contributions[:sourceIndex]}}, contribution.destination)
-		if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}]; found {
-			if projected, ok := assigned.outputForInput(0); ok {
-				itemIndex = projected
-			}
-		}
 		key := contribution.destination.String()
+		assigned := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: key}]
+		itemIndex, found := assigned.outputForInput(0)
+		if !found {
+			// Normal composition supplies the assignment directly. For a
+			// handoff without ranges, scan each preceding tree only once.
+			if prefixLengths == nil {
+				prefixLengths = make(map[string]int)
+				for _, previous := range scope.contributions[:sourceIndex] {
+					addSequencePrefixLengths(previous.document, Path{}, prefixLengths)
+				}
+			}
+			itemIndex = prefixLengths[key]
+		}
 		base, initialized := baseOffsets[key]
 		policy, _ := composed.collectionPolicies.at(contribution.destination)
 		if !initialized {
@@ -986,6 +997,20 @@ func projectNestedCollectionPolicies(composed *composedDocument, scope acquisiti
 	}
 }
 
+func addSequencePrefixLengths(node *document.Node, path Path, lengths map[string]int) {
+	if node == nil {
+		return
+	}
+	if node.Kind == document.Sequence {
+		lengths[path.String()] += len(node.Sequence)
+		return
+	}
+	// Contribution destinations select mapping paths, not sequence members.
+	for _, entry := range node.Mapping {
+		addSequencePrefixLengths(entry.Value, path.Key(entry.Key), lengths)
+	}
+}
+
 func projectNestedOrigins(target map[string]Origin, root *document.Node, origins map[string]Origin, prefix Path) {
 	var visit func(*document.Node, Path)
 	visit = func(node *document.Node, path Path) {