Replay explanations through shared history selections

internal/engine/history_source.go

@@ -19,12 +19,14 @@ const (
 // historySourcePayload keeps source values and replacement facts under the
 // shared model's ownership. Application documents remain shared when immutable.
 type historySourcePayload struct {
-	kind          historySourcePayloadKind
-	application   historyApplication
-	overlay       overlayRecord
-	path          Path
-	origin        Origin
-	contributions []historyPayloadRef
+	kind              historySourcePayloadKind
+	application       historyApplication
+	overlay           overlayRecord
+	applicationRef    historyPayloadRef
+	hasApplicationRef bool
+	path              Path
+	origin            Origin
+	contributions     []historyPayloadRef
 }
 
 func (builder *historyBuilder) observeBaseline(root *document.Node) {
@@ -53,11 +55,22 @@ func (builder *historyBuilder) observeSourceEvidence(root *document.Node, origin
 	if len(applications) == 0 && len(records) == 0 {
 		return
 	}
+	applicationsBySource := make(map[string]historyPayloadRef)
+	applicationsByLayer := make(map[Layer][]historyPayloadRef)
 	for index := range applications {
 		application := applications[index]
 		payload := builder.addSourcePayload(&application, nil)
+		applicationsByLayer[application.layer] = append(applicationsByLayer[application.layer], payload)
 		walkHistoryApplication(application, func(_, effective Path, node *document.Node) {
+			if node.Location.SourceID != "" {
+				applicationsBySource[node.Location.SourceID] = payload
+			}
 			origin := sourceApplicationOrigin(application, node)
+			for _, location := range origin.Locations {
+				if location.SourceID != "" {
+					applicationsBySource[location.SourceID] = payload
+				}
+			}
 			attribution := builder.addAttribution(origin, nil, nil)
 			builder.addObservation(historyObservation{
 				path: effective, phase: historyPhaseSources, payload: payload,
@@ -70,6 +83,12 @@ func (builder *historyBuilder) observeSourceEvidence(root *document.Node, origin
 	for index := range records {
 		record := records[index]
 		payload := builder.addSourcePayload(nil, &record)
+		applicationRef, hasApplication := sourceApplicationForRecord(record, applicationsBySource, applicationsByLayer)
+		if hasApplication {
+			owned := &builder.model.sourcePayloads[payload.id]
+			owned.applicationRef = applicationRef
+			owned.hasApplicationRef = true
+		}
 		for displacedIndex, displaced := range record.displaced {
 			definitionPayload := payload
 			definitionPayload.substep = displacedIndex + 1
@@ -119,6 +138,19 @@ func (builder *historyBuilder) observeSourceEvidence(root *document.Node, origin
 	})
 }
 
+func sourceApplicationForRecord(record overlayRecord, bySource map[string]historyPayloadRef, byLayer map[Layer][]historyPayloadRef) (historyPayloadRef, bool) {
+	for _, location := range record.change.Origin.Locations {
+		if reference, found := bySource[location.SourceID]; found && location.SourceID != "" {
+			return reference, true
+		}
+	}
+	applications := byLayer[record.change.Origin.Layer]
+	if len(applications) == 1 {
+		return applications[0], true
+	}
+	return historyPayloadRef{}, false
+}
+
 func (builder *historyBuilder) addSourcePayload(application *historyApplication, overlay *overlayRecord) historyPayloadRef {
 	payload := builder.nextPayload(historyPhaseSources)
 	if payload.id != len(builder.model.sourcePayloads) {