Share sequence history construction

docs/roadmap/implementation.md

@@ -978,6 +978,14 @@ record replacement/removal, modes, origin locations, and sensitivity for both
 precedence and inheritance.
 Focused check: `go test ./internal/engine -run 'Sequence|Collection|Inheritance|History|Explain|Sensitivity'`.
 
+Status: **complete** (E03). Prior completed commits include `8832449`,
+`549fca2`, `3a1b6ff`, and `d172a5e`. Validation passed:
+`env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test ./internal/engine -run 'Sequence|Collection|Inheritance|History|Explain|Sensitivity' -count=1`,
+`env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test ./internal/engine -count=1`,
+`env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test ./... -count=1`,
+`env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test -race ./...`,
+and `git diff --check`.
+
 ## Stage 30 — Index Original Tier-History Values Lazily
 
 Finding: **E04**.

internal/engine/history.go

@@ -413,6 +413,27 @@ func applySequenceHistory(target **document.Node, incoming *document.Node, contr
 	}
 }
 
+func sequenceHistoryContributionForCombination(
+	destination Path,
+	operation sequenceMode,
+	incoming *document.Node,
+	combined sequenceCombinationResult,
+) sequenceHistoryContribution {
+	contribution := sequenceHistoryContribution{
+		path: destination.clone(), mode: sequenceHistoryMapped,
+	}
+	if operation == sequenceModeReplace {
+		contribution.mode = sequenceHistoryReplace
+		if incoming != nil && incoming.Kind == document.Sequence {
+			contribution.length = len(incoming.Sequence)
+		}
+		return contribution
+	}
+	contribution.items = append([]sequenceItemProjection(nil), combined.higherItems...)
+	contribution.replacements = append([]sequenceItemReplacement(nil), combined.replacements...)
+	return contribution
+}
+
 func cloneSequenceHistoryContributions(values []sequenceHistoryContribution) []sequenceHistoryContribution {
 	if values == nil {
 		return nil

internal/engine/history_sequence_test.go

@@ -469,21 +469,18 @@ func TestSequenceHistoryReplaysCombinationEvidence(t *testing.T) {
 				t.Fatalf("combine diagnostics = %#v", diagnostics)
 			}
 
-			mode := sequenceHistoryMapped
-			if test.operation == sequenceModeReplace {
-				mode = sequenceHistoryReplace
-			}
-			length := 0
-			if mode == sequenceHistoryReplace {
-				length = len(combined.higherItems)
-			}
 			application := historyApplication{
 				layer: LayerOverlay,
 				value: higherRoot,
-				sequences: []sequenceHistoryContribution{{
+				sequences: []sequenceHistoryContribution{
-					path: Keys("records"), mode: mode, length: length,
+					sequenceHistoryContributionForCombination(Keys("records"), test.operation, higher, combined),
-					items: combined.higherItems, replacements: combined.replacements,
+				},
-				}},
+			}
+			if len(combined.higherItems) != 0 {
+				combined.higherItems[0].outputIndex++
+			}
+			if len(combined.replacements) != 0 {
+				combined.replacements[0].outputIndex++
 			}
 			application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
 			replayed := document.Clone(lowerRoot)

internal/engine/inheritance_combine.go

@@ -85,17 +85,9 @@ func combineInheritanceTemplate(
 			result.replacements = append(result.replacements, applyInheritanceSequenceOrigins(
 				mergedOrigins, child.origins, childPath, lowerValue, incoming, combined, resolution.operation,
 			)...)
-			history := sequenceHistoryContribution{path: childPath.clone(), mode: sequenceHistoryMapped}
+			sequenceContributions = append(sequenceContributions, sequenceHistoryContributionForCombination(
-			if resolution.operation == sequenceModeReplace {
+				childPath, resolution.operation, incoming, combined,
-				history.mode = sequenceHistoryReplace
+			))
-				if incoming != nil && incoming.Kind == document.Sequence {
-					history.length = len(incoming.Sequence)
-				}
-			} else {
-				history.items = append([]sequenceItemProjection(nil), combined.higherItems...)
-				history.replacements = append([]sequenceItemReplacement(nil), combined.replacements...)
-			}
-			sequenceContributions = append(sequenceContributions, history)
 			return combined.value, true
 		},
 	})

internal/engine/overlay.go

@@ -274,17 +274,9 @@ func (operation *overlayMergeOperation) merge(path Path, existing, incoming *doc
 		operation.incomingOrigins, operation.incomingPolicies, operation.layer,
 	)
 	operation.valueDestinations[path.String()] = true
-	history := sequenceHistoryContribution{path: path.clone(), mode: sequenceHistoryMapped}
+	operation.sequenceApplications[path.String()] = sequenceHistoryContributionForCombination(
-	if resolution.operation == sequenceModeReplace {
+		path, resolution.operation, incoming, combined,
-		history.mode = sequenceHistoryReplace
+	)
-		if incoming != nil && incoming.Kind == document.Sequence {
-			history.length = len(incoming.Sequence)
-		}
-	} else {
-		history.items = append([]sequenceItemProjection(nil), combined.higherItems...)
-		history.replacements = append([]sequenceItemReplacement(nil), combined.replacements...)
-	}
-	operation.sequenceApplications[path.String()] = history
 	return combined.value, true
 }