Project sequence evidence through item mappings

2026-10-23 18:07:15 +00:00
parent 7b04dc6a50
commit f6b02dec03
10 changed files with 374 additions and 66 deletions
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -1,6 +1,6 @@
 # Sequence Identity And Combination Implementation Plan

-Status: implementation in progress; Stages 1–7 complete.
+Status: implementation in progress; Stages 1–8 complete.

 Audience: a Luna coding agent implementing one numbered stage per prompt, in
 order. This plan starts at commit
@@ -359,6 +359,10 @@ Scope: use the prepared item mappings consistently in metadata consumers.
 Validation: metadata/history/sequence/provenance/sensitivity tests,
 `go test -race ./internal/engine`, and the everyday check.

+**Completion:** Implemented and verified with focused projection, history,
+sequence, provenance, and sensitivity tests, `go test -race ./internal/engine`,
+and `./scripts/check.sh`.
+
 ## Stage 9 — Separate Per-Input Acquisition From Composition

 Scope: a behavior-preserving acquired-input handoff; grouped underlay coordination
--- a/internal/engine/compose.go
+++ b/internal/engine/compose.go
@@ -83,6 +83,48 @@ type sequenceRange struct {
 	path        Path
 	start       int
 	end         int
+	items       []sequenceItemProjection
+	projection  sequenceItemProjectionIndex
+}
+
+func (assigned sequenceRange) projections() []sequenceItemProjection {
+	if assigned.items != nil {
+		return append([]sequenceItemProjection{}, assigned.items...)
+	}
+	return sequenceItemProjections(assigned.end-assigned.start, assigned.start)
+}
+
+func (assigned sequenceRange) outputForInput(inputIndex int) (int, bool) {
+	if assigned.projection.outputByInput != nil {
+		return assigned.projection.outputForInput(inputIndex)
+	}
+	if assigned.items != nil {
+		return newSequenceItemProjectionIndex(assigned.items).outputForInput(inputIndex)
+	}
+	if inputIndex < 0 || inputIndex >= assigned.end-assigned.start {
+		return 0, false
+	}
+	return assigned.start + inputIndex, true
+}
+
+func (assigned sequenceRange) inputForOutput(outputIndex int) (int, bool) {
+	if assigned.projection.inputByOutput != nil {
+		return assigned.projection.inputForOutput(outputIndex)
+	}
+	if assigned.items != nil {
+		return newSequenceItemProjectionIndex(assigned.items).inputForOutput(outputIndex)
+	}
+	if outputIndex < assigned.start || outputIndex >= assigned.end {
+		return 0, false
+	}
+	return outputIndex - assigned.start, true
+}
+
+func indexSequenceRange(assigned sequenceRange) sequenceRange {
+	if assigned.projection.outputByInput == nil && assigned.projection.inputByOutput == nil {
+		assigned.projection = newSequenceItemProjectionIndex(assigned.projections())
+	}
+	return assigned
 }

 // composeDocuments recursively joins disjoint mapping entries. Source order
@@ -371,6 +413,11 @@ func cloneSequenceRanges(ranges []sequenceRange) []sequenceRange {
 	for i, assigned := range ranges {
 		cloned[i] = assigned
 		cloned[i].path = assigned.path.clone()
+		if assigned.items != nil {
+			cloned[i].items = append([]sequenceItemProjection{}, assigned.items...)
+		}
+		cloned[i].projection.outputByInput = append([]int(nil), assigned.projection.outputByInput...)
+		cloned[i].projection.inputByOutput = append([]int(nil), assigned.projection.inputByOutput...)
 	}
 	return cloned
 }
@@ -410,12 +457,22 @@ func documentEntryIndex(entries []document.Entry, key string) int {

 func projectedCompositionOrigins(inputs []*document.Node, ranges []sequenceRange) map[string]Origin {
 	origins := make(map[string]Origin)
-	rangeBySourcePath := make(map[sequenceRangeKey]int, len(ranges))
-	for index, assigned := range ranges {
-		rangeBySourcePath[sequenceRangeKey{sourceIndex: assigned.sourceIndex, path: assigned.path.String()}] = index
+	sequencesBySource := make([][]sequenceHistoryContribution, len(inputs))
+	for _, assigned := range ranges {
+		sequencesBySource[assigned.sourceIndex] = append(sequencesBySource[assigned.sourceIndex], sequenceHistoryContribution{
+			path: assigned.path.clone(), mode: sequenceHistoryMapped, items: assigned.projections(),
+		})
 	}
 	for sourceIndex, input := range inputs {
-		collectProjectedCompositionOrigins(input, Path{}, Path{}, sourceIndex, ranges, rangeBySourcePath, origins)
+		application := historyApplication{value: input, sequences: sequencesBySource[sourceIndex]}
+		application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+		walkHistoryApplication(application, func(_, effective Path, node *document.Node) {
+			pointer := effective.String()
+			origin := origins[pointer]
+			origin.Layer = LayerBase
+			origin.Locations = append(origin.Locations, publicLocation(node.Location))
+			origins[pointer] = origin
+		})
 	}
 	return origins
 }
@@ -433,39 +490,12 @@ func indexSequenceRanges(ranges []sequenceRange) sequenceRangeIndex {
 	}
 	index := make(sequenceRangeIndex, len(ranges))
 	for _, assigned := range ranges {
+		assigned = indexSequenceRange(assigned)
 		index[sequenceRangeKey{sourceIndex: assigned.sourceIndex, path: assigned.path.String()}] = assigned
 	}
 	return index
 }

-// Keep local paths for matching Stage 2 ranges and effective paths for emitted
-// evidence. The paths are transformed segment by segment; pointer strings are
-// only used as map keys after projection is complete.
-func collectProjectedCompositionOrigins(node *document.Node, localPath, effectivePath Path, sourceIndex int, ranges []sequenceRange, rangeBySourcePath map[sequenceRangeKey]int, origins map[string]Origin) {
-	pointer := effectivePath.String()
-	origin := origins[pointer]
-	origin.Layer = LayerBase
-	origin.Locations = append(origin.Locations, publicLocation(node.Location))
-	origins[pointer] = origin
-
-	switch node.Kind {
-	case document.Mapping:
-		for _, entry := range node.Mapping {
-			collectProjectedCompositionOrigins(entry.Value, localPath.Key(entry.Key), effectivePath.Key(entry.Key), sourceIndex, ranges, rangeBySourcePath, origins)
-		}
-	case document.Sequence:
-		start := 0
-		if rangeIndex, ok := rangeBySourcePath[sequenceRangeKey{sourceIndex: sourceIndex, path: localPath.String()}]; ok {
-			start = ranges[rangeIndex].start
-		}
-		for index, child := range node.Sequence {
-			localChild := localPath.Index(index)
-			effectiveChild := effectivePath.Index(start + index)
-			collectProjectedCompositionOrigins(child, localChild, effectiveChild, sourceIndex, ranges, rangeBySourcePath, origins)
-		}
-	}
-}
-
 func applyCompositionOrigins(decoded *decodedConfiguration, origins map[string]Origin) {
 	for pointer, origin := range origins {
 		if _, present := decoded.presence[pointer]; present {
--- a/internal/engine/compose_test.go
+++ b/internal/engine/compose_test.go
@@ -357,6 +357,78 @@ escaped/~:
 	}
 }

+func TestProjectedCompositionOriginsUseNoncontiguousItemMappings(t *testing.T) {
+	lowerRoot := parseCompositionDocument(t, "lower", `records:
+  - id: first
+    label: equal
+  - id: middle
+  - id: last
+    nested: {old: lower}
+`)
+	higherRoot := parseCompositionDocument(t, "higher", `records:
+  - id: last
+    nested: {new: higher}
+  - id: first
+    label: equal
+  - id: added
+`)
+	lower, _ := documentAtPath(lowerRoot, Keys("records"))
+	higher, _ := documentAtPath(higherRoot, Keys("records"))
+	combined, diagnostics := combineSequence(
+		lower, higher, sequenceCombinationPolicy(sequenceModeUpsert, "id"),
+		sequenceCombinationContext{destination: Keys("records")},
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("combine diagnostics = %#v", diagnostics)
+	}
+	inputs := []*document.Node{lowerRoot, higherRoot}
+	ranges := []sequenceRange{
+		{
+			sourceIndex: 0, path: Keys("records"), start: 0, end: 3,
+			items: combined.lowerItems,
+		}, {
+			sourceIndex: 1, path: Keys("records"), start: 0, end: 3,
+			items: combined.higherItems,
+		},
+	}
+	origins := projectedCompositionOrigins(inputs, ranges)
+	if got, want := sequenceIdentityValues(combined.value, "id"), []string{"first", "middle", "last", "added"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("combined IDs = %#v, want %#v", got, want)
+	}
+	for path, want := range map[string][]string{
+		"/records/0/label":      {"higher"},
+		"/records/1/id":         {"lower"},
+		"/records/2/nested/new": {"higher"},
+		"/records/3/id":         {"higher"},
+		"/records/0/nested/new": nil,
+		"/records/2/nested/old": nil,
+	} {
+		origin := origins[path]
+		var got []string
+		for _, location := range origin.Locations {
+			got = append(got, location.SourceID)
+		}
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("origin sources at %s = %#v, want %#v", path, got, want)
+		}
+	}
+}
+
+func TestCloneSequenceRangesPreservesPreparedProjection(t *testing.T) {
+	ranges := []sequenceRange{{
+		start: 0, end: 1,
+		projection: newSequenceItemProjectionIndex([]sequenceItemProjection{{inputIndex: 0, outputIndex: 4}}),
+	}}
+	cloned := cloneSequenceRanges(ranges)
+	if output, ok := cloned[0].outputForInput(0); !ok || output != 4 {
+		t.Fatalf("cloned projection = %d, %v; want output index 4", output, ok)
+	}
+	cloned[0].projection.outputByInput[0] = 9
+	if output, ok := ranges[0].outputForInput(0); !ok || output != 4 {
+		t.Fatalf("cloned projection aliases original: %d, %v", output, ok)
+	}
+}
+
 func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
 	tests := []struct {
 		name     string
--- a/internal/engine/directives.go
+++ b/internal/engine/directives.go
@@ -165,10 +165,11 @@ func fragmentVersionClaimWithIndex(root *document.Node, rule *VersionRule, sourc
 				return current
 			}
 			if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: currentPath.String()}]; found {
-				if index < assigned.start || index >= assigned.end {
+				projected, ok := assigned.inputForOutput(index)
+				if !ok {
 					return nil
 				}
-				index -= assigned.start
+				index = projected
 			}
 			if index < 0 || index >= len(current.Sequence) {
 				return nil
--- a/internal/engine/history.go
+++ b/internal/engine/history.go
@@ -273,8 +273,7 @@ func sequenceHistoryAt(contributions map[string]sequenceHistoryContributionIndex

 type sequenceHistoryContributionIndex struct {
 	sequenceHistoryContribution
-	outputIndexes   []int
-	inputIndexes    []int
+	projection      sequenceItemProjectionIndex
 	replacedOutputs map[int]struct{}
 }

@@ -286,26 +285,7 @@ func indexSequenceHistoryContributions(values []sequenceHistoryContribution) map
 	for _, value := range values {
 		contribution := sequenceHistoryContributionIndex{sequenceHistoryContribution: value}
 		if value.mode == sequenceHistoryMapped {
-			maxOutput := -1
-			for _, projection := range value.items {
-				if projection.outputIndex > maxOutput {
-					maxOutput = projection.outputIndex
-				}
-			}
-			contribution.outputIndexes = make([]int, len(value.items))
-			for inputIndex := range contribution.outputIndexes {
-				contribution.outputIndexes[inputIndex] = -1
-			}
-			contribution.inputIndexes = make([]int, maxOutput+1)
-			for outputIndex := range contribution.inputIndexes {
-				contribution.inputIndexes[outputIndex] = -1
-			}
-			for _, projection := range value.items {
-				if projection.inputIndex >= 0 && projection.inputIndex < len(contribution.outputIndexes) && projection.outputIndex >= 0 && projection.outputIndex < len(contribution.inputIndexes) {
-					contribution.outputIndexes[projection.inputIndex] = projection.outputIndex
-					contribution.inputIndexes[projection.outputIndex] = projection.inputIndex
-				}
-			}
+			contribution.projection = newSequenceItemProjectionIndex(value.items)
 			contribution.replacedOutputs = make(map[int]struct{}, len(value.replacements))
 			for _, replacement := range value.replacements {
 				contribution.replacedOutputs[replacement.outputIndex] = struct{}{}
@@ -317,17 +297,19 @@ func indexSequenceHistoryContributions(values []sequenceHistoryContribution) map
 }

 func (contribution sequenceHistoryContributionIndex) outputForInput(inputIndex int) int {
-	if inputIndex < 0 || inputIndex >= len(contribution.outputIndexes) {
+	outputIndex, ok := contribution.projection.outputForInput(inputIndex)
+	if !ok {
 		return -1
 	}
-	return contribution.outputIndexes[inputIndex]
+	return outputIndex
 }

 func (contribution sequenceHistoryContributionIndex) inputForOutput(outputIndex int) int {
-	if outputIndex < 0 || outputIndex >= len(contribution.inputIndexes) {
+	inputIndex, ok := contribution.projection.inputForOutput(outputIndex)
+	if !ok {
 		return -1
 	}
-	return contribution.inputIndexes[outputIndex]
+	return inputIndex
 }

 func applySequenceHistory(target **document.Node, incoming *document.Node, contribution sequenceHistoryContributionIndex) {
--- a/internal/engine/history_sequence_test.go
+++ b/internal/engine/history_sequence_test.go
@@ -230,6 +230,77 @@ func TestReplacedProjectedSensitiveRecordRemainsRedacted(t *testing.T) {
 	}
 }

+func TestMappedUpsertProjectsSensitiveReplacementEvidence(t *testing.T) {
+	lowerRoot := parseCompositionDocument(t, "lower", `records:
+  - id: first
+  - id: middle
+  - id: last
+    token: MAPPED-SECRET-CANARY
+`)
+	higherRoot := parseCompositionDocument(t, "higher", `records:
+  - id: last
+    token: visible
+  - id: first
+  - id: added
+`)
+	lower, _ := documentAtPath(lowerRoot, Keys("records"))
+	higher, _ := documentAtPath(higherRoot, Keys("records"))
+	combined, diagnostics := combineSequence(
+		lower, higher,
+		sequenceCombinationPolicy(sequenceModeUpsert, "id"),
+		sequenceCombinationContext{destination: Keys("records")},
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("combine diagnostics = %#v", diagnostics)
+	}
+	secretPath := Keys("records").Index(2).Key("token")
+	secretNode, present := documentAtPath(lowerRoot, Keys("records").Index(2).Key("token"))
+	if !present {
+		t.Fatal("sensitive source token is missing")
+	}
+	secretNode.Sensitive = true
+	application := historyApplication{
+		layer: LayerOverlay, value: higherRoot,
+		sequences: []sequenceHistoryContribution{{
+			path: Keys("records"), mode: sequenceHistoryMapped,
+			items: combined.higherItems, replacements: combined.replacements,
+		}},
+	}
+	application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+	applications := []historyApplication{{value: lowerRoot}, application}
+	replayed := &document.Node{Kind: document.Mapping}
+	for _, current := range applications {
+		applyHistoryApplication(&replayed, current, new(mappingIndex))
+	}
+	replayedRecords, _ := documentAtPath(replayed, Keys("records"))
+	if !reflect.DeepEqual(replayedRecords.Sequence, combined.value.Sequence) {
+		t.Fatalf("replayed value = %#v, want combination = %#v", replayedRecords.Sequence, combined.value.Sequence)
+	}
+	finalRoot := document.Clone(lowerRoot)
+	finalRecords, _ := documentAtPath(finalRoot, Keys("records"))
+	finalRecords.Sequence = document.Clone(combined.value).Sequence
+
+	var metadata Metadata
+	finalizeMetadataPaths(&metadata, finalRoot, applications)
+	inspection := &Inspection{
+		metadata: metadata, baseline: &document.Node{Kind: document.Mapping},
+		input: finalRoot, final: finalRoot, applications: applications,
+	}
+	secretRule := func(candidate Path) bool { return reflect.DeepEqual(candidate, secretPath) }
+	inspection.secrets = classifySecretPaths(nil, secretRule, metadata, finalRoot, applications)
+	explanation, ok := inspection.Explain(secretPath)
+	if !ok || !explanation.Effective.Redacted {
+		t.Fatalf("mapped replacement explanation = %#v, %v", explanation, ok)
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, secretPath); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(rendered.String(), "MAPPED-SECRET-CANARY") || !strings.Contains(rendered.String(), "[REDACTED]") {
+		t.Fatalf("mapped replacement history leaked or was not redacted: %s", rendered.String())
+	}
+}
+
 func TestSequenceHistoryReplaysCombinationEvidence(t *testing.T) {
 	tests := []struct {
 		name      string
--- a/internal/engine/imports.go
+++ b/internal/engine/imports.go
@@ -817,8 +817,13 @@ func projectNestedEvidence(composed *composedDocument, scope acquisitionScope) {
 			projectedApplications = append(projectedApplications, applications[sourceIndex])
 			continue
 		}
-		start := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}].start
-		prefix := contribution.destination.Index(start)
+		itemIndex := 0
+		if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}]; found {
+			if projected, ok := assigned.outputForInput(0); ok {
+				itemIndex = projected
+			}
+		}
+		prefix := contribution.destination.Index(itemIndex)
 		if sourceIndex < len(applications) {
 			outer := applications[sourceIndex]
 			structural := document.Clone(outer.value)
@@ -875,7 +880,12 @@ func projectAcquiredDestinations(composed *composedDocument, scope acquisitionSc
 			}
 			contributionPrefix := prefix
 			contributionInsideItem := insideItem
-			start := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}].start
+			itemIndex := 0
+			if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}]; found {
+				if projected, ok := assigned.outputForInput(0); ok {
+					itemIndex = projected
+				}
+			}
 			if contribution.source.placementMode == placementModeSequenceItem {
 				container := appendDestination(contributionPrefix, contribution.destination)
 				contribution.source.Destination = container.clone()
@@ -885,7 +895,7 @@ func projectAcquiredDestinations(composed *composedDocument, scope acquisitionSc
 					sources[position].Destination = container
 					sources[position].destinationUnresolved = false
 				}
-				contributionPrefix = appendDestination(container, Path{}.Index(start))
+				contributionPrefix = appendDestination(container, Path{}.Index(itemIndex))
 				contributionInsideItem = true
 				projectSourceVersionEvidence(contribution.processing, contributionPrefix, nil, sourceIndex)
 			} else if contributionInsideItem {
@@ -928,7 +938,9 @@ func projectSourceVersionEvidence(processing *sourceVersionProcessingResult, des
 				}
 				item, ok := schemaIndex(path.segments[position])
 				if ok && path.segments[position].kind != pathSegmentKey {
-					path.segments[position] = pathSegment{kind: pathSegmentIndex, index: assigned.start + item}
+					if outputIndex, projected := assigned.outputForInput(item); projected {
+						path.segments[position] = pathSegment{kind: pathSegmentIndex, index: outputIndex}
+					}
 				}
 			}
 			paths[index] = path
--- a/internal/engine/imports_test.go
+++ b/internal/engine/imports_test.go
@@ -525,6 +525,82 @@ func TestLoadMainItemPlacementKeepsItsHelpersInTheItem(t *testing.T) {
 	}
 }

+func TestProjectAcquiredDestinationsUsesMappedSequenceItemIndex(t *testing.T) {
+	child := &composedDocument{}
+	childScope := acquisitionScope{contributions: []acquiredContribution{{
+		source: SourceInfo{ID: "nested-source"}, destination: Keys("settings"),
+	}}}
+	child.acquisition = []acquisitionScope{childScope}
+	composed := &composedDocument{sequenceRanges: []sequenceRange{{
+		sourceIndex: 0, path: Keys("records"), start: 0, end: 1,
+		items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+	}}}
+	scope := acquisitionScope{contributions: []acquiredContribution{{
+		source:      SourceInfo{ID: "outer-source", placementMode: placementModeSequenceItem},
+		destination: Keys("records"), nested: child,
+	}}}
+	sources := []SourceInfo{{ID: "outer-source"}, {ID: "nested-source"}}
+
+	projectAcquiredDestinations(composed, scope, sources)
+	if got, want := sources[0].Destination.String(), "/records"; got != want {
+		t.Errorf("outer source destination = %q, want %q", got, want)
+	}
+	if got, want := sources[1].Destination.String(), "/records/2/settings"; got != want {
+		t.Errorf("nested source destination = %q, want %q", got, want)
+	}
+}
+
+func TestProjectNestedEvidenceUsesMappedSequenceItemIndex(t *testing.T) {
+	outer := historyApplication{
+		value: parseCompositionDocument(t, "outer-source", "records: [{id: outer}]\n"),
+		sequences: []sequenceHistoryContribution{{
+			path: Keys("records"), mode: sequenceHistoryMapped,
+			items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+		}},
+	}
+	outer.sequencesByPath = indexSequenceHistoryContributions(outer.sequences)
+	nestedRoot := parseCompositionDocument(t, "nested-source", "settings: {value: nested}\n")
+	nestedOrigin := Origin{Layer: LayerBase, Locations: []Location{{SourceID: "nested-source"}}}
+	nested := &composedDocument{
+		root: nestedRoot, applications: []historyApplication{{value: nestedRoot}},
+		origins: map[string]Origin{"/settings/value": nestedOrigin},
+	}
+	scope := acquisitionScope{contributions: []acquiredContribution{{
+		source: SourceInfo{ID: "outer-source"}, destination: Keys("records"), nested: nested,
+	}}}
+	composed := &composedDocument{
+		applications: []historyApplication{outer},
+		sequenceRanges: []sequenceRange{{
+			sourceIndex: 0, path: Keys("records"), start: 0, end: 1,
+			items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+		}},
+		origins: make(map[string]Origin),
+	}
+
+	projectNestedEvidence(composed, scope)
+	if got, want := composed.applications[1].path.String(), "/records/2"; got != want {
+		t.Fatalf("nested history application path = %q, want %q", got, want)
+	}
+	if origin := composed.origins["/records/2/settings/value"]; len(origin.Locations) != 1 || origin.Locations[0].SourceID != "nested-source" {
+		t.Fatalf("nested origin = %#v", origin)
+	}
+	query := Keys("records").Index(2).Key("settings").Key("value")
+	if operation, relevant := applicationOperationPath(composed.applications[1], query, nil); !relevant || operation.String() != query.String() {
+		t.Fatalf("nested historical query = %s, %v", operation.String(), relevant)
+	}
+	replayed := parseCompositionDocument(t, "lower", "records: [{id: first}, {id: middle}, {id: last}]\n")
+	for _, application := range composed.applications {
+		applyHistoryApplication(&replayed, application, new(mappingIndex))
+	}
+	if value, present := documentAtPath(replayed, query); !present || value.Text != "nested" {
+		t.Fatalf("replayed nested value = %#v, %v", value, present)
+	}
+	composed.applications[1].path.segments[0].key = "detached"
+	if got := nested.applications[0].path.String(); got != "" {
+		t.Fatalf("projected nested history path aliases its source: %q", got)
+	}
+}
+
 func TestItemImportCanJoinAnAuthoredListUnderRegisteredComposition(t *testing.T) {
 	t.Parallel()
 	type config struct {
--- a/internal/engine/sequence_projection.go
+++ b/internal/engine/sequence_projection.go
@@ -0,0 +1,50 @@
+package engine
+
+type sequenceItemProjectionIndex struct {
+	outputByInput []int
+	inputByOutput []int
+}
+
+func newSequenceItemProjectionIndex(projections []sequenceItemProjection) sequenceItemProjectionIndex {
+	maxInput := -1
+	maxOutput := -1
+	for _, projection := range projections {
+		if projection.inputIndex > maxInput {
+			maxInput = projection.inputIndex
+		}
+		if projection.outputIndex > maxOutput {
+			maxOutput = projection.outputIndex
+		}
+	}
+	index := sequenceItemProjectionIndex{
+		outputByInput: make([]int, maxInput+1),
+		inputByOutput: make([]int, maxOutput+1),
+	}
+	for input := range index.outputByInput {
+		index.outputByInput[input] = -1
+	}
+	for output := range index.inputByOutput {
+		index.inputByOutput[output] = -1
+	}
+	for _, projection := range projections {
+		if projection.inputIndex >= 0 && projection.inputIndex < len(index.outputByInput) && projection.outputIndex >= 0 && projection.outputIndex < len(index.inputByOutput) {
+			index.outputByInput[projection.inputIndex] = projection.outputIndex
+			index.inputByOutput[projection.outputIndex] = projection.inputIndex
+		}
+	}
+	return index
+}
+
+func (index sequenceItemProjectionIndex) outputForInput(inputIndex int) (int, bool) {
+	if inputIndex < 0 || inputIndex >= len(index.outputByInput) || index.outputByInput[inputIndex] < 0 {
+		return 0, false
+	}
+	return index.outputByInput[inputIndex], true
+}
+
+func (index sequenceItemProjectionIndex) inputForOutput(outputIndex int) (int, bool) {
+	if outputIndex < 0 || outputIndex >= len(index.inputByOutput) || index.inputByOutput[outputIndex] < 0 {
+		return 0, false
+	}
+	return index.inputByOutput[outputIndex], true
+}
--- a/internal/engine/source_version_test.go
+++ b/internal/engine/source_version_test.go
@@ -48,6 +48,16 @@ func TestFragmentVersionClaimUsesAssignedSequenceProjection(t *testing.T) {
 	if node := fragmentVersionClaimProjected(fragment, &VersionRule{Path: Keys("items").Index(1).Key("version")}, 1, rangeForPeer); node == nil || node.Text != "v1" {
 		t.Fatalf("assigned peer index 1 claim = %#v, want its version node", node)
 	}
+	mappedRange := []sequenceRange{{
+		sourceIndex: 1, path: Keys("items"), start: 0, end: 3,
+		items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+	}}
+	if node := fragmentVersionClaimProjected(fragment, &VersionRule{Path: Keys("items").Index(0).Key("version")}, 1, mappedRange); node != nil {
+		t.Fatalf("unassigned mapped peer index claimed a version: %#v", node)
+	}
+	if node := fragmentVersionClaimProjected(fragment, &VersionRule{Path: Keys("items").Index(2).Key("version")}, 1, mappedRange); node == nil || node.Text != "v1" {
+		t.Fatalf("mapped peer index 2 claim = %#v, want its version node", node)
+	}
 	nested := sourceVersionTestDocument(t, "items: [{nested: [{version: nested-v}]}]\n")
 	nestedRanges := []sequenceRange{
 		{sourceIndex: 1, path: Keys("items"), start: 1, end: 2},