Implemented an overlap detection module in the postprocessing chain

README.md

@@ -148,10 +148,21 @@ The merged output uses the current seriatim envelope:
       "speaker": "Eric Rakestraw",
       "start": 1.25,
       "end": 3.5,
-      "text": "Hello there."
+      "text": "Hello there.",
+      "overlap_group_id": 1
     }
   ],
-  "overlap_groups": []
+  "overlap_groups": [
+    {
+      "id": 1,
+      "start": 1.25,
+      "end": 4.0,
+      "segments": ["eric.json#0", "mike.json#0"],
+      "speakers": ["Eric Rakestraw", "Mike Brown"],
+      "class": "unknown",
+      "resolution": "unresolved"
+    }
+  ]
 }
 ```
 
@@ -163,6 +174,21 @@ Segments are sorted deterministically by:
 
 Final segment IDs are assigned after sorting and start at `1`.
 
+## Overlap Detection
+
+The default postprocessing pipeline detects overlapping segment groups.
+
+Overlap behavior:
+
+- A strict timing overlap is required: `next.start < current_group_end`.
+- Segments that only touch at a boundary are not grouped.
+- Groups require at least two distinct speakers.
+- Transitive overlaps are grouped together.
+- Segments in detected groups receive `overlap_group_id`.
+- `overlap_groups[].segments` contains stable references in `source#source_segment_index` format.
+- `class` is currently `unknown`.
+- `resolution` is currently `unresolved`; overlap resolution is still a no-op.
+
 ## Autocorrect
 
 Autocorrect is included in the default postprocessing pipeline. If `--autocorrect` is omitted, the module leaves transcript text unchanged and records a skip event in the optional report.
@@ -202,5 +228,5 @@ Matching behavior:
 
 - Only JSON input is supported.
 - Word-level timing data is not preserved yet.
-- Overlap detection and overlap resolution are currently no-op modules.
+- Overlap resolution is currently a no-op module.
 - Coalescing and alternate output formats are not implemented yet.

internal/builtin/postprocess.go

@@ -7,6 +7,7 @@ import (
 	"gitea.maximumdirect.net/eric/seriatim/internal/autocorrect"
 	"gitea.maximumdirect.net/eric/seriatim/internal/config"
 	"gitea.maximumdirect.net/eric/seriatim/internal/model"
+	"gitea.maximumdirect.net/eric/seriatim/internal/overlap"
 	"gitea.maximumdirect.net/eric/seriatim/internal/report"
 )
 
@@ -48,6 +49,23 @@ func (assignIDs) Process(ctx context.Context, in model.MergedTranscript, cfg con
 	}, nil
 }
 
+type detectOverlaps struct{}
+
+func (detectOverlaps) Name() string {
+	return "detect-overlaps"
+}
+
+func (detectOverlaps) Process(ctx context.Context, in model.MergedTranscript, cfg config.Config) (model.MergedTranscript, []report.Event, error) {
+	if err := ctx.Err(); err != nil {
+		return model.MergedTranscript{}, nil, err
+	}
+
+	in = overlap.Detect(in)
+	return in, []report.Event{
+		report.Info("postprocessing", "detect-overlaps", fmt.Sprintf("detected %d overlap group(s)", len(in.OverlapGroups))),
+	}, nil
+}
+
 type autocorrectPostprocessor struct{}
 
 func (autocorrectPostprocessor) Name() string {

internal/builtin/registry.go

@@ -11,7 +11,7 @@ func NewRegistry() *pipeline.Registry {
 	registry.RegisterPreprocessor(normalizeSpeakers{})
 	registry.RegisterPreprocessor(trimText{})
 	registry.RegisterMerger(placeholderMerger{})
-	registry.RegisterPostprocessor(noopPostprocessor{name: "detect-overlaps"})
+	registry.RegisterPostprocessor(detectOverlaps{})
 	registry.RegisterPostprocessor(noopPostprocessor{name: "resolve-overlaps"})
 	registry.RegisterPostprocessor(assignIDs{})
 	registry.RegisterPostprocessor(noopPostprocessor{name: "validate-output"})

internal/cli/merge_test.go

@@ -149,6 +149,74 @@ func TestMergeTieBreakOrder(t *testing.T) {
 	}
 }
 
+func TestMergeDetectsOverlapGroups(t *testing.T) {
+	dir := t.TempDir()
+	inputA := writeJSONFile(t, dir, "a.json", `{
+		"segments": [
+			{"start": 1, "end": 5, "text": "alice long"},
+			{"start": 2, "end": 3, "text": "alice nested"}
+		]
+	}`)
+	inputB := writeJSONFile(t, dir, "b.json", `{
+		"segments": [
+			{"start": 4, "end": 6, "text": "bob overlap"}
+		]
+	}`)
+	speakers := writeYAMLFile(t, dir, "speakers.yml", `match:
+  - speaker: Alice
+    match: ["a.json"]
+  - speaker: Bob
+    match: ["b.json"]
+`)
+	output := filepath.Join(dir, "merged.json")
+	reportPath := filepath.Join(dir, "report.json")
+
+	err := executeMerge(
+		"--input-file", inputB,
+		"--input-file", inputA,
+		"--speakers", speakers,
+		"--output-file", output,
+		"--report-file", reportPath,
+	)
+	if err != nil {
+		t.Fatalf("merge failed: %v", err)
+	}
+
+	var transcript model.FinalTranscript
+	readJSON(t, output, &transcript)
+	if len(transcript.OverlapGroups) != 1 {
+		t.Fatalf("overlap group count = %d, want 1", len(transcript.OverlapGroups))
+	}
+	group := transcript.OverlapGroups[0]
+	if group.ID != 1 {
+		t.Fatalf("group ID = %d, want 1", group.ID)
+	}
+	if group.Start != 1 || group.End != 6 {
+		t.Fatalf("group bounds = %f-%f, want 1-6", group.Start, group.End)
+	}
+	wantRefs := []string{inputA + "#0", inputA + "#1", inputB + "#0"}
+	if !equalStrings(group.Segments, wantRefs) {
+		t.Fatalf("group refs = %v, want %v", group.Segments, wantRefs)
+	}
+	if !equalStrings(group.Speakers, []string{"Alice", "Bob"}) {
+		t.Fatalf("group speakers = %v, want [Alice Bob]", group.Speakers)
+	}
+	if group.Class != "unknown" || group.Resolution != "unresolved" {
+		t.Fatalf("unexpected group class/resolution: %q/%q", group.Class, group.Resolution)
+	}
+	for index, segment := range transcript.Segments {
+		if segment.OverlapGroupID != 1 {
+			t.Fatalf("segment %d overlap group ID = %d, want 1", index, segment.OverlapGroupID)
+		}
+	}
+
+	var rpt report.Report
+	readJSON(t, reportPath, &rpt)
+	if !hasReportEvent(rpt, "postprocessing", "detect-overlaps", "detected 1 overlap group(s)") {
+		t.Fatal("expected detect-overlaps report event")
+	}
+}
+
 func TestSpeakerMatchingUsesFirstMatchingRuleCaseInsensitive(t *testing.T) {
 	dir := t.TempDir()
 	input := writeJSONFile(t, dir, "2026-04-19-Adam_Rakestraw.json", `{

internal/config/config.go

@@ -137,6 +137,7 @@ func normalizeInputFiles(paths []string) ([]string, error) {
 	}
 
 	normalized := make([]string, 0, len(paths))
+	seen := make(map[string]struct{}, len(paths))
 	for _, path := range paths {
 		path = strings.TrimSpace(path)
 		if path == "" {
@@ -147,6 +148,10 @@ func normalizeInputFiles(paths []string) ([]string, error) {
 		if err := requireFile(clean, "--input-file"); err != nil {
 			return nil, err
 		}
+		if _, exists := seen[clean]; exists {
+			return nil, fmt.Errorf("duplicate --input-file %q", clean)
+		}
+		seen[clean] = struct{}{}
 		normalized = append(normalized, clean)
 	}
 	sort.Strings(normalized)

internal/config/config_test.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -24,6 +25,27 @@ func TestOmittingNormalizeSpeakersDoesNotRequireSpeakers(t *testing.T) {
 	}
 }
 
+func TestDuplicateInputFilesFailValidation(t *testing.T) {
+	dir := t.TempDir()
+	input := writeTempFile(t, dir, "input.json")
+	output := filepath.Join(dir, "merged.json")
+
+	_, err := NewMergeConfig(MergeOptions{
+		InputFiles:            []string{input, input},
+		OutputFile:            output,
+		InputReader:           DefaultInputReader,
+		OutputModules:         DefaultOutputModules,
+		PreprocessingModules:  DefaultPreprocessingModules,
+		PostprocessingModules: DefaultPostprocessingModules,
+	})
+	if err == nil {
+		t.Fatal("expected duplicate input error")
+	}
+	if !strings.Contains(err.Error(), "duplicate --input-file") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func writeTempFile(t *testing.T, dir string, name string) string {
 	t.Helper()
 

internal/overlap/detect.go

@@ -0,0 +1,118 @@
+package overlap
+
+import (
+	"fmt"
+
+	"gitea.maximumdirect.net/eric/seriatim/internal/model"
+)
+
+const (
+	defaultClass      = "unknown"
+	defaultResolution = "unresolved"
+)
+
+// Detect annotates overlapping segment groups in an already sorted merged transcript.
+func Detect(in model.MergedTranscript) model.MergedTranscript {
+	clearExisting(&in)
+	if len(in.Segments) < 2 {
+		return in
+	}
+
+	var groupID int
+	var candidate overlapCandidate
+	for index := range in.Segments {
+		segment := in.Segments[index]
+		if !candidate.active {
+			candidate = newCandidate(index, segment)
+			continue
+		}
+
+		if segment.Start < candidate.end {
+			candidate.add(index, segment)
+			continue
+		}
+
+		groupID = finalizeCandidate(&in, candidate, groupID)
+		candidate = newCandidate(index, segment)
+	}
+
+	finalizeCandidate(&in, candidate, groupID)
+	return in
+}
+
+type overlapCandidate struct {
+	active  bool
+	indices []int
+	start   float64
+	end     float64
+}
+
+func newCandidate(index int, segment model.Segment) overlapCandidate {
+	return overlapCandidate{
+		active:  true,
+		indices: []int{index},
+		start:   segment.Start,
+		end:     segment.End,
+	}
+}
+
+func (c *overlapCandidate) add(index int, segment model.Segment) {
+	c.indices = append(c.indices, index)
+	if segment.End > c.end {
+		c.end = segment.End
+	}
+}
+
+func finalizeCandidate(in *model.MergedTranscript, candidate overlapCandidate, currentGroupID int) int {
+	if !candidate.active || len(candidate.indices) < 2 {
+		return currentGroupID
+	}
+
+	speakers := distinctSpeakers(in.Segments, candidate.indices)
+	if len(speakers) < 2 {
+		return currentGroupID
+	}
+
+	groupID := currentGroupID + 1
+	refs := make([]string, 0, len(candidate.indices))
+	for _, index := range candidate.indices {
+		in.Segments[index].OverlapGroupID = groupID
+		refs = append(refs, segmentRef(in.Segments[index]))
+	}
+
+	in.OverlapGroups = append(in.OverlapGroups, model.OverlapGroup{
+		ID:         groupID,
+		Start:      candidate.start,
+		End:        candidate.end,
+		Segments:   refs,
+		Speakers:   speakers,
+		Class:      defaultClass,
+		Resolution: defaultResolution,
+	})
+	return groupID
+}
+
+func distinctSpeakers(segments []model.Segment, indices []int) []string {
+	seen := make(map[string]struct{}, len(indices))
+	speakers := make([]string, 0, len(indices))
+	for _, index := range indices {
+		speaker := segments[index].Speaker
+		if _, exists := seen[speaker]; exists {
+			continue
+		}
+		seen[speaker] = struct{}{}
+		speakers = append(speakers, speaker)
+	}
+	return speakers
+}
+
+func segmentRef(segment model.Segment) string {
+	return fmt.Sprintf("%s#%d", segment.Source, segment.SourceSegmentIndex)
+}
+
+func clearExisting(in *model.MergedTranscript) {
+	in.OverlapGroups = make([]model.OverlapGroup, 0)
+	for index := range in.Segments {
+		in.Segments[index].OverlapGroupID = 0
+	}
+}

internal/overlap/detect_test.go

@@ -0,0 +1,203 @@
+package overlap
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/seriatim/internal/model"
+)
+
+func TestDetectNoOverlaps(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 2),
+			segment("b.json", 0, "Bob", 2, 3),
+		},
+	}
+
+	got := Detect(merged)
+	if len(got.OverlapGroups) != 0 {
+		t.Fatalf("expected no overlap groups, got %d", len(got.OverlapGroups))
+	}
+	assertNoSegmentAnnotations(t, got)
+}
+
+func TestDetectSimpleTwoSpeakerOverlap(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 3),
+			segment("b.json", 0, "Bob", 2, 4),
+		},
+	}
+
+	got := Detect(merged)
+	assertGroup(t, got, 0, 1, 1, 4, []string{"a.json#0", "b.json#0"}, []string{"Alice", "Bob"})
+	assertSegmentGroupIDs(t, got, []int{1, 1})
+}
+
+func TestDetectTransitiveOverlap(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 10, 14),
+			segment("b.json", 0, "Bob", 12, 13),
+			segment("c.json", 0, "Carol", 13.5, 15),
+		},
+	}
+
+	got := Detect(merged)
+	assertGroup(t, got, 0, 1, 10, 15, []string{"a.json#0", "b.json#0", "c.json#0"}, []string{"Alice", "Bob", "Carol"})
+	assertSegmentGroupIDs(t, got, []int{1, 1, 1})
+}
+
+func TestDetectBoundaryContactDoesNotOverlap(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 2),
+			segment("b.json", 0, "Bob", 2, 3),
+		},
+	}
+
+	got := Detect(merged)
+	if len(got.OverlapGroups) != 0 {
+		t.Fatalf("expected no overlap groups, got %d", len(got.OverlapGroups))
+	}
+	assertNoSegmentAnnotations(t, got)
+}
+
+func TestDetectSameSpeakerOnlyOverlapDoesNotCreateGroup(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 3),
+			segment("a.json", 1, "Alice", 2, 4),
+		},
+	}
+
+	got := Detect(merged)
+	if len(got.OverlapGroups) != 0 {
+		t.Fatalf("expected no overlap groups, got %d", len(got.OverlapGroups))
+	}
+	assertNoSegmentAnnotations(t, got)
+}
+
+func TestDetectIncludesSameSpeakerSegmentInsideMultiSpeakerOverlap(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 5),
+			segment("a.json", 1, "Alice", 2, 3),
+			segment("b.json", 0, "Bob", 4, 6),
+		},
+	}
+
+	got := Detect(merged)
+	assertGroup(t, got, 0, 1, 1, 6, []string{"a.json#0", "a.json#1", "b.json#0"}, []string{"Alice", "Bob"})
+	assertSegmentGroupIDs(t, got, []int{1, 1, 1})
+}
+
+func TestDetectMultipleGroups(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 3),
+			segment("b.json", 0, "Bob", 2, 4),
+			segment("c.json", 0, "Carol", 5, 7),
+			segment("d.json", 0, "Dan", 6, 8),
+		},
+	}
+
+	got := Detect(merged)
+	if len(got.OverlapGroups) != 2 {
+		t.Fatalf("group count = %d, want 2", len(got.OverlapGroups))
+	}
+	assertGroup(t, got, 0, 1, 1, 4, []string{"a.json#0", "b.json#0"}, []string{"Alice", "Bob"})
+	assertGroup(t, got, 1, 2, 5, 8, []string{"c.json#0", "d.json#0"}, []string{"Carol", "Dan"})
+	assertSegmentGroupIDs(t, got, []int{1, 1, 2, 2})
+}
+
+func TestDetectIsIdempotent(t *testing.T) {
+	merged := model.MergedTranscript{
+		Segments: []model.Segment{
+			segment("a.json", 0, "Alice", 1, 3),
+			segment("b.json", 0, "Bob", 2, 4),
+		},
+		OverlapGroups: []model.OverlapGroup{
+			{ID: 99},
+		},
+	}
+	merged.Segments[0].OverlapGroupID = 99
+
+	once := Detect(merged)
+	twice := Detect(once)
+	assertGroup(t, twice, 0, 1, 1, 4, []string{"a.json#0", "b.json#0"}, []string{"Alice", "Bob"})
+	assertSegmentGroupIDs(t, twice, []int{1, 1})
+}
+
+func segment(source string, sourceIndex int, speaker string, start float64, end float64) model.Segment {
+	return model.Segment{
+		Source:             source,
+		SourceSegmentIndex: sourceIndex,
+		Speaker:            speaker,
+		Start:              start,
+		End:                end,
+		Text:               speaker,
+	}
+}
+
+func assertGroup(t *testing.T, merged model.MergedTranscript, groupIndex int, id int, start float64, end float64, refs []string, speakers []string) {
+	t.Helper()
+	if len(merged.OverlapGroups) <= groupIndex {
+		t.Fatalf("missing overlap group index %d", groupIndex)
+	}
+	group := merged.OverlapGroups[groupIndex]
+	if group.ID != id {
+		t.Fatalf("group ID = %d, want %d", group.ID, id)
+	}
+	if group.Start != start {
+		t.Fatalf("group start = %f, want %f", group.Start, start)
+	}
+	if group.End != end {
+		t.Fatalf("group end = %f, want %f", group.End, end)
+	}
+	if group.Class != "unknown" {
+		t.Fatalf("group class = %q, want unknown", group.Class)
+	}
+	if group.Resolution != "unresolved" {
+		t.Fatalf("group resolution = %q, want unresolved", group.Resolution)
+	}
+	if !equalStrings(group.Segments, refs) {
+		t.Fatalf("group refs = %v, want %v", group.Segments, refs)
+	}
+	if !equalStrings(group.Speakers, speakers) {
+		t.Fatalf("group speakers = %v, want %v", group.Speakers, speakers)
+	}
+}
+
+func assertSegmentGroupIDs(t *testing.T, merged model.MergedTranscript, ids []int) {
+	t.Helper()
+	if len(merged.Segments) != len(ids) {
+		t.Fatalf("segment count = %d, want %d", len(merged.Segments), len(ids))
+	}
+	for index, id := range ids {
+		if merged.Segments[index].OverlapGroupID != id {
+			t.Fatalf("segment %d overlap group ID = %d, want %d", index, merged.Segments[index].OverlapGroupID, id)
+		}
+	}
+}
+
+func assertNoSegmentAnnotations(t *testing.T, merged model.MergedTranscript) {
+	t.Helper()
+	for index, segment := range merged.Segments {
+		if segment.OverlapGroupID != 0 {
+			t.Fatalf("segment %d overlap group ID = %d, want 0", index, segment.OverlapGroupID)
+		}
+	}
+}
+
+func equalStrings(left []string, right []string) bool {
+	if len(left) != len(right) {
+		return false
+	}
+	for index := range left {
+		if left[index] != right[index] {
+			return false
+		}
+	}
+	return true
+}