seriatim/internal/coalesce/coalesce.go

package coalesce

import (
	"fmt"
	"strings"

	"gitea.maximumdirect.net/eric/seriatim/internal/model"
)

// Summary records deterministic counters for a coalesce pass.
type Summary struct {
	OriginalSegmentsMerged int
	CoalescedSegments      int
}

// Apply merges adjacent same-speaker segments in the transcript's current order.
func Apply(in model.MergedTranscript, gap float64) (model.MergedTranscript, Summary) {
	if len(in.Segments) < 2 {
		return in, Summary{}
	}

	out := model.MergedTranscript{
		Segments:      make([]model.Segment, 0, len(in.Segments)),
		OverlapGroups: in.OverlapGroups,
	}
	summary := Summary{}
	coalescedID := 0

	current := newRun(in.Segments[0])
	pendingSkipped := make([]model.Segment, 0)
	for _, segment := range in.Segments[1:] {
		if segment.Speaker == current.speaker() {
			if current.canMerge(segment, gap) {
				current.add(segment)
				continue
			}
		} else if isSkippableInterjection(segment) {
			pendingSkipped = append(pendingSkipped, segment)
			continue
		}

		coalescedID = appendRun(&out, current, coalescedID, &summary)
		if seeded, remaining, ok := seedRunFromPending(pendingSkipped, segment, gap); ok {
			pendingSkipped = remaining
			current = seeded
			continue
		}
		out.Segments = append(out.Segments, pendingSkipped...)
		pendingSkipped = pendingSkipped[:0]
		current = newRun(segment)
	}
	coalescedID = appendRun(&out, current, coalescedID, &summary)
	out.Segments = append(out.Segments, pendingSkipped...)

	return out, summary
}

type run struct {
	segments []model.Segment
	start    float64
	end      float64
}

func newRun(segment model.Segment) run {
	return run{
		segments: []model.Segment{segment},
		start:    segment.Start,
		end:      segment.End,
	}
}

func (r run) canMerge(next model.Segment, gap float64) bool {
	return r.speaker() == next.Speaker && next.Start-r.end <= gap
}

func (r run) speaker() string {
	return r.segments[0].Speaker
}

func (r *run) add(segment model.Segment) {
	r.segments = append(r.segments, segment)
	if segment.Start < r.start {
		r.start = segment.Start
	}
	if segment.End > r.end {
		r.end = segment.End
	}
}

func seedRunFromPending(pending []model.Segment, segment model.Segment, gap float64) (run, []model.Segment, bool) {
	for start := range pending {
		if pending[start].Speaker != segment.Speaker {
			continue
		}

		candidate := newRun(pending[start])
		selected := map[int]struct{}{start: {}}
		for index := start + 1; index < len(pending); index++ {
			if pending[index].Speaker == segment.Speaker && candidate.canMerge(pending[index], gap) {
				candidate.add(pending[index])
				selected[index] = struct{}{}
			}
		}
		if !candidate.canMerge(segment, gap) {
			continue
		}

		candidate.add(segment)
		remaining := make([]model.Segment, 0, len(pending)-len(selected))
		for index, skipped := range pending {
			if _, ok := selected[index]; ok {
				continue
			}
			remaining = append(remaining, skipped)
		}
		return candidate, remaining, true
	}
	return run{}, pending, false
}

func appendRun(out *model.MergedTranscript, current run, coalescedID int, summary *Summary) int {
	if len(current.segments) == 1 {
		out.Segments = append(out.Segments, current.segments[0])
		return coalescedID
	}

	coalescedID++
	out.Segments = append(out.Segments, current.coalescedSegment(coalescedID))
	summary.OriginalSegmentsMerged += len(current.segments)
	summary.CoalescedSegments++
	return coalescedID
}

func (r run) coalescedSegment(id int) model.Segment {
	first := r.segments[0]
	merged := model.Segment{
		Source:      first.Source,
		SourceRef:   fmt.Sprintf("coalesce:%d", id),
		DerivedFrom: make([]string, 0, len(r.segments)),
		Speaker:     first.Speaker,
		Start:       r.start,
		End:         r.end,
		Words:       make([]model.Word, 0),
	}

	text := make([]string, 0, len(r.segments))
	for _, segment := range r.segments {
		if segment.Source != merged.Source {
			merged.Source = "derived"
		}
		if trimmed := strings.TrimSpace(segment.Text); trimmed != "" {
			text = append(text, trimmed)
		}
		merged.Words = append(merged.Words, segment.Words...)
		merged.DerivedFrom = append(merged.DerivedFrom, segmentRef(segment))
	}

	merged.Text = strings.Join(text, " ")
	return merged
}

func segmentRef(segment model.Segment) string {
	if segment.SourceSegmentIndex != nil {
		return fmt.Sprintf("%s#%d", segment.Source, *segment.SourceSegmentIndex)
	}
	if segment.SourceRef != "" {
		return segment.SourceRef
	}
	return segment.Source
}

func isSkippableInterjection(segment model.Segment) bool {
	return hasAnyCategory(segment, "backchannel", "filler")
}

func hasAnyCategory(segment model.Segment, categories ...string) bool {
	for _, existing := range segment.Categories {
		for _, category := range categories {
			if existing == category {
				return true
			}
		}
	}
	return false
}