fix(diarization): channel-total split attribution so mic speech can't be swallowed by a far span (FR-SPK)

src-tauri/src/diarization/mod.rs

@@ -58,6 +58,55 @@ pub fn assign_by_overlap(segments: &mut [TranscriptSegment], spans: &[SpeakerSpa
     }
 }
 
+/// Split-layout attribution (FR-SPK): decides per segment between "You" (mic
+/// channel voice activity) and the far side's diarized speakers by comparing
+/// the *total* voiced overlap on each channel, not by picking the single
+/// longest span — a long far-side diarizer span could otherwise swallow a
+/// segment the user spoke most of, showing their words under "Speaker N".
+/// The mic channel is physically the user's voice alone, so channel evidence
+/// outranks cluster evidence; ties go to "You" (mislabeling the user's own
+/// words as someone else is the worse failure). A segment with no voiced
+/// overlap on either channel keeps its prior label rather than guessing.
+// ponytail: whole-segment labels — a segment genuinely containing both sides
+// still gets one speaker; the upgrade path is transcribing each channel
+// separately so segments can never mix voices.
+pub fn assign_split(
+    segments: &mut [TranscriptSegment],
+    you_spans: &[(u64, u64)],
+    far_vad: &[(u64, u64)],
+    far_spans: &[SpeakerSpan],
+) {
+    fn overlap(a0: u64, a1: u64, b0: u64, b1: u64) -> u64 {
+        a1.min(b1).saturating_sub(a0.max(b0))
+    }
+    for seg in segments.iter_mut() {
+        let mic_ms: u64 = you_spans
+            .iter()
+            .map(|&(s, e)| overlap(seg.start_ms, seg.end_ms, s, e))
+            .sum();
+        let far_ms: u64 = far_vad
+            .iter()
+            .map(|&(s, e)| overlap(seg.start_ms, seg.end_ms, s, e))
+            .sum();
+        if mic_ms == 0 && far_ms == 0 {
+            continue;
+        }
+        if mic_ms >= far_ms {
+            seg.speaker = "You".to_string();
+        } else if let Some(span) = far_spans
+            .iter()
+            .map(|sp| (overlap(seg.start_ms, seg.end_ms, sp.start_ms, sp.end_ms), sp))
+            .filter(|(o, _)| *o > 0)
+            .max_by_key(|(o, _)| *o)
+            .map(|(_, sp)| sp)
+        {
+            seg.speaker = span.speaker.clone();
+        }
+        // Far side voiced but no diarizer span overlaps (e.g. a sub-700ms span
+        // was filtered): keep the prior label rather than guess.
+    }
+}
+
 /// sherpa-onnx-backed diarizer: pyannote segmentation + speaker-embedding +
 /// fast clustering (ADR-0005, T4.1). `Diarize::compute` needs `&mut self`; it's
 /// wrapped in a `Mutex` to satisfy `Diarizer: Sync` — diarization is a
@@ -210,6 +259,58 @@ mod overlap_tests {
         assign_by_overlap(&mut segments, &[]);
         assert_eq!(segments[0].speaker, "S1");
     }
+
+    #[test]
+    fn split_labels_a_mic_dominant_segment_you_even_against_a_longer_far_span() {
+        // The user spoke 0-4000ms; the far side 4000-6000ms — but the far
+        // cluster span covers the whole window, so the old merged max-overlap
+        // pick handed the entire segment (the user's words included) to the
+        // far speaker. Channel totals must side with the mic instead.
+        let mut segments = vec![segment(0, 6000)];
+        let you = vec![(0u64, 4000u64)];
+        let far_vad = vec![(4000u64, 6000u64)];
+        let far_spans = vec![span(0, 6000, "S1")]; // long far cluster span
+        assign_split(&mut segments, &you, &far_vad, &far_spans);
+        assert_eq!(segments[0].speaker, "You");
+    }
+
+    #[test]
+    fn split_ties_go_to_you() {
+        let mut segments = vec![segment(0, 2000)];
+        let you = vec![(0u64, 1000u64)];
+        let far_vad = vec![(1000u64, 2000u64)];
+        let far_spans = vec![span(1000, 2000, "S1")];
+        assign_split(&mut segments, &you, &far_vad, &far_spans);
+        assert_eq!(segments[0].speaker, "You");
+    }
+
+    #[test]
+    fn split_assigns_the_best_far_span_when_the_far_side_dominates() {
+        let mut segments = vec![segment(0, 3000)];
+        let you = vec![(0u64, 500u64)];
+        let far_vad = vec![(500u64, 3000u64)];
+        let far_spans = vec![span(500, 1000, "S1"), span(1000, 3000, "S2")];
+        assign_split(&mut segments, &you, &far_vad, &far_spans);
+        assert_eq!(segments[0].speaker, "S2");
+    }
+
+    #[test]
+    fn split_keeps_the_prior_label_when_both_channels_are_silent() {
+        let mut segments = vec![segment(5000, 6000)];
+        segments[0].speaker = "S9".to_string();
+        assign_split(&mut segments, &[(0, 1000)], &[(0, 1000)], &[span(0, 1000, "S1")]);
+        assert_eq!(segments[0].speaker, "S9");
+    }
+
+    #[test]
+    fn split_keeps_the_prior_label_when_far_is_voiced_but_no_far_span_overlaps() {
+        // Far VAD hears speech but every diarizer span was filtered (sub-700ms):
+        // don't guess a label.
+        let mut segments = vec![segment(0, 1000)];
+        segments[0].speaker = "S3".to_string();
+        assign_split(&mut segments, &[], &[(0, 1000)], &[span(2000, 3000, "S1")]);
+        assert_eq!(segments[0].speaker, "S3");
+    }
 }
 
 #[cfg(all(test, feature = "diarization"))]