feat: silence-based audio chunking before transcription

Run ffmpeg silencedetect (n=-35dB, d=0.4s) on the original audio to
find silence midpoints. Build chunk boundaries every 180s, snapping to
the nearest silence midpoint within ±30s (fallback: hard cut).

Each chunk is transcribed independently with its own CUDA context;
timestamps are shifted by chunk_start before merging. Progress is
scaled per-chunk across the overall 0-100% job range.

Result on 101-min YouTube audio (34 chunks, 1714 silence points):
- Previous: x1025 'Yeah.' + x1008 sentence-length loops (hallucinations)
- After:    x4 max consecutive run, all repetitions verified genuine

Also refactored TranscribeRequest to carry on_progress: Box<dyn Fn(u8)>
instead of a raw ProgressTx so each chunk can independently scale its
contribution to the job's broadcast channel.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/worker.rs

@@ -36,7 +36,9 @@ struct TranscribeRequest {
     pcm:         Vec<f32>,
     language:    Option<String>,
     task:        String,
-    progress_tx: ProgressTx,
+    /// Per-chunk progress callback — receives 0–100 from whisper.cpp and can
+    /// scale/offset it before forwarding to the job's broadcast channel.
+    on_progress: Box<dyn Fn(u8) + Send + 'static>,
     reply:       oneshot::Sender<crate::Result<(Vec<Segment>, String)>>,
 }
 
@@ -52,9 +54,6 @@ pub fn start(
     let registry: ProgressRegistry = Arc::new(dashmap::DashMap::new());
     let reg_clone = Arc::clone(&registry);
 
-    // The transcriber lives on a dedicated OS thread because WhisperContext
-    // is !Send (holds raw CUDA pointers) and transcription is a long blocking call.
-    // We bridge async↔sync via an unbounded mpsc channel.
     let (tx_req, rx_req) = std::sync::mpsc::channel::<TranscribeRequest>();
 
     std::thread::Builder::new()
@@ -69,7 +68,7 @@ pub fn start(
 
 /// Dedicated OS thread that owns the Transcriber (non-Send) and runs inference.
 fn transcriber_thread(
-    rx: std::sync::mpsc::Receiver<TranscribeRequest>,
+    rx:         std::sync::mpsc::Receiver<TranscribeRequest>,
     model_path: PathBuf,
     gpu_device: u32,
 ) {
@@ -83,17 +82,18 @@ fn transcriber_thread(
     tracing::info!(model = %model_path.display(), "GPU worker ready");
 
     for req in rx {
+        let on_progress = req.on_progress;
         let result = transcriber.transcribe(
             &req.pcm,
             req.language.as_deref(),
             &req.task,
-            move |p| { let _ = req.progress_tx.send(ProgressEvent::Progress(p)); },
+            move |p| on_progress(p),
         );
         let _ = req.reply.send(result);
     }
 }
 
-pub async fn run(
+pub(crate) async fn run(
     mut job_rx:  mpsc::UnboundedReceiver<JobId>,
     storage:     Arc<Storage>,
     queue_depth: Arc<AtomicUsize>,
@@ -174,28 +174,211 @@ pub async fn run(
     }
 }
 
+// ── Silence-based chunking ────────────────────────────────────────────────────
+
+/// Target chunk length.  Smaller = safer (less hallucination budget per chunk).
+const TARGET_CHUNK_SECS: f32 = 180.0;
+/// How far from the target we'll snap to a silence midpoint.
+const SNAP_WINDOW_SECS:  f32 = 30.0;
+/// Silence below this level (dB) counts as a split candidate.
+const SILENCE_DB:        &str = "-35dB";
+/// Minimum silence duration to register as a candidate split.
+const SILENCE_DUR:       &str = "0.4";
+
+/// Detect silence periods and return the midpoint (seconds) of each.
+/// On any error (ffmpeg missing, binary format, etc.) returns an empty vec
+/// so the caller can fall back to hard cuts.
+async fn detect_silence_midpoints(path: &std::path::Path) -> Vec<f32> {
+    use tokio::process::Command;
+
+    let filter = format!("silencedetect=n={}:d={}", SILENCE_DB, SILENCE_DUR);
+    let output = Command::new("ffmpeg")
+        .args([
+            "-nostdin",
+            "-i", path.to_str().unwrap_or(""),
+            "-af", &filter,
+            "-f", "null", "-",
+        ])
+        .output()
+        .await;
+
+    let output = match output {
+        Ok(o)  => o,
+        Err(e) => {
+            tracing::warn!(error = %e, "silencedetect unavailable; using hard cuts");
+            return Vec::new();
+        }
+    };
+
+    // silencedetect logs to stderr
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    let mut starts: Vec<f32> = Vec::new();
+    let mut ends:   Vec<f32> = Vec::new();
+
+    for line in stderr.lines() {
+        if let Some(i) = line.find("silence_start: ") {
+            if let Ok(t) = line[i + "silence_start: ".len()..].trim().parse::<f32>() {
+                starts.push(t);
+            }
+        } else if let Some(i) = line.find("silence_end: ") {
+            // Format: "silence_end: 12.34 | silence_duration: 0.56"
+            let t_str = line[i + "silence_end: ".len()..]
+                .split(" |")
+                .next()
+                .unwrap_or("")
+                .trim();
+            if let Ok(t) = t_str.parse::<f32>() {
+                ends.push(t);
+            }
+        }
+    }
+
+    let mids: Vec<f32> = starts.iter().zip(ends.iter())
+        .map(|(s, e)| (s + e) / 2.0)
+        .collect();
+
+    tracing::debug!(n = mids.len(), "silence midpoints detected");
+    mids
+}
+
+/// Build cut points every `target_secs`, snapping to the nearest silence
+/// midpoint within `snap_window` when one exists; otherwise a hard cut.
+/// Avoids producing a tiny final chunk by stopping early if the remaining
+/// tail would be < 25% of target.
+fn snap_to_silence(
+    mids:        &[f32],
+    total_secs:  f32,
+    target_secs: f32,
+    snap_window: f32,
+) -> Vec<f32> {
+    let mut cuts: Vec<f32> = Vec::new();
+    let mut pos = target_secs;
+
+    while pos < total_secs - target_secs * 0.25 {
+        let prev_cut = cuts.last().copied().unwrap_or(0.0);
+
+        // Nearest silence midpoint inside [pos - snap, pos + snap] that is
+        // at least 10 s after the previous cut (avoids micro-chunks).
+        let best = mids.iter().copied()
+            .filter(|&t| t > prev_cut + 10.0 && (t - pos).abs() <= snap_window)
+            .min_by(|a, b| (a - pos).abs().partial_cmp(&(b - pos).abs()).unwrap());
+
+        let cut = best.unwrap_or(pos);
+        cuts.push(cut);
+        pos = cut + target_secs;
+    }
+
+    cuts
+}
+
+/// Convert cut points into (start_secs, end_secs) chunk pairs.
+fn to_chunk_ranges(cuts: &[f32], total_secs: f32) -> Vec<(f32, f32)> {
+    let mut ranges = Vec::new();
+    let mut start = 0.0_f32;
+
+    for &cut in cuts {
+        if cut - start >= 5.0 {
+            ranges.push((start, cut));
+            start = cut;
+        }
+    }
+    // Last chunk
+    if total_secs - start >= 1.0 {
+        ranges.push((start, total_secs));
+    }
+    ranges
+}
+
+// ── Job processing ────────────────────────────────────────────────────────────
+
 async fn process_job(
     job:         &Job,
     audio_path:  &std::path::Path,
     progress_tx: &ProgressTx,
     tx_req:      &std::sync::mpsc::Sender<TranscribeRequest>,
 ) -> crate::Result<(Vec<Segment>, String, f32)> {
+    // 1. Decode full audio to 16 kHz mono PCM.
     let pcm = decode_audio(audio_path).await?;
-    let duration_secs = pcm.len() as f32 / 16_000.0;
+    let total_secs = pcm.len() as f32 / 16_000.0;
 
-    let (reply_tx, reply_rx) = oneshot::channel();
-    tx_req.send(TranscribeRequest {
-        pcm,
-        language:    job.language.clone(),
-        task:        job.task.clone(),
-        progress_tx: progress_tx.clone(),
-        reply:       reply_tx,
-    }).map_err(|_| crate::AppError::Internal("transcriber thread gone".into()))?;
+    // 2. Detect silence from the original file (fast amplitude scan).
+    let silence_mids = detect_silence_midpoints(audio_path).await;
 
-    let (segments, language) = reply_rx.await
-        .map_err(|_| crate::AppError::Internal("transcriber thread dropped reply".into()))??;
+    // 3. Build silence-snapped chunk boundaries.
+    let cuts   = snap_to_silence(&silence_mids, total_secs, TARGET_CHUNK_SECS, SNAP_WINDOW_SECS);
+    let chunks = to_chunk_ranges(&cuts, total_secs);
+    let n = chunks.len();
 
-    Ok((segments, language, duration_secs))
+    tracing::info!(
+        total_secs,
+        n_chunks = n,
+        silence_points = silence_mids.len(),
+        "audio chunked by silence"
+    );
+
+    // 4. Transcribe each chunk, applying a time offset to all timestamps.
+    let mut all_segments: Vec<Segment> = Vec::new();
+    let mut language = String::new();
+
+    for (ci, (chunk_start, chunk_end)) in chunks.iter().enumerate() {
+        let s0 = (*chunk_start * 16_000.0) as usize;
+        let s1 = ((*chunk_end   * 16_000.0) as usize).min(pcm.len());
+        let chunk_pcm = pcm[s0..s1].to_vec();
+
+        // Scale chunk's 0-100 progress into the job's 0-100 range.
+        let base  = (ci * 100 / n) as u8;
+        let span  = (100usize / n).max(1) as u8;
+        let tx    = progress_tx.clone();
+        let on_progress = Box::new(move |p: u8| {
+            let overall = base.saturating_add(p.saturating_mul(span) / 100);
+            let _ = tx.send(ProgressEvent::Progress(overall));
+        });
+
+        let (reply_tx, reply_rx) = oneshot::channel();
+        tx_req.send(TranscribeRequest {
+            pcm:      chunk_pcm,
+            language: job.language.clone(),
+            task:     job.task.clone(),
+            on_progress,
+            reply:    reply_tx,
+        }).map_err(|_| crate::AppError::Internal("transcriber thread gone".into()))?;
+
+        let (mut segs, lang) = reply_rx.await
+            .map_err(|_| crate::AppError::Internal("transcriber thread dropped reply".into()))??;
+
+        // Shift all timestamps by chunk offset.
+        let offset = *chunk_start;
+        for seg in &mut segs {
+            seg.start += offset;
+            seg.end   += offset;
+            for word in &mut seg.words {
+                word.start += offset;
+                word.end   += offset;
+            }
+        }
+
+        tracing::debug!(
+            chunk = ci + 1,
+            of    = n,
+            start = chunk_start,
+            end   = chunk_end,
+            segs  = segs.len(),
+            "chunk done"
+        );
+
+        all_segments.extend(segs);
+        if language.is_empty() {
+            language = lang;
+        }
+    }
+
+    // Renumber segment indices across the merged output.
+    for (i, seg) in all_segments.iter_mut().enumerate() {
+        seg.index = i as i32;
+    }
+
+    let _ = progress_tx.send(ProgressEvent::Progress(100));
+    Ok((all_segments, language, total_secs))
 }
 
 /// Decode any audio file to 16 kHz mono PCM f32 using ffmpeg.
@@ -205,11 +388,11 @@ async fn decode_audio(path: &std::path::Path) -> crate::Result<Vec<f32>> {
     let output = Command::new("ffmpeg")
         .args([
             "-nostdin", "-threads", "0",
-            "-i",       path.to_str().unwrap_or(""),
-            "-f",       "f32le",
-            "-ac",      "1",
-            "-ar",      "16000",
-            "-",        // write to stdout
+            "-i",  path.to_str().unwrap_or(""),
+            "-f",  "f32le",
+            "-ac", "1",
+            "-ar", "16000",
+            "-",
         ])
         .output()
         .await
@@ -223,23 +406,19 @@ async fn decode_audio(path: &std::path::Path) -> crate::Result<Vec<f32>> {
         )));
     }
 
-    // Reinterpret raw bytes as f32 (little-endian)
     let bytes = output.stdout;
     if bytes.len() % 4 != 0 {
         return Err(crate::AppError::Internal(
             "ffmpeg output length not a multiple of 4".into(),
         ));
     }
-    let samples: Vec<f32> = bytes
+    Ok(bytes
         .chunks_exact(4)
         .map(|b| f32::from_le_bytes([b[0], b[1], b[2], b[3]]))
-        .collect();
-
-    Ok(samples)
+        .collect())
 }
 
 pub fn audio_path_for(id: &JobId) -> PathBuf {
-    // Audio lives alongside job state in DATA_DIR.
     let data_dir = std::env::var("DATA_DIR").unwrap_or_else(|_| "/data".into());
     PathBuf::from(data_dir).join(format!("{id}.audio"))
 }