Feat: tweak and fix from last night workshop

src/engine/sequencer.rs

@@ -531,6 +531,7 @@ impl KeyCache {
 pub(crate) struct SequencerState {
     audio_state: AudioState,
     pattern_cache: PatternCache,
+    pending_updates: HashMap<(usize, usize), PatternSnapshot>,
     runs_counter: RunsCounter,
     step_traces: Arc<StepTracesMap>,
     event_count: usize,
@@ -559,6 +560,7 @@ impl SequencerState {
         Self {
             audio_state: AudioState::new(),
             pattern_cache: PatternCache::new(),
+            pending_updates: HashMap::new(),
             runs_counter: RunsCounter::new(),
             step_traces: Arc::new(HashMap::new()),
             event_count: 0,
@@ -596,7 +598,14 @@ impl SequencerState {
                     pattern,
                     data,
                 } => {
-                    self.pattern_cache.set(bank, pattern, data);
+                    let id = PatternId { bank, pattern };
+                    let is_active = self.audio_state.active_patterns.contains_key(&id);
+                    let has_cache = self.pattern_cache.get(bank, pattern).is_some();
+                    if is_active && has_cache {
+                        self.pending_updates.insert((bank, pattern), data);
+                    } else {
+                        self.pattern_cache.set(bank, pattern, data);
+                    }
                 }
                 SeqCommand::PatternStart {
                     bank,
@@ -652,6 +661,10 @@ impl SequencerState {
                     }
                 }
                 SeqCommand::StopAll => {
+                    // Flush pending updates so cache stays current for future launches
+                    for ((bank, pattern), snapshot) in self.pending_updates.drain() {
+                        self.pattern_cache.set(bank, pattern, snapshot);
+                    }
                     self.audio_state.active_patterns.clear();
                     self.audio_state.pending_starts.clear();
                     self.audio_state.pending_stops.clear();
@@ -715,6 +728,10 @@ impl SequencerState {
             Arc::make_mut(&mut self.step_traces).retain(|&(bank, pattern, _), _| {
                 bank != pending.id.bank || pattern != pending.id.pattern
             });
+            let key = (pending.id.bank, pending.id.pattern);
+            if let Some(snapshot) = self.pending_updates.remove(&key) {
+                self.pattern_cache.set(key.0, key.1, snapshot);
+            }
         }
         self.audio_state.pending_starts.clear();
         self.audio_state.prev_beat = -1.0;
@@ -773,6 +790,11 @@ impl SequencerState {
                 Arc::make_mut(&mut self.step_traces).retain(|&(bank, pattern, _), _| {
                     bank != pending.id.bank || pattern != pending.id.pattern
                 });
+                // Flush pending update so cache stays current for future launches
+                let key = (pending.id.bank, pending.id.pattern);
+                if let Some(snapshot) = self.pending_updates.remove(&key) {
+                    self.pattern_cache.set(key.0, key.1, snapshot);
+                }
                 self.buf_stopped.push(pending.id);
             }
         }
@@ -921,6 +943,14 @@ impl SequencerState {
             }
         }
 
+        // Apply deferred updates for patterns that just completed an iteration
+        for completed_id in &self.buf_completed_iterations {
+            let key = (completed_id.bank, completed_id.pattern);
+            if let Some(snapshot) = self.pending_updates.remove(&key) {
+                self.pattern_cache.set(key.0, key.1, snapshot);
+            }
+        }
+
         result
     }
 
@@ -1872,8 +1902,9 @@ mod tests {
         let ap = state.audio_state.active_patterns.get(&pid(0, 0)).unwrap();
         assert_eq!(ap.step_index, 0);
 
-        // Update pattern to length 2 while running — step_index wraps via modulo
-        // beat=1.25: beat_int=5, prev=4, fires 1 step. step_index=0%2=0 fires, advances to 1
+        // Update pattern to length 2 while running — deferred until iteration boundary
+        // beat=1.25: update is deferred (pattern active), still length 4
+        // step_index=0 fires, advances to 1
         state.tick(tick_with(
             vec![SeqCommand::PatternUpdate {
                 bank: 0,
@@ -1883,10 +1914,21 @@ mod tests {
             1.25,
         ));
         let ap = state.audio_state.active_patterns.get(&pid(0, 0)).unwrap();
+        assert_eq!(ap.step_index, 1); // still length 4
+
+        // Advance through remaining steps of original length-4 pattern
+        state.tick(tick_at(1.5, true)); // step 1→2
+        state.tick(tick_at(1.75, true)); // step 2→3
+        state.tick(tick_at(2.0, true)); // step 3→wraps to 0, iteration completes, update applies
+
+        // Now length=2 is applied. Next tick uses new length.
+        // beat=2.25: step 0 fires, advances to 1
+        state.tick(tick_at(2.25, true));
+        let ap = state.audio_state.active_patterns.get(&pid(0, 0)).unwrap();
         assert_eq!(ap.step_index, 1);
 
-        // beat=1.5: beat_int=6, prev=5, step fires. step_index=1 fires, wraps to 0
-        state.tick(tick_at(1.5, true));
+        // beat=2.5: step 1 fires, wraps to 0 (length 2)
+        state.tick(tick_at(2.5, true));
         let ap = state.audio_state.active_patterns.get(&pid(0, 0)).unwrap();
         assert_eq!(ap.step_index, 0);
     }