First attempt at interpolation

audio_engine/src/main.rs

@@ -51,6 +51,9 @@ use track::Track;
 use track::TrackState;
 use track_matrix::TrackMatrix;
 
+const COLS: usize = 5;
+const ROWS: usize = 5;
+
 pub struct JackPorts {
     pub audio_in: jack::Port<jack::AudioIn>,
     pub audio_out: jack::Port<jack::AudioOut>,
@@ -66,10 +69,6 @@ async fn main() {
         .init()
         .expect("Could not initialize logger");
 
-    let (mut osc, osc_controller) = Osc::new(&args.socket)
-        .await
-        .expect("Could not create OSC server");
-
     let (jack_client, ports) = setup_jack();
 
     let mut allocator = Allocator::spawn(jack_client.sample_rate(), 3);
@@ -84,11 +83,19 @@ async fn main() {
         PersistenceManager::new(&args, notification_handler.subscribe());
     let state = persistence_manager.state();
 
+    // Calculate tempo from state and jack client
+    let tempo_bpm = (jack_client.sample_rate() as f32 * 60.0) 
+                    / state.borrow().metronome.frames_per_beat as f32;
+
+    let (mut osc, osc_controller) = Osc::new(&args.socket, COLS, ROWS, tempo_bpm)
+        .await
+        .expect("Could not create OSC server");
+
     // Create post-record handler and get controller for ProcessHandler
     let (mut post_record_handler, post_record_controller) =
         PostRecordHandler::new(&args).expect("Could not create post-record handler");
 
-    let process_handler = ProcessHandler::new(
+    let process_handler = ProcessHandler::<_, COLS, ROWS>::new(
         &jack_client,
         ports,
         allocator,

audio_engine/src/midi.rs

@@ -1,8 +1,8 @@
 use crate::*;
 
 /// Process MIDI events
-pub fn process_events<F: ChunkFactory>(
-    process_handler: &mut ProcessHandler<F>,
+pub fn process_events<F: ChunkFactory, const COLS: usize, const ROWS: usize>(
+    process_handler: &mut ProcessHandler<F, COLS, ROWS>,
     ps: &jack::ProcessScope,
 ) -> Result<()> {
     // First, collect all MIDI events into a fixed-size array

audio_engine/src/osc_server/server.rs

@@ -11,8 +11,13 @@ pub struct Osc {
 }
 
 impl Osc {
-    /// Create new OSC server and controller with same interface as before
-    pub async fn new(socket_path: &str) -> Result<(Self, OscController)> {
+    /// Create new OSC server and controller with configurable matrix size and tempo
+    pub async fn new(
+        socket_path: &str,
+        columns: usize,
+        rows: usize,
+        tempo: f32,
+    ) -> Result<(Self, OscController)> {
         // Create platform listener (server)
         let listener = osc_server::platform::create_listener(socket_path).await?;
 
@@ -27,7 +32,7 @@ impl Osc {
             receiver,
             listener,
             broadcaster,
-            shadow_state: osc::State::new(5, 5), // Default 5x5 matrix
+            shadow_state: osc::State::new(columns, rows, tempo),
         };
 
         Ok((server, controller))

audio_engine/src/process_handler.rs

@@ -1,9 +1,6 @@
 use crate::*;
 
-const COLS: usize = 5;
-const ROWS: usize = 5;
-
-pub struct ProcessHandler<F: ChunkFactory> {
+pub struct ProcessHandler<F: ChunkFactory, const COLS: usize, const ROWS: usize> {
     post_record_controller: PostRecordController,
     osc: OscController,
     persistence: PersistenceManagerController,
@@ -16,7 +13,7 @@ pub struct ProcessHandler<F: ChunkFactory> {
     sample_rate: usize,
 }
 
-impl<F: ChunkFactory> ProcessHandler<F> {
+impl<F: ChunkFactory, const COLS: usize, const ROWS: usize> ProcessHandler<F, COLS, ROWS> {
     pub fn new(
         client: &jack::Client,
         ports: JackPorts,
@@ -153,7 +150,7 @@ impl<F: ChunkFactory> ProcessHandler<F> {
     }
 }
 
-impl<F: ChunkFactory> jack::ProcessHandler for ProcessHandler<F> {
+impl<F: ChunkFactory, const COLS: usize, const ROWS: usize> jack::ProcessHandler for ProcessHandler<F, COLS, ROWS> {
     fn process(&mut self, _client: &jack::Client, ps: &jack::ProcessScope) -> jack::Control {
         if let Err(e) = self.process_with_error_handling(ps) {
             log::error!("Error processing audio: {}", e);
@@ -164,7 +161,7 @@ impl<F: ChunkFactory> jack::ProcessHandler for ProcessHandler<F> {
     }
 }
 
-impl<F: ChunkFactory> ProcessHandler<F> {
+impl<F: ChunkFactory, const COLS: usize, const ROWS: usize> ProcessHandler<F, COLS, ROWS> {
     fn process_with_error_handling(&mut self, ps: &jack::ProcessScope) -> Result<()> {
         // Process metronome and get beat timing information
         let timing = self.metronome.process(ps, &mut self.ports, &self.osc)?;

gui/src/interpolation.rs

@@ -0,0 +1,549 @@
+use std::time::Instant;
+
+/// Result of interpolation calculation for one frame
+#[derive(Debug, Clone)]
+pub struct Interpolated {
+    /// Position within current beat (0.0 = start of beat, 1.0 = end of beat)
+    pub position_in_beat: f32,
+    /// True if a beat boundary occurred since the last frame
+    pub beat_boundary: bool,
+}
+
+/// Handles interpolation of time-based values between OSC updates
+#[derive(Debug)]
+pub struct Interpolation {
+    /// Last received metronome position from OSC
+    last_osc_position: f32,
+    /// When the last OSC position was received
+    last_osc_timestamp: Instant,
+    /// Current tempo in BPM
+    tempo: f32,
+    /// When we last calculated interpolated values
+    last_frame_timestamp: Instant,
+    /// The position we calculated in the last frame
+    last_frame_position: f32,
+    /// Total number of beat boundaries we've reported (to avoid double-counting)
+    beats_reported: u64,
+    /// Whether we've received any OSC data yet
+    initialized: bool,
+}
+
+impl Interpolation {
+    pub fn new(current_time: Instant) -> Self {
+        Self {
+            last_osc_position: 0.0,
+            last_osc_timestamp: current_time,
+            tempo: 120.0,
+            last_frame_timestamp: current_time,
+            last_frame_position: 0.0,
+            beats_reported: 0,
+            initialized: false,
+        }
+    }
+
+    /// Update interpolation with current state and return interpolated values
+    pub fn update(&mut self, state: &osc::State, current_time: Instant) -> Interpolated {
+        if !self.initialized {
+            // First call - just initialize
+            self.last_osc_position = state.metronome_position;
+            self.last_osc_timestamp = state.metronome_timestamp;
+            self.tempo = state.tempo;
+            self.last_frame_timestamp = current_time;
+            self.last_frame_position = state.metronome_position;
+            self.beats_reported = 0;
+            self.initialized = true;
+
+            return Interpolated {
+                position_in_beat: state.metronome_position,
+                beat_boundary: false,
+            };
+        }
+
+        let beat_duration_secs = 60.0 / self.tempo;
+
+        // Check if we have new OSC data
+        let has_new_osc_data = state.metronome_timestamp != self.last_osc_timestamp;
+
+        let current_position;
+        let mut beat_boundary = false;
+
+        if has_new_osc_data {
+            // Update tempo first in case it changed
+            self.tempo = state.tempo;
+            let beat_duration_secs = 60.0 / self.tempo;
+
+            // Check for beat boundary in the OSC data itself (position wraparound)
+            if state.metronome_position < self.last_osc_position && self.last_osc_position > 0.7 {
+                beat_boundary = true;
+            }
+
+            // Use OSC position as our new reference and interpolate forward to current frame time
+            let elapsed_since_osc = current_time.duration_since(state.metronome_timestamp);
+            let advancement = elapsed_since_osc.as_secs_f32() / beat_duration_secs;
+            current_position = state.metronome_position + advancement;
+
+            // Update our reference point
+            self.last_osc_position = state.metronome_position;
+            self.last_osc_timestamp = state.metronome_timestamp;
+        } else {
+            // No new OSC data - interpolate from our last reference point
+            let elapsed_since_osc = current_time.duration_since(self.last_osc_timestamp);
+            let advancement = elapsed_since_osc.as_secs_f32() / beat_duration_secs;
+            current_position = self.last_osc_position + advancement;
+        }
+
+        // Detect beat boundary by comparing with last frame position
+        if !beat_boundary {
+            // Check if we crossed an integer boundary (beat boundary)
+            let last_beat = self.last_frame_position.floor();
+            let current_beat = current_position.floor();
+
+            if current_beat > last_beat {
+                beat_boundary = true;
+            }
+        }
+
+        // Normalize position to [0.0, 1.0) within current beat
+        let position_in_beat = current_position.fract();
+
+        // Update tracking for next frame
+        self.last_frame_timestamp = current_time;
+        self.last_frame_position = current_position;
+
+        if beat_boundary {
+            self.beats_reported += 1;
+        }
+
+        Interpolated {
+            position_in_beat,
+            beat_boundary,
+        }
+    }
+}
+
+impl Default for Interpolation {
+    fn default() -> Self {
+        Self::new(Instant::now())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use osc::State;
+    use std::time::Duration;
+
+    fn create_test_state(metronome_position: f32, tempo: f32, timestamp: Instant) -> State {
+        let mut state = State::new(5, 5, tempo);
+        state.set_metronome_position(metronome_position);
+        // Manually set the timestamp to make tests deterministic
+        state.metronome_timestamp = timestamp;
+        state
+    }
+
+    #[test]
+    fn test_initial_state() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+        let state = create_test_state(0.0, 120.0, start_time);
+
+        let result = interpolation.update(&state, start_time);
+
+        // First call should initialize but not report beat boundary
+        assert_eq!(result.position_in_beat, 0.0);
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_normal_interpolation() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+        let state = create_test_state(0.5, 120.0, start_time);
+
+        // First update to initialize
+        interpolation.update(&state, start_time);
+
+        // Simulate 100ms delay (at 120 BPM, one beat = 500ms, so this is 0.2 of a beat)
+        let later_time = start_time + Duration::from_millis(100);
+
+        let result = interpolation.update(&state, later_time);
+
+        // Position should be interpolated forward
+        assert!(result.position_in_beat > 0.5);
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_beat_boundary_detection() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Start at position 0.9
+        let state1 = create_test_state(0.9, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // New OSC update shows we're at beginning of next beat
+        let later_time = start_time + Duration::from_millis(100);
+        let state2 = create_test_state(0.1, 120.0, later_time);
+        let result = interpolation.update(&state2, later_time);
+
+        assert_eq!(result.beat_boundary, true);
+        assert_eq!(result.position_in_beat, 0.1);
+    }
+
+    #[test]
+    fn test_single_missed_osc_update() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize at 0.1
+        let state1 = create_test_state(0.1, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // Frame update with no new OSC data - should interpolate
+        let frame_time = start_time + Duration::from_millis(50);
+        let result = interpolation.update(&state1, frame_time);
+
+        // Should interpolate forward from 0.1
+        assert!(result.position_in_beat > 0.1);
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_single_late_osc_update() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize
+        let state1 = create_test_state(0.1, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // Late OSC update arrives after we've been interpolating
+        let osc_time = start_time + Duration::from_millis(100);
+        let frame_time = start_time + Duration::from_millis(150);
+        let state2 = create_test_state(0.3, 120.0, osc_time);
+
+        let result = interpolation.update(&state2, frame_time);
+
+        // Should sync to OSC data and continue interpolating
+        assert!(result.position_in_beat >= 0.3);
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_multiple_missed_updates() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize
+        let state1 = create_test_state(0.8, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // Multiple frame updates without new OSC data, crossing beat boundary
+        let frame1_time = start_time + Duration::from_millis(100);
+        let result1 = interpolation.update(&state1, frame1_time);
+        // At 120 BPM: 100ms = 0.2 beats, so 0.8 + 0.2 = 1.0 (beat boundary)
+        assert_eq!(result1.beat_boundary, true);
+
+        let frame2_time = start_time + Duration::from_millis(200);
+        let result2 = interpolation.update(&state1, frame2_time);
+        // No new beat boundary since we already crossed at 100ms
+        assert_eq!(result2.beat_boundary, false);
+
+        let frame3_time = start_time + Duration::from_millis(600);
+        let result3 = interpolation.update(&state1, frame3_time);
+        // Another beat boundary should be detected (at 500ms mark)
+        assert_eq!(result3.beat_boundary, true);
+    }
+
+    #[test]
+    fn test_full_missed_beat_several_updates() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize at 0.9
+        let state1 = create_test_state(0.9, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // OSC update arrives showing we've crossed a full beat
+        let osc_time = start_time + Duration::from_millis(600); // > 1 beat at 120 BPM
+        let frame_time = start_time + Duration::from_millis(650);
+        let state2 = create_test_state(0.1, 120.0, osc_time);
+
+        let result = interpolation.update(&state2, frame_time);
+
+        // Should only report one beat boundary, not multiple
+        assert_eq!(result.beat_boundary, true);
+        // Position should be interpolated forward from OSC time to frame time
+        // 50ms at 120 BPM = 50/500 = 0.1 beat advancement
+        assert!((result.position_in_beat - 0.2).abs() < 0.01); // 0.1 + 0.1 = 0.2
+    }
+
+    #[test]
+    fn test_missing_updates_for_several_beats() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize with OSC data
+        let state1 = create_test_state(0.1, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        let mut beat_count = 0;
+
+        // Simulate regular frame updates (every 16ms) but OSC data becomes stale
+        // OSC data stops updating, but GUI keeps calling update()
+        for i in 1..=94 {
+            // 94 * 16ms = ~1500ms = 3 beats at 120 BPM
+            let frame_time = start_time + Duration::from_millis(i * 16);
+
+            // Keep using the same stale OSC data (same timestamp)
+            let result = interpolation.update(&state1, frame_time);
+
+            if result.beat_boundary {
+                beat_count += 1;
+            }
+        }
+
+        // Should detect beat boundaries through time interpolation even with stale OSC data
+        // Starting at 0.1, over 1500ms at 120 BPM, should cross 3 beat boundaries
+        // (at ~450ms, ~950ms, ~1450ms from 0.1 starting point)
+        assert_eq!(beat_count, 3);
+
+        // Finally, new OSC data arrives (simulating network recovery)
+        let late_osc_time = start_time + Duration::from_millis(1500);
+        let frame_time = start_time + Duration::from_millis(1500);
+        let state2 = create_test_state(0.7, 120.0, late_osc_time);
+        let result = interpolation.update(&state2, frame_time);
+
+        // Should not trigger additional beat boundary since we already tracked them
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_no_double_beat_counting() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize
+        let state1 = create_test_state(0.9, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // First frame after beat boundary
+        let later_time = start_time + Duration::from_millis(100);
+        let state2 = create_test_state(0.1, 120.0, later_time);
+        let result1 = interpolation.update(&state2, later_time);
+        assert_eq!(result1.beat_boundary, true);
+
+        // Second frame with same OSC data (no new timestamp)
+        let frame_time = later_time + Duration::from_millis(16);
+        let result2 = interpolation.update(&state2, frame_time);
+        assert_eq!(result2.beat_boundary, false); // Should not double-count
+
+        // Third frame with same OSC data
+        let frame_time2 = later_time + Duration::from_millis(32);
+        let result3 = interpolation.update(&state2, frame_time2);
+        assert_eq!(result3.beat_boundary, false); // Should still not double-count
+    }
+
+    #[test]
+    fn test_beat_counting_across_multiple_beats() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize
+        let state1 = create_test_state(0.1, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        let mut beat_count = 0;
+
+        // Simulate several beats passing through time interpolation
+        // At 120 BPM: 250ms = 0.5 beats, 500ms = 1 beat
+        // Starting at 0.1, beat boundaries at: 500ms, 1000ms, 1500ms, 2000ms, 2500ms
+        for i in 1..=10 {
+            let frame_time = start_time + Duration::from_millis(i * 250); // Half beat intervals
+            let result = interpolation.update(&state1, frame_time);
+            if result.beat_boundary {
+                beat_count += 1;
+            }
+        }
+
+        // Should detect exactly 5 beat boundaries (at 500ms, 1000ms, 1500ms, 2000ms, 2500ms)
+        assert_eq!(beat_count, 5);
+    }
+
+    #[test]
+    fn test_tempo_change_during_interpolation() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Start with 120 BPM at position 0.5
+        let state1 = create_test_state(0.5, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // 100ms later, tempo changes to 60 BPM but no position update
+        let frame_time = start_time + Duration::from_millis(100);
+        let state2 = create_test_state(0.5, 60.0, start_time); // Same OSC timestamp, new tempo
+        let result = interpolation.update(&state2, frame_time);
+
+        // Should still advance based on old tempo until new OSC data arrives
+        assert!(result.position_in_beat > 0.5);
+        assert_eq!(result.beat_boundary, false);
+
+        // Now new OSC data arrives with the new tempo
+        let osc_time = start_time + Duration::from_millis(150);
+        let frame_time2 = start_time + Duration::from_millis(200);
+        let state3 = create_test_state(0.7, 60.0, osc_time);
+        let result2 = interpolation.update(&state3, frame_time2);
+
+        // Should interpolate with new tempo: 50ms at 60 BPM = 50/1000 = 0.05 beat
+        assert!((result2.position_in_beat - 0.75).abs() < 0.01);
+    }
+
+    #[test]
+    fn test_osc_data_at_exact_beat_boundary() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize at 0.9
+        let state1 = create_test_state(0.9, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // OSC data arrives showing exact beat boundary (position 0.0)
+        let later_time = start_time + Duration::from_millis(100);
+        let state2 = create_test_state(0.0, 120.0, later_time);
+        let result = interpolation.update(&state2, later_time);
+
+        assert_eq!(result.beat_boundary, true);
+        assert_eq!(result.position_in_beat, 0.0);
+    }
+
+    #[test]
+    fn test_large_position_jump_without_beat() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize at 0.2
+        let state1 = create_test_state(0.2, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // Large jump within same beat (shouldn't trigger beat boundary)
+        let later_time = start_time + Duration::from_millis(50);
+        let state2 = create_test_state(0.8, 120.0, later_time);
+        let result = interpolation.update(&state2, later_time);
+
+        assert_eq!(result.beat_boundary, false);
+        assert!((result.position_in_beat - 0.8).abs() < 0.1);
+    }
+
+    #[test]
+    fn test_position_wraparound() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Position near end of beat
+        let state1 = create_test_state(0.95, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // Position wraps to beginning
+        let later_time = start_time + Duration::from_millis(50);
+        let state2 = create_test_state(0.05, 120.0, later_time);
+        let result = interpolation.update(&state2, later_time);
+
+        assert_eq!(result.beat_boundary, true);
+        assert_eq!(result.position_in_beat, 0.05);
+    }
+
+    #[test]
+    fn test_backward_time_jump() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize at some position
+        let state1 = create_test_state(0.7, 120.0, start_time);
+        interpolation.update(&state1, start_time);
+
+        // OSC update shows earlier position (shouldn't happen normally)
+        let later_time = start_time + Duration::from_millis(100);
+        let state2 = create_test_state(0.3, 120.0, later_time);
+        let result = interpolation.update(&state2, later_time);
+
+        // Should handle gracefully without reporting false beat boundary
+        assert_eq!(result.position_in_beat, 0.3);
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_multiple_consecutive_updates() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Series of rapid updates
+        for i in 0..10 {
+            let position = (i as f32) * 0.1;
+            let time = start_time + Duration::from_millis(i * 50);
+            let state = create_test_state(position, 120.0, time);
+            let result = interpolation.update(&state, time);
+
+            if i == 0 {
+                assert_eq!(result.beat_boundary, false); // First update
+            }
+            // Beat boundary should only occur when crossing 1.0 -> 0.0
+        }
+    }
+
+    #[test]
+    fn test_rapid_frame_updates_between_osc() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Initialize
+        let state = create_test_state(0.2, 120.0, start_time);
+        interpolation.update(&state, start_time);
+
+        // Multiple rapid frame updates without new OSC data
+        for i in 1..5 {
+            let frame_time = start_time + Duration::from_millis(i * 16); // ~60fps
+            let result = interpolation.update(&state, frame_time);
+
+            // Each frame should advance position slightly
+            assert!(result.position_in_beat >= 0.2);
+            // No beat boundaries unless we cross a full beat
+        }
+    }
+
+    #[test]
+    fn test_very_slow_tempo() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Very slow tempo (30 BPM = 2 second beats)
+        let state = create_test_state(0.1, 30.0, start_time);
+        interpolation.update(&state, start_time);
+
+        // Small time advancement
+        let later_time = start_time + Duration::from_millis(100);
+        let result = interpolation.update(&state, later_time);
+
+        // Position should advance very slowly
+        assert!(result.position_in_beat > 0.1);
+        assert!(result.position_in_beat < 0.2);
+        assert_eq!(result.beat_boundary, false);
+    }
+
+    #[test]
+    fn test_very_fast_tempo() {
+        let start_time = Instant::now();
+        let mut interpolation = Interpolation::new(start_time);
+
+        // Very fast tempo (240 BPM = 250ms beats)
+        let state = create_test_state(0.8, 240.0, start_time);
+        interpolation.update(&state, start_time);
+
+        // Time advancement that crosses beat boundary
+        let later_time = start_time + Duration::from_millis(100);
+        let result = interpolation.update(&state, later_time);
+
+        // Should detect beat boundary quickly
+        assert_eq!(result.beat_boundary, true);
+    }
+}

gui/src/main.rs

@@ -1,4 +1,6 @@
+mod interpolation;
 mod osc_client;
+mod pendulum;
 mod ui;
 
 use anyhow::anyhow;

gui/src/osc_client.rs

@@ -23,7 +23,7 @@ pub struct Client {
 
 impl Client {
     pub fn new(socket_path: &str, columns: usize, rows: usize) -> Result<Self> {
-        let initial_state = osc::State::new(columns, rows);
+        let initial_state = osc::State::new(columns, rows, 120.0);
         // Test data
         let initial_state = {
             let mut initial_state = initial_state;

gui/src/pendulum.rs

@@ -0,0 +1,40 @@
+use crate::interpolation::Interpolated;
+
+/// Handles pendulum swing direction and position calculation
+#[derive(Debug)]
+pub struct Pendulum {
+    /// Current swing direction (true = left to right, false = right to left)
+    going_right: bool,
+}
+
+impl Pendulum {
+    /// Create a new pendulum starting in the right direction
+    pub fn new() -> Self {
+        Self { going_right: true }
+    }
+
+    /// Update pendulum with interpolated timing data and return current position
+    /// 
+    /// Returns position on rail where 0.0 = left side, 1.0 = right side
+    pub fn update(&mut self, interpolated: &Interpolated) -> f32 {
+        // Change direction on beat boundary
+        if interpolated.beat_boundary {
+            self.going_right = !self.going_right;
+        }
+
+        // Calculate position based on current direction
+        if self.going_right {
+            // Left to right: 0.0 -> 0.0, 1.0 -> 1.0
+            interpolated.position_in_beat
+        } else {
+            // Right to left: 0.0 -> 1.0, 1.0 -> 0.0
+            1.0 - interpolated.position_in_beat
+        }
+    }
+}
+
+impl Default for Pendulum {
+    fn default() -> Self {
+        Self::new()
+    }
+}

gui/src/ui/app.rs

@@ -1,16 +1,26 @@
 pub struct App {
     state_receiver: tokio::sync::watch::Receiver<osc::State>,
+    interpolation: crate::interpolation::Interpolation,
+    pendulum: crate::pendulum::Pendulum,
 }
 
 impl App {
     pub fn new(state_receiver: tokio::sync::watch::Receiver<osc::State>) -> Self {
-        Self { state_receiver }
+        let interpolation = crate::interpolation::Interpolation::new(std::time::Instant::now());
+        let pendulum = crate::pendulum::Pendulum::new();
+        Self {
+            state_receiver,
+            interpolation,
+            pendulum,
+        }
     }
 }
 
 impl eframe::App for App {
     fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
         let state = self.state_receiver.borrow_and_update().clone();
+        let interpolated = self.interpolation.update(&state, std::time::Instant::now());
+        let pendulum_position = self.pendulum.update(&interpolated);
 
         ctx.style_mut(|style| {
             style.visuals.panel_fill = egui::Color32::from_rgb(17, 17, 17);
@@ -20,7 +30,7 @@ impl eframe::App for App {
 
         egui::CentralPanel::default().show(ctx, |ui| {
             ui.vertical(|ui| {
-                ui.add(super::Metronome::new(state.metronome_position));
+                ui.add(super::Metronome::new(pendulum_position));
                 ui.add(super::Matrix::new(
                     &state.cells,
                     state.selected_column,

osc/src/message.rs

@@ -21,6 +21,9 @@ pub enum Message {
     MetronomePosition {
         position: f32, // 0.0 - 1.0 position within current beat
     },
+    Tempo {
+        bpm: f32,
+    },
 }
 
 impl Message {
@@ -59,6 +62,9 @@ impl Message {
             Message::MetronomePosition { position } => {
                 state.set_metronome_position(*position);
             }
+            Message::Tempo { bpm } => {
+                state.set_tempo(*bpm);
+            }
         }
     }
 
@@ -117,6 +123,11 @@ impl Message {
                     position: *position,
                 }));
             }
+        } else if addr == "/looper/tempo" {
+            if let Some(rosc::OscType::Float(bpm)) = args.first() {
+                log::trace!("Tempo: bpm={bpm}");
+                return Ok(Some(Message::Tempo { bpm: *bpm }));
+            }
         }
         // Unknown or unsupported message
         Ok(None)
@@ -168,6 +179,14 @@ impl Message {
                     args: vec![rosc::OscType::Float(position)],
                 })
             }
+            Message::Tempo { bpm } => {
+                let address = "/looper/tempo".to_string();
+
+                rosc::OscPacket::Message(rosc::OscMessage {
+                    addr: address,
+                    args: vec![rosc::OscType::Float(bpm)],
+                })
+            }
         }
     }
 }

osc/src/state.rs

@@ -23,10 +23,11 @@ pub struct State {
     pub rows: usize,
     pub metronome_position: f32, // 0.0 - 1.0 position within current beat
     pub metronome_timestamp: std::time::Instant, // When position was last updated
+    pub tempo: f32, // BPM
 }
 
 impl State {
-    pub fn new(columns: usize, rows: usize) -> Self {
+    pub fn new(columns: usize, rows: usize, tempo: f32) -> Self {
         let cells = (0..columns)
             .map(|_| {
                 vec![
@@ -47,6 +48,7 @@ impl State {
             rows,
             metronome_position: 0.0,
             metronome_timestamp: std::time::Instant::now(),
+            tempo,
         }
     }
 
@@ -79,12 +81,18 @@ impl State {
             Message::MetronomePosition { position } => {
                 self.set_metronome_position(*position);
             }
+            Message::Tempo { bpm } => {
+                self.set_tempo(*bpm);
+            }
         }
     }
 
     pub fn create_state_dump(&self) -> Vec<Message> {
         let mut messages = Vec::new();
 
+        // Send tempo
+        messages.push(Message::Tempo { bpm: self.tempo });
+
         // Send current selections
         messages.push(Message::SelectedColumnChanged {
             column: self.selected_column,
@@ -140,4 +148,8 @@ impl State {
         self.metronome_position = position;
         self.metronome_timestamp = std::time::Instant::now();
     }
+
+    pub fn set_tempo(&mut self, bpm: f32) {
+        self.tempo = bpm;
+    }
 }