laser-dac 0.11.7

//! FrameSession and FrameSessionConfig — public frame-mode API.

use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::mpsc;
use std::sync::{Arc, Mutex};
use std::thread::JoinHandle;
use std::time::{Duration, Instant};

use crate::backend::{BackendKind, WriteOutcome};
use crate::error::{Error, Result};
use crate::reconnect::{reconnect_backend_with_retry, ReconnectPolicy};
use crate::scheduler;
use crate::stream::StreamControl;
use crate::types::{DacInfo, LaserPoint, RunExit};

use super::engine::{ColorDelayLine, PresentationEngine};
use super::{
    default_transition, Frame, OutputFilter, OutputFilterContext, OutputResetReason,
    PresentedSliceKind, TransitionFn,
};

/// Convert a duration to a point count at the given PPS rate.
fn duration_to_points(d: Duration, pps: u32) -> usize {
    if d.is_zero() {
        0
    } else {
        (d.as_secs_f64() * pps as f64).ceil() as usize
    }
}

// =============================================================================
// FrameSessionConfig
// =============================================================================

/// Configuration for a frame-mode streaming session.
pub struct FrameSessionConfig {
    /// Points per second output rate.
    pub pps: u32,
    /// Transition function for blanking between frames.
    pub transition_fn: TransitionFn,
    /// Duration of forced blanking after arming (default: 1ms).
    pub startup_blank: std::time::Duration,
    /// Number of points to shift RGB relative to XY (0 = disabled).
    ///
    /// Delays color channels relative to XY coordinates, compensating for
    /// the difference in galvo mirror and laser modulation response times.
    /// Applied at composition time. Set to 0 to disable.
    pub color_delay_points: usize,
    /// Reconnection configuration (default: disabled).
    ///
    /// Set via [`with_reconnect`](Self::with_reconnect) to enable automatic
    /// reconnection when the device disconnects.
    pub reconnect: Option<crate::types::ReconnectConfig>,
    /// Policy for what to output when the stream is idle (disarmed).
    ///
    /// Controls scanner behavior when disarmed. Default: [`Blank`](crate::types::IdlePolicy::Blank)
    /// (park at origin with laser off). Use [`Park`](crate::types::IdlePolicy::Park) to park at a
    /// specific position.
    pub idle_policy: crate::types::IdlePolicy,
    /// Optional hook for processing the final presented output.
    pub output_filter: Option<Box<dyn OutputFilter>>,
}

impl FrameSessionConfig {
    const DEFAULT_COLOR_DELAY: std::time::Duration = std::time::Duration::from_micros(150);

    /// Create a new config with the given PPS and default transition.
    pub fn new(pps: u32) -> Self {
        let color_delay_points =
            (Self::DEFAULT_COLOR_DELAY.as_secs_f64() * pps as f64).ceil() as usize;
        Self {
            pps,
            transition_fn: default_transition(pps),
            startup_blank: std::time::Duration::from_millis(1),
            color_delay_points,
            idle_policy: crate::types::IdlePolicy::default(),
            reconnect: None,
            output_filter: None,
        }
    }

    /// Set the transition function (builder pattern).
    pub fn with_transition_fn(mut self, f: TransitionFn) -> Self {
        self.transition_fn = f;
        self
    }

    /// Set the startup blank duration (builder pattern).
    pub fn with_startup_blank(mut self, duration: std::time::Duration) -> Self {
        self.startup_blank = duration;
        self
    }

    /// Set the color delay in points (builder pattern).
    pub fn with_color_delay_points(mut self, n: usize) -> Self {
        self.color_delay_points = n;
        self
    }

    /// Enable automatic reconnection (builder pattern).
    ///
    /// Requires the device to have been opened via [`open_device`](crate::open_device).
    pub fn with_reconnect(mut self, config: crate::types::ReconnectConfig) -> Self {
        self.reconnect = Some(config);
        self
    }

    /// Set the idle policy (builder pattern).
    ///
    /// Controls scanner behavior when disarmed. See [`crate::types::IdlePolicy`].
    pub fn with_idle_policy(mut self, policy: crate::types::IdlePolicy) -> Self {
        self.idle_policy = policy;
        self
    }

    /// Install a final-output filter (builder pattern).
    pub fn with_output_filter(mut self, filter: Box<dyn OutputFilter>) -> Self {
        self.output_filter = Some(filter);
        self
    }
}

// =============================================================================
// FrameSession
// =============================================================================

/// Read-only liveness and connectivity metrics for a [`FrameSession`].
#[derive(Clone)]
pub struct FrameSessionMetrics {
    inner: Arc<FrameSessionMetricsInner>,
}

struct FrameSessionMetricsInner {
    connected: AtomicBool,
    origin: Instant,
    last_loop_activity_nanos: AtomicU64,
    last_write_success_nanos: AtomicU64,
}

impl FrameSessionMetrics {
    fn new(connected: bool) -> Self {
        let metrics = Self {
            inner: Arc::new(FrameSessionMetricsInner {
                connected: AtomicBool::new(connected),
                origin: Instant::now(),
                last_loop_activity_nanos: AtomicU64::new(0),
                last_write_success_nanos: AtomicU64::new(0),
            }),
        };
        metrics.mark_loop_activity();
        metrics
    }

    /// Returns whether the session currently has a connected backend.
    pub fn connected(&self) -> bool {
        self.inner.connected.load(Ordering::SeqCst)
    }

    /// Returns the last time the scheduler thread showed progress.
    pub fn last_loop_activity(&self) -> Option<Instant> {
        self.instant_from_nanos(self.inner.last_loop_activity_nanos.load(Ordering::SeqCst))
    }

    /// Returns the last time a backend write completed successfully.
    pub fn last_write_success(&self) -> Option<Instant> {
        self.instant_from_nanos(self.inner.last_write_success_nanos.load(Ordering::SeqCst))
    }

    fn instant_from_nanos(&self, nanos: u64) -> Option<Instant> {
        if nanos == 0 {
            None
        } else {
            self.inner.origin.checked_add(Duration::from_nanos(nanos))
        }
    }

    fn now_nanos(&self) -> u64 {
        (self.inner.origin.elapsed().as_nanos().min(u64::MAX as u128) as u64).max(1)
    }

    fn mark_loop_activity(&self) {
        self.inner
            .last_loop_activity_nanos
            .store(self.now_nanos(), Ordering::SeqCst);
    }

    fn mark_write_success(&self) {
        let now = self.now_nanos();
        self.inner
            .last_loop_activity_nanos
            .store(now, Ordering::SeqCst);
        self.inner
            .last_write_success_nanos
            .store(now, Ordering::SeqCst);
    }

    fn set_connected(&self, connected: bool) {
        self.inner.connected.store(connected, Ordering::SeqCst);
        self.mark_loop_activity();
    }
}

struct MetricsDisconnectGuard(FrameSessionMetrics);

impl Drop for MetricsDisconnectGuard {
    fn drop(&mut self) {
        self.0.set_connected(false);
    }
}

/// A frame-mode streaming session.
///
/// Owns a scheduler thread that reads frames from a channel and writes them
/// to the DAC backend using the appropriate strategy (FIFO or frame-swap).
///
/// # Example
///
/// ```ignore
/// use laser_dac::{open_device, FrameSessionConfig, Frame, LaserPoint};
///
/// let device = open_device("my-device")?;
/// let config = FrameSessionConfig::new(30_000);
/// let (session, _info) = device.start_frame_session(config)?;
///
/// session.control().arm()?;
/// session.send_frame(Frame::new(vec![
///     LaserPoint::new(0.0, 0.0, 65535, 0, 0, 65535),
/// ]));
/// ```
pub struct FrameSession {
    control: StreamControl,
    thread: Option<JoinHandle<Result<RunExit>>>,
    frame_slot: Arc<Mutex<Option<Frame>>>,
    metrics: FrameSessionMetrics,
}

impl FrameSession {
    /// Start a frame session on the given backend.
    pub(crate) fn start(
        mut backend: BackendKind,
        config: FrameSessionConfig,
        reconnect_policy: Option<ReconnectPolicy>,
    ) -> Result<Self> {
        // Connect if needed
        if !backend.is_connected() {
            backend.connect()?;
        }

        let (control_tx, control_rx) = mpsc::channel();
        let initial_color_delay = if config.color_delay_points > 0 {
            Duration::from_secs_f64(config.color_delay_points as f64 / config.pps as f64)
        } else {
            Duration::ZERO
        };
        let control = StreamControl::new(control_tx, initial_color_delay, config.pps);
        let frame_slot: Arc<Mutex<Option<Frame>>> = Arc::new(Mutex::new(None));
        let metrics = FrameSessionMetrics::new(backend.is_connected());

        let control_clone = control.clone();
        let is_frame_swap = backend.is_frame_swap();
        let slot_clone = frame_slot.clone();
        let metrics_clone = metrics.clone();

        let thread = std::thread::spawn(move || {
            let _disconnect_guard = MetricsDisconnectGuard(metrics_clone.clone());
            if is_frame_swap {
                Self::run_frame_swap_loop(
                    backend,
                    config,
                    control_clone,
                    control_rx,
                    slot_clone,
                    metrics_clone,
                    reconnect_policy,
                )
            } else {
                Self::run_fifo_loop(
                    backend,
                    config,
                    control_clone,
                    control_rx,
                    slot_clone,
                    metrics_clone,
                    reconnect_policy,
                )
            }
        });

        Ok(Self {
            control,
            thread: Some(thread),
            frame_slot,
            metrics,
        })
    }

    /// Returns a control handle for arm/disarm/stop.
    pub fn control(&self) -> StreamControl {
        self.control.clone()
    }

    /// Submit a frame for display. Latest-wins: overwrites any unconsumed
    /// pending frame immediately, with no buffering or memory growth.
    pub fn send_frame(&self, frame: Frame) {
        *self.frame_slot.lock().unwrap() = Some(frame);
    }

    /// Returns true if the scheduler thread has finished.
    pub fn is_finished(&self) -> bool {
        self.thread.as_ref().is_some_and(|h| h.is_finished())
    }

    /// Returns a metrics handle for observing scheduler liveness.
    pub fn metrics(&self) -> FrameSessionMetrics {
        self.metrics.clone()
    }

    /// Wait for the session thread to finish and return the exit reason.
    pub fn join(mut self) -> Result<RunExit> {
        if let Some(handle) = self.thread.take() {
            handle
                .join()
                .unwrap_or(Err(Error::disconnected("thread panicked")))
        } else {
            Ok(RunExit::Stopped)
        }
    }

    // =========================================================================
    // FIFO scheduler loop
    // =========================================================================

    fn run_fifo_loop(
        backend: BackendKind,
        config: FrameSessionConfig,
        control: StreamControl,
        control_rx: mpsc::Receiver<crate::stream::ControlMsg>,
        frame_slot: Arc<Mutex<Option<Frame>>>,
        metrics: FrameSessionMetrics,
        reconnect_policy: Option<ReconnectPolicy>,
    ) -> Result<RunExit> {
        let is_udp_timed = backend.caps().output_model == crate::types::OutputModel::UdpTimed;

        if is_udp_timed {
            Self::run_udp_timed_loop(
                backend,
                config,
                control,
                control_rx,
                frame_slot,
                metrics,
                reconnect_policy,
            )
        } else {
            Self::run_fifo_estimation_loop(
                backend,
                config,
                control,
                control_rx,
                frame_slot,
                metrics,
                reconnect_policy,
            )
        }
    }

    /// Fixed-rate loop for UdpTimed backends (LaserCube WiFi).
    ///
    /// Sends a fixed chunk every cycle at precise intervals. No buffer
    /// estimation — just metronomic pacing. This avoids all the overflow
    /// and estimation issues that plague UDP-based laser DACs.
    fn run_udp_timed_loop(
        mut backend: BackendKind,
        config: FrameSessionConfig,
        control: StreamControl,
        control_rx: mpsc::Receiver<crate::stream::ControlMsg>,
        frame_slot: Arc<Mutex<Option<Frame>>>,
        metrics: FrameSessionMetrics,
        reconnect_policy: Option<ReconnectPolicy>,
    ) -> Result<RunExit> {
        let FrameSessionConfig {
            pps: initial_pps,
            transition_fn,
            startup_blank,
            color_delay_points,
            idle_policy,
            reconnect: _,
            output_filter,
        } = config;
        let initial_pps_f64 = initial_pps as f64;
        // Fixed chunk: ~10ms worth of points (small enough to avoid overflow,
        // large enough for efficient packets). At 30kpps = 300 points = ~2 packets.
        let mut chunk_points =
            ((initial_pps_f64 * 0.010).ceil() as usize).min(backend.caps().max_points_per_chunk);
        let mut chunk_duration = Duration::from_secs_f64(chunk_points as f64 / initial_pps_f64);

        let mut engine = PresentationEngine::new(transition_fn);
        let mut chunk_buffer = vec![LaserPoint::default(); chunk_points];
        let mut color_delay = ColorDelayLine::new(color_delay_points);
        let mut shutter_open = false;
        let mut startup_blank_remaining: usize = 0;
        let mut last_armed = false;
        let mut next_send = Instant::now();
        let mut last_frame: Option<Frame> = None;
        let mut retry_points: Option<usize> = None;
        let mut output_filter = output_filter;

        Self::reset_output_filter(&mut output_filter, OutputResetReason::SessionStart);

        loop {
            metrics.mark_loop_activity();
            // 1. High-precision wait until next send time
            Self::sleep_until_precise(
                &control,
                &control_rx,
                next_send,
                &mut shutter_open,
                &mut backend,
                &metrics,
            )?;
            next_send += chunk_duration;

            // Catch up if we fell behind (don't accumulate debt)
            let now = Instant::now();
            if next_send < now {
                next_send = now;
            }

            let pps = control.pps();
            let pps_f64 = pps as f64;
            let new_chunk_points =
                ((pps_f64 * 0.010).ceil() as usize).min(backend.caps().max_points_per_chunk);
            if new_chunk_points != chunk_points {
                chunk_points = new_chunk_points;
                chunk_buffer.resize(chunk_points, LaserPoint::default());
                chunk_duration = Duration::from_secs_f64(chunk_points as f64 / pps_f64);
            }
            let startup_blank_points = duration_to_points(startup_blank, pps);
            color_delay.resize(duration_to_points(control.color_delay(), pps));

            // 2. Check stop
            if control.is_stop_requested() {
                return Ok(RunExit::Stopped);
            }

            // 3. Check for new frame (latest-wins slot)
            if let Some(frame) = frame_slot.lock().unwrap().take() {
                last_frame = Some(frame.clone());
                engine.set_pending(frame);
            }

            // 4. Check connection
            if !backend.is_connected() {
                if let Some(ref policy) = reconnect_policy {
                    match Self::try_reconnect(
                        &mut backend,
                        policy,
                        &control,
                        &mut shutter_open,
                        &mut last_armed,
                        &mut engine,
                        &last_frame,
                        false,
                        &metrics,
                    ) {
                        Ok(_info) => {
                            next_send = Instant::now();
                            color_delay.reset();
                            Self::reset_output_filter(
                                &mut output_filter,
                                OutputResetReason::Reconnect,
                            );
                            retry_points = None;
                            continue;
                        }
                        Err(exit) => return Ok(exit),
                    }
                }
                return Ok(RunExit::Disconnected);
            }

            // 5. Process control messages
            if Self::process_control_messages(&control_rx, &mut shutter_open, &mut backend) {
                return Ok(RunExit::Stopped);
            }

            // 6. Handle arm/disarm transitions
            let is_armed = control.is_armed();
            if let Some(reason) = Self::handle_shutter_transition(
                is_armed,
                &mut last_armed,
                &mut shutter_open,
                &mut startup_blank_remaining,
                startup_blank_points,
                &mut backend,
            ) {
                Self::reset_output_filter(&mut output_filter, reason);
            }

            // 7. Fill chunk from engine
            let n = if let Some(n) = retry_points {
                n
            } else {
                let n = engine.fill_chunk(&mut chunk_buffer, chunk_points);
                if n == 0 {
                    continue;
                }

                // Apply blanking modifications
                Self::apply_blanking(
                    is_armed,
                    &mut startup_blank_remaining,
                    &mut chunk_buffer[..n],
                    &idle_policy,
                );

                // Apply color delay (stateful — carries across chunks)
                color_delay.apply(&mut chunk_buffer[..n]);
                if engine.has_logical_frame() {
                    Self::apply_output_filter(
                        &mut output_filter,
                        &mut chunk_buffer[..n],
                        OutputFilterContext {
                            pps,
                            kind: PresentedSliceKind::FifoChunk,
                            is_cyclic: false,
                        },
                    );
                }
                retry_points = Some(n);
                n
            };

            // 8. Write. On backpressure, retry the exact same transmit
            // buffer next cycle instead of advancing frame state.
            match backend.try_write(pps, &chunk_buffer[..n]) {
                Ok(WriteOutcome::Written) => {
                    metrics.mark_write_success();
                    retry_points = None;
                }
                Ok(WriteOutcome::WouldBlock) => {
                    metrics.mark_loop_activity();
                    // Device busy — keep retry_points set and retry next cycle
                }
                Err(e) if e.is_disconnected() => {
                    if let Some(ref policy) = reconnect_policy {
                        match Self::try_reconnect(
                            &mut backend,
                            policy,
                            &control,
                            &mut shutter_open,
                            &mut last_armed,
                            &mut engine,
                            &last_frame,
                            false,
                            &metrics,
                        ) {
                            Ok(_info) => {
                                next_send = Instant::now();
                                color_delay.reset();
                                Self::reset_output_filter(
                                    &mut output_filter,
                                    OutputResetReason::Reconnect,
                                );
                                retry_points = None;
                                continue;
                            }
                            Err(exit) => return Ok(exit),
                        }
                    }
                    return Ok(RunExit::Disconnected);
                }
                Err(_) => {}
            }
        }
    }

    /// Buffer-estimation loop for standard FIFO backends (Ether Dream, IDN, etc).
    fn run_fifo_estimation_loop(
        mut backend: BackendKind,
        config: FrameSessionConfig,
        control: StreamControl,
        control_rx: mpsc::Receiver<crate::stream::ControlMsg>,
        frame_slot: Arc<Mutex<Option<Frame>>>,
        metrics: FrameSessionMetrics,
        reconnect_policy: Option<ReconnectPolicy>,
    ) -> Result<RunExit> {
        let FrameSessionConfig {
            transition_fn,
            startup_blank,
            color_delay_points,
            idle_policy,
            output_filter,
            ..
        } = config;
        let mut max_points = backend.caps().max_points_per_chunk;
        let target_buffer_secs = 0.020_f64;

        let mut engine = PresentationEngine::new(transition_fn);
        let mut scheduled_ahead: u64 = 0;
        let mut fractional_consumed: f64 = 0.0;
        let mut last_iteration = Instant::now();
        let mut chunk_buffer = vec![LaserPoint::default(); max_points];
        let mut color_delay = ColorDelayLine::new(color_delay_points);
        let mut shutter_open = false;
        let mut startup_blank_remaining: usize = 0;
        let mut last_armed = false;
        let mut last_frame: Option<Frame> = None;
        let mut output_filter = output_filter;

        Self::reset_output_filter(&mut output_filter, OutputResetReason::SessionStart);

        loop {
            metrics.mark_loop_activity();
            // 1. Check stop
            if control.is_stop_requested() {
                return Ok(RunExit::Stopped);
            }

            let pps = control.pps();
            let pps_f64 = pps as f64;
            let target_buffer_points = (target_buffer_secs * pps_f64) as u64;
            let startup_blank_points = duration_to_points(startup_blank, pps);
            color_delay.resize(duration_to_points(control.color_delay(), pps));

            // Time-based decay of scheduled_ahead
            let now = Instant::now();
            scheduler::advance_scheduled_ahead(
                &mut scheduled_ahead,
                &mut fractional_consumed,
                &mut last_iteration,
                now,
                pps_f64,
            );

            // 2. Check for new frame (latest-wins slot)
            if let Some(frame) = frame_slot.lock().unwrap().take() {
                last_frame = Some(frame.clone());
                engine.set_pending(frame);
            }

            // 3. Estimate buffer
            let buffered =
                scheduler::conservative_buffered_points(scheduled_ahead, backend.queued_points());

            // 4. Sleep if buffer healthy
            if buffered > target_buffer_points {
                let excess = buffered - target_buffer_points;
                let sleep_time = Duration::from_secs_f64(excess as f64 / pps_f64);
                Self::sleep_with_control_check(
                    &control,
                    &control_rx,
                    sleep_time,
                    &mut shutter_open,
                    &mut backend,
                    &metrics,
                )?;
                continue;
            }

            // 5. Check connection
            if !backend.is_connected() {
                if let Some(ref policy) = reconnect_policy {
                    match Self::try_reconnect(
                        &mut backend,
                        policy,
                        &control,
                        &mut shutter_open,
                        &mut last_armed,
                        &mut engine,
                        &last_frame,
                        false,
                        &metrics,
                    ) {
                        Ok(info) => {
                            scheduled_ahead = 0;
                            fractional_consumed = 0.0;
                            last_iteration = Instant::now();
                            max_points = info.caps.max_points_per_chunk;
                            chunk_buffer.resize(max_points, LaserPoint::default());
                            color_delay.reset();
                            Self::reset_output_filter(
                                &mut output_filter,
                                OutputResetReason::Reconnect,
                            );
                            continue;
                        }
                        Err(exit) => return Ok(exit),
                    }
                }
                return Ok(RunExit::Disconnected);
            }

            // 6. Process control messages
            if Self::process_control_messages(&control_rx, &mut shutter_open, &mut backend) {
                return Ok(RunExit::Stopped);
            }

            // 7. Handle arm/disarm transitions
            let is_armed = control.is_armed();
            if let Some(reason) = Self::handle_shutter_transition(
                is_armed,
                &mut last_armed,
                &mut shutter_open,
                &mut startup_blank_remaining,
                startup_blank_points,
                &mut backend,
            ) {
                Self::reset_output_filter(&mut output_filter, reason);
            }

            // 8. Fill chunk from engine
            let deficit = (target_buffer_secs - buffered as f64 / pps_f64).max(0.0);
            let target_points = ((deficit * pps_f64).ceil() as usize).min(max_points);
            if target_points == 0 {
                Self::sleep_and_mark_activity(Duration::from_millis(1), &metrics);
                continue;
            }

            let n = engine.fill_chunk(&mut chunk_buffer, target_points);
            if n == 0 {
                Self::sleep_and_mark_activity(Duration::from_millis(1), &metrics);
                continue;
            }

            // Apply blanking modifications
            Self::apply_blanking(
                is_armed,
                &mut startup_blank_remaining,
                &mut chunk_buffer[..n],
                &idle_policy,
            );

            // Apply color delay (stateful — carries across chunks)
            color_delay.apply(&mut chunk_buffer[..n]);
            if engine.has_logical_frame() {
                Self::apply_output_filter(
                    &mut output_filter,
                    &mut chunk_buffer[..n],
                    OutputFilterContext {
                        pps,
                        kind: PresentedSliceKind::FifoChunk,
                        is_cyclic: false,
                    },
                );
            }

            // 9. Write to backend with retry on WouldBlock
            loop {
                match backend.try_write(pps, &chunk_buffer[..n]) {
                    Ok(WriteOutcome::Written) => {
                        metrics.mark_write_success();
                        scheduled_ahead += n as u64;
                        break;
                    }
                    Ok(WriteOutcome::WouldBlock) => {
                        metrics.mark_loop_activity();
                        std::thread::yield_now();
                        if control.is_stop_requested() {
                            return Ok(RunExit::Stopped);
                        }
                        Self::sleep_and_mark_activity(Duration::from_micros(100), &metrics);
                    }
                    Err(e) if e.is_disconnected() => {
                        if let Some(ref policy) = reconnect_policy {
                            match Self::try_reconnect(
                                &mut backend,
                                policy,
                                &control,
                                &mut shutter_open,
                                &mut last_armed,
                                &mut engine,
                                &last_frame,
                                false,
                                &metrics,
                            ) {
                                Ok(info) => {
                                    scheduled_ahead = 0;
                                    fractional_consumed = 0.0;
                                    last_iteration = Instant::now();
                                    max_points = info.caps.max_points_per_chunk;
                                    chunk_buffer.resize(max_points, LaserPoint::default());
                                    color_delay.reset();
                                    Self::reset_output_filter(
                                        &mut output_filter,
                                        OutputResetReason::Reconnect,
                                    );
                                    break;
                                }
                                Err(exit) => return Ok(exit),
                            }
                        }
                        return Ok(RunExit::Disconnected);
                    }
                    Err(_) => {
                        break;
                    }
                }
            }
        }
    }

    // =========================================================================
    // Frame-swap scheduler loop
    // =========================================================================

    fn run_frame_swap_loop(
        mut backend: BackendKind,
        config: FrameSessionConfig,
        control: StreamControl,
        control_rx: mpsc::Receiver<crate::stream::ControlMsg>,
        frame_slot: Arc<Mutex<Option<Frame>>>,
        metrics: FrameSessionMetrics,
        reconnect_policy: Option<ReconnectPolicy>,
    ) -> Result<RunExit> {
        let FrameSessionConfig {
            transition_fn,
            startup_blank,
            color_delay_points,
            idle_policy,
            output_filter,
            ..
        } = config;
        let mut engine = PresentationEngine::new(transition_fn);
        engine.set_frame_capacity(backend.frame_capacity());
        let mut shutter_open = false;
        let mut startup_blank_remaining: usize = 0;
        let mut last_armed = false;
        let mut last_frame: Option<Frame> = None;
        let mut frame_buf: Vec<LaserPoint> = Vec::new();
        let mut color_delay = ColorDelayLine::new(color_delay_points);
        let mut write_pending = false;
        let mut output_filter = output_filter;

        Self::reset_output_filter(&mut output_filter, OutputResetReason::SessionStart);

        loop {
            metrics.mark_loop_activity();
            // 1. Check stop
            if control.is_stop_requested() {
                return Ok(RunExit::Stopped);
            }

            let pps = control.pps();
            let startup_blank_points = duration_to_points(startup_blank, pps);
            color_delay.resize(duration_to_points(control.color_delay(), pps));

            // 2. Check for new frame (latest-wins slot)
            if let Some(frame) = frame_slot.lock().unwrap().take() {
                last_frame = Some(frame.clone());
                engine.set_pending(frame);
            }

            // 3. Check connection
            if !backend.is_connected() {
                if let Some(ref policy) = reconnect_policy {
                    match Self::try_reconnect(
                        &mut backend,
                        policy,
                        &control,
                        &mut shutter_open,
                        &mut last_armed,
                        &mut engine,
                        &last_frame,
                        true,
                        &metrics,
                    ) {
                        Ok(_info) => {
                            engine.set_frame_capacity(backend.frame_capacity());
                            color_delay.reset();
                            Self::reset_output_filter(
                                &mut output_filter,
                                OutputResetReason::Reconnect,
                            );
                            write_pending = false;
                            continue;
                        }
                        Err(exit) => return Ok(exit),
                    }
                }
                return Ok(RunExit::Disconnected);
            }

            // 4. Process control messages
            if Self::process_control_messages(&control_rx, &mut shutter_open, &mut backend) {
                return Ok(RunExit::Stopped);
            }

            // 5. Handle arm/disarm transitions
            let is_armed = control.is_armed();
            if let Some(reason) = Self::handle_shutter_transition(
                is_armed,
                &mut last_armed,
                &mut shutter_open,
                &mut startup_blank_remaining,
                startup_blank_points,
                &mut backend,
            ) {
                Self::reset_output_filter(&mut output_filter, reason);
            }

            // 6. Check if device is ready
            if !write_pending && !backend.is_ready_for_frame() {
                Self::sleep_and_mark_activity(Duration::from_millis(1), &metrics);
                continue;
            }

            // 7. Compose frame and copy to reusable buffer
            if !write_pending {
                let composed = engine.compose_hardware_frame();
                if composed.is_empty() {
                    Self::sleep_and_mark_activity(Duration::from_millis(1), &metrics);
                    continue;
                }
                frame_buf.clear();
                frame_buf.extend_from_slice(composed);

                // Apply blanking modifications
                Self::apply_blanking(
                    is_armed,
                    &mut startup_blank_remaining,
                    &mut frame_buf,
                    &idle_policy,
                );

                color_delay.apply(&mut frame_buf);
                Self::apply_output_filter(
                    &mut output_filter,
                    &mut frame_buf,
                    OutputFilterContext {
                        pps,
                        kind: PresentedSliceKind::FrameSwapFrame,
                        is_cyclic: true,
                    },
                );
            }

            // 8. Write frame
            match backend.try_write(pps, &frame_buf) {
                Ok(WriteOutcome::Written) => {
                    metrics.mark_write_success();
                    write_pending = false;
                }
                Ok(WriteOutcome::WouldBlock) => {
                    metrics.mark_loop_activity();
                    write_pending = true;
                    Self::sleep_and_mark_activity(Duration::from_millis(1), &metrics);
                }
                Err(e) if e.is_disconnected() => {
                    if let Some(ref policy) = reconnect_policy {
                        match Self::try_reconnect(
                            &mut backend,
                            policy,
                            &control,
                            &mut shutter_open,
                            &mut last_armed,
                            &mut engine,
                            &last_frame,
                            true,
                            &metrics,
                        ) {
                            Ok(_info) => {
                                engine.set_frame_capacity(backend.frame_capacity());
                                color_delay.reset();
                                Self::reset_output_filter(
                                    &mut output_filter,
                                    OutputResetReason::Reconnect,
                                );
                                write_pending = false;
                                continue;
                            }
                            Err(exit) => return Ok(exit),
                        }
                    }
                    return Ok(RunExit::Disconnected);
                }
                Err(_) => {}
            }
        }
    }

    // =========================================================================
    // Shared helpers
    // =========================================================================

    /// Attempt to reconnect the backend using the reconnection policy.
    ///
    /// On success, replaces `backend` and resets scheduler state.
    /// Returns the new `DacInfo` on success.
    #[allow(clippy::too_many_arguments)]
    fn try_reconnect(
        backend: &mut BackendKind,
        policy: &ReconnectPolicy,
        control: &StreamControl,
        shutter_open: &mut bool,
        last_armed: &mut bool,
        engine: &mut PresentationEngine,
        last_frame: &Option<Frame>,
        expected_frame_swap: bool,
        metrics: &FrameSessionMetrics,
    ) -> std::result::Result<DacInfo, RunExit> {
        metrics.set_connected(false);
        let current_pps = control.pps();
        let (info, new_backend) = reconnect_backend_with_retry(
            policy,
            || control.is_stop_requested(),
            |info, new_backend| {
                if current_pps < info.caps.pps_min || current_pps > info.caps.pps_max {
                    log::error!(
                        "'{}' PPS {} outside new device range [{}, {}]",
                        policy.target.device_id,
                        current_pps,
                        info.caps.pps_min,
                        info.caps.pps_max
                    );
                    return Err(RunExit::Disconnected);
                }

                if new_backend.is_frame_swap() != expected_frame_swap {
                    log::error!(
                        "'{}' reconnected device has incompatible backend type",
                        policy.target.device_id
                    );
                    return Err(RunExit::Disconnected);
                }

                Ok(())
            },
            || metrics.mark_loop_activity(),
        )?;

        // Swap backend
        *backend = new_backend;
        *shutter_open = false;
        *last_armed = false;
        metrics.set_connected(true);

        // Reset engine and replay last frame
        engine.reset();
        if let Some(frame) = last_frame {
            engine.set_pending(frame.clone());
        }

        // Fire on_reconnect
        if let Some(cb) = policy.on_reconnect.lock().unwrap().as_mut() {
            cb(&info);
        }

        Ok(info)
    }

    /// Handle arm/disarm shutter transitions.
    fn handle_shutter_transition(
        is_armed: bool,
        last_armed: &mut bool,
        shutter_open: &mut bool,
        startup_blank_remaining: &mut usize,
        startup_blank_points: usize,
        backend: &mut BackendKind,
    ) -> Option<OutputResetReason> {
        if !*last_armed && is_armed {
            *startup_blank_remaining = startup_blank_points;
            if !*shutter_open {
                let _ = backend.set_shutter(true);
                *shutter_open = true;
            }
            *last_armed = is_armed;
            return Some(OutputResetReason::Arm);
        } else if *last_armed && !is_armed {
            if *shutter_open {
                let _ = backend.set_shutter(false);
                *shutter_open = false;
            }
            *last_armed = is_armed;
            return Some(OutputResetReason::Disarm);
        }
        *last_armed = is_armed;
        None
    }

    /// Apply disarm blanking and startup blanking to a buffer.
    fn apply_blanking(
        is_armed: bool,
        startup_blank_remaining: &mut usize,
        buffer: &mut [LaserPoint],
        idle_policy: &crate::types::IdlePolicy,
    ) {
        if !is_armed {
            // When disarmed, apply idle policy: park scanners instead of tracing shapes
            let park = match idle_policy {
                crate::types::IdlePolicy::Park { x, y } => LaserPoint::blanked(*x, *y),
                // RepeatLast falls back to Blank when disarmed
                _ => LaserPoint::blanked(0.0, 0.0),
            };
            buffer.fill(park);
        } else if *startup_blank_remaining > 0 {
            let blank_count = buffer.len().min(*startup_blank_remaining);
            for p in &mut buffer[..blank_count] {
                p.r = 0;
                p.g = 0;
                p.b = 0;
                p.intensity = 0;
            }
            *startup_blank_remaining -= blank_count;
        }
    }

    fn process_control_messages(
        control_rx: &mpsc::Receiver<crate::stream::ControlMsg>,
        shutter_open: &mut bool,
        backend: &mut BackendKind,
    ) -> bool {
        use std::sync::mpsc::TryRecvError;
        loop {
            match control_rx.try_recv() {
                Ok(crate::stream::ControlMsg::Arm) => {
                    if !*shutter_open {
                        let _ = backend.set_shutter(true);
                        *shutter_open = true;
                    }
                }
                Ok(crate::stream::ControlMsg::Disarm) => {
                    if *shutter_open {
                        let _ = backend.set_shutter(false);
                        *shutter_open = false;
                    }
                }
                Ok(crate::stream::ControlMsg::Stop) => {
                    return true;
                }
                Err(TryRecvError::Empty) | Err(TryRecvError::Disconnected) => break,
            }
        }
        false
    }

    fn reset_output_filter(
        output_filter: &mut Option<Box<dyn OutputFilter>>,
        reason: OutputResetReason,
    ) {
        if let Some(filter) = output_filter.as_deref_mut() {
            filter.reset(reason);
        }
    }

    fn apply_output_filter(
        output_filter: &mut Option<Box<dyn OutputFilter>>,
        points: &mut [LaserPoint],
        ctx: OutputFilterContext,
    ) {
        if let Some(filter) = output_filter.as_deref_mut() {
            filter.filter(points, &ctx);
        }
    }

    fn sleep_with_control_check(
        control: &StreamControl,
        control_rx: &mpsc::Receiver<crate::stream::ControlMsg>,
        duration: std::time::Duration,
        shutter_open: &mut bool,
        backend: &mut BackendKind,
        metrics: &FrameSessionMetrics,
    ) -> Result<()> {
        const SLICE: std::time::Duration = std::time::Duration::from_millis(2);
        let mut remaining = duration;
        while remaining > std::time::Duration::ZERO {
            let slice = remaining.min(SLICE);
            std::thread::sleep(slice);
            remaining = remaining.saturating_sub(slice);
            metrics.mark_loop_activity();
            if control.is_stop_requested() {
                return Err(Error::Stopped);
            }
            Self::process_control_messages(control_rx, shutter_open, backend);
        }
        Ok(())
    }

    /// High-precision sleep for UdpTimed backends.
    ///
    /// Uses coarse sleeps first, then busy-waits near the deadline to
    /// minimize wake-up jitter. Matches `Stream::sleep_until_with_control_check`.
    fn sleep_until_precise(
        control: &StreamControl,
        control_rx: &mpsc::Receiver<crate::stream::ControlMsg>,
        deadline: std::time::Instant,
        shutter_open: &mut bool,
        backend: &mut BackendKind,
        metrics: &FrameSessionMetrics,
    ) -> Result<()> {
        const BUSY_WAIT_THRESHOLD: std::time::Duration = std::time::Duration::from_micros(500);
        const SLEEP_SLICE: std::time::Duration = std::time::Duration::from_millis(1);

        loop {
            let now = std::time::Instant::now();
            if now >= deadline {
                return Ok(());
            }

            let remaining = deadline.duration_since(now);
            if remaining > BUSY_WAIT_THRESHOLD {
                let slice = remaining
                    .saturating_sub(BUSY_WAIT_THRESHOLD)
                    .min(SLEEP_SLICE);
                std::thread::sleep(slice);
            } else {
                std::thread::yield_now();
            }
            metrics.mark_loop_activity();

            if control.is_stop_requested() {
                return Err(Error::Stopped);
            }
            Self::process_control_messages(control_rx, shutter_open, backend);
        }
    }

    fn sleep_and_mark_activity(duration: Duration, metrics: &FrameSessionMetrics) {
        std::thread::sleep(duration);
        metrics.mark_loop_activity();
    }
}

impl Drop for FrameSession {
    fn drop(&mut self) {
        let _ = self.control.stop();
        if let Some(handle) = self.thread.take() {
            let _ = handle.join();
        }
    }
}