use std::thread;
use std::time::{Duration, Instant};

#[derive(Clone, Debug)]
pub struct Config {
    pub target_frame_hz: Option<f64>,
    pub target_update_hz: f64,
    pub max_updates_per_frame: u32,
    pub max_accumulated_steps: u32,
    pub sleep_tolerance: Duration,
}

impl Default for Config {
    fn default() -> Self {
        Self {
            target_frame_hz: Some(144.0),
            target_update_hz: 60.0,
            max_updates_per_frame: 5,
            max_accumulated_steps: 8,
            sleep_tolerance: Duration::from_micros(500),
        }
    }
}

#[derive(Debug)]
pub struct Time {
    cfg: Config,
    last_instant: Instant,
    next_frame_due: Instant,
    frame_interval: Option<Duration>,
    fixed_dt: Duration,

    // tracking
    frame_dt: Duration,
    accumulator: Duration,

    // counters
    pub frame_count: u64,
    pub update_count: u64,
}

pub struct TickPlan {
    /// How many fixed updates to run this frame
    pub updates: u32,
    /// Interpolation factor for rendering between previous/next sim states
    pub alpha: f32,
    /// Measured last frame delta (seconds)
    pub frame_dt: f32,
    /// Fixed timestep (seconds)
    pub fixed_dt: f32,
}

impl Time {
    pub fn new(cfg: Config) -> Self {
        let now = Instant::now();
        let frame_interval = cfg.target_frame_hz.map(|hz| Duration::from_secs_f64(1.0 / hz));
        let fixed_dt = Duration::from_secs_f64(1.0 / cfg.target_update_hz);
        Self {
            cfg,
            last_instant: now,
            next_frame_due: now,
            frame_interval,
            fixed_dt,
            frame_dt: Duration::ZERO,
            accumulator: Duration::ZERO,
            frame_count: 0,
            update_count: 0,
        }
    }

    pub fn reconfigure(&mut self, cfg: Config) {
        self.cfg = cfg.clone();
        self.frame_interval = cfg.target_frame_hz.map(|hz| Duration::from_secs_f64(1.0 / hz));
        self.fixed_dt = Duration::from_secs_f64(1.0 / cfg.target_update_hz);
    }

    pub fn begin_frame_blocking(&mut self) -> TickPlan {
        // 1) If there's a frame cap, block until next frame deadline
        if let Some(interval) = self.frame_interval {
            let mut now = Instant::now();
            if now < self.next_frame_due {
                // Sleep most of the remainder, then spin the last tiny bit for precision
                let total_remaining = self.next_frame_due - now;
                if total_remaining > self.cfg.sleep_tolerance {
                    let sleep_for = total_remaining - self.cfg.sleep_tolerance;
                    thread::sleep(sleep_for);
                }
                // Short spin-wait for precision
                while Instant::now() < self.next_frame_due {
                    std::hint::spin_loop();
                }
                now = self.next_frame_due;
            }
            self.next_frame_due = self.next_frame_due + interval;
            // In case we fell far behind (e.g., debugger pause), resync.
            if self.next_frame_due < now {
                self.next_frame_due = now + interval;
            }
        }

        // 2) Measure frame dt
        let now = Instant::now();
        self.frame_dt = now.saturating_duration_since(self.last_instant);
        self.last_instant = now;
        self.frame_count += 1;

        // 3) Accumulate for fixed updates
        self.accumulator += self.frame_dt;

        // Clamp accumulator to avoid doing a huge number of updates after a stall
        let max_accumulated = self.fixed_dt * self.cfg.max_accumulated_steps;
        if self.accumulator > max_accumulated {
            self.accumulator = max_accumulated;
        }

        // 4) Determine how many updates to run this frame
        let mut updates = 0u32;
        while self.accumulator >= self.fixed_dt && updates < self.cfg.max_updates_per_frame {
            self.accumulator -= self.fixed_dt;
            updates += 1;
            self.update_count += 1;
        }

        // 5) Compute interpolation factor for rendering (0..1)
        let alpha = if self.fixed_dt.is_zero() {
            1.0
        } else {
            (self.accumulator.as_secs_f32() / self.fixed_dt.as_secs_f32()).clamp(0.0, 1.0)
        };

        TickPlan {
            updates,
            alpha,
            frame_dt: self.frame_dt.as_secs_f32(),
            fixed_dt: self.fixed_dt.as_secs_f32(),
        }
    }

    pub fn frame_dt_seconds(&self) -> f32 { self.frame_dt.as_secs_f32() }
    pub fn fixed_dt_seconds(&self) -> f32 { self.fixed_dt.as_secs_f32() }
    pub fn alpha(&self) -> f32 {
        if self.fixed_dt.is_zero() { 1.0 } else { (self.accumulator.as_secs_f32() / self.fixed_dt.as_secs_f32()).clamp(0.0, 1.0) }
    }
}