use std::collections::VecDeque;

use prometeu_bytecode::HeapRef;

/// Deterministic cooperative scheduler core.
///
/// Policy:
/// - FIFO for ready coroutines.
/// - Sleeping coroutines are ordered by `wake_tick` and moved to ready when `wake_tick <= current_tick`.
/// - No randomness, no preemption, no context switching here.
#[derive(Debug, Default)]
pub struct Scheduler {
    /// Queue of runnable coroutines (FIFO)
    ready_queue: VecDeque<HeapRef>,
    /// Sleeping list kept sorted by (wake_tick, insertion_order)
    sleeping: Vec<SleepEntry>,
    /// Currently selected coroutine (purely informational here)
    current: Option<HeapRef>,
    /// Monotonic counter to preserve deterministic order for same wake_tick
    next_seq: u64,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct SleepEntry {
    wake_tick: u64,
    seq: u64,
    coro: HeapRef,
}

impl Scheduler {
    pub fn new() -> Self {
        Self::default()
    }

    // ---------- Ready queue operations ----------

    /// Enqueue a coroutine to the ready FIFO.
    pub fn enqueue_ready(&mut self, coro: HeapRef) {
        self.ready_queue.push_back(coro);
    }

    /// Dequeue next ready coroutine (front of FIFO).
    pub fn dequeue_next(&mut self) -> Option<HeapRef> {
        self.ready_queue.pop_front()
    }

    #[cfg(test)]
    pub fn is_ready_empty(&self) -> bool {
        self.ready_queue.is_empty()
    }
    #[cfg(test)]
    pub fn ready_len(&self) -> usize {
        self.ready_queue.len()
    }

    // ---------- Current tracking (no switching here) ----------
    pub fn set_current(&mut self, coro: Option<HeapRef>) {
        self.current = coro;
    }
    #[cfg(test)]
    pub fn current(&self) -> Option<HeapRef> {
        self.current
    }
    pub fn clear_current(&mut self) {
        self.current = None;
    }

    // ---------- Sleeping operations ----------

    /// Put a coroutine to sleep until `wake_tick`.
    /// A coroutine is woken when `current_tick >= wake_tick`.
    /// The sleeping list is kept stably ordered to guarantee determinism.
    pub fn sleep_until(&mut self, coro: HeapRef, wake_tick: u64) {
        let entry = SleepEntry { wake_tick, seq: self.next_seq, coro };
        self.next_seq = self.next_seq.wrapping_add(1);

        // Binary search insertion point by wake_tick, then by seq to keep total order deterministic
        let idx = match self.sleeping.binary_search_by(|e| {
            if e.wake_tick == entry.wake_tick {
                e.seq.cmp(&entry.seq)
            } else {
                e.wake_tick.cmp(&entry.wake_tick)
            }
        }) {
            Ok(i) => i, // equal element position; insert after to preserve FIFO among equals
            Err(i) => i,
        };
        self.sleeping.insert(idx, entry);
    }

    /// Move all sleeping coroutines with `wake_tick <= current_tick` to ready queue (FIFO by wake order).
    pub fn wake_ready(&mut self, current_tick: u64) {
        // Find split point where wake_tick > current_tick
        let split = self.sleeping.partition_point(|e| e.wake_tick <= current_tick);
        if split == 0 {
            return;
        }
        let mut ready_slice: Vec<SleepEntry> = self.sleeping.drain(0..split).collect();
        // Already in order by (wake_tick, seq); push in that order to preserve determinism
        for e in ready_slice.drain(..) {
            self.ready_queue.push_back(e.coro);
        }
    }

    /// Returns true if there are any sleeping coroutines.
    pub fn has_sleeping(&self) -> bool {
        !self.sleeping.is_empty()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn hr(id: u32) -> HeapRef {
        HeapRef(id)
    }

    #[test]
    fn fifo_ready_queue_is_deterministic() {
        let mut s = Scheduler::new();
        s.enqueue_ready(hr(1));
        s.enqueue_ready(hr(2));
        s.enqueue_ready(hr(3));

        assert_eq!(s.ready_len(), 3);
        assert_eq!(s.dequeue_next(), Some(hr(1)));
        assert_eq!(s.dequeue_next(), Some(hr(2)));
        assert_eq!(s.dequeue_next(), Some(hr(3)));
        assert_eq!(s.dequeue_next(), None);
    }

    #[test]
    fn sleeping_wake_is_stable_and_fifo_by_insertion() {
        let mut s = Scheduler::new();
        s.sleep_until(hr(10), 5); // first with wake at 5
        s.sleep_until(hr(11), 5); // second with same wake
        s.sleep_until(hr(12), 6); // later wake

        // Before tick 5: nothing wakes
        s.wake_ready(4);
        assert!(s.is_ready_empty());

        // At tick 5: first two wake in insertion order
        s.wake_ready(5);
        assert_eq!(s.dequeue_next(), Some(hr(10)));
        assert_eq!(s.dequeue_next(), Some(hr(11)));
        assert!(s.is_ready_empty());

        // At tick 6: last one wakes
        s.wake_ready(6);
        assert_eq!(s.dequeue_next(), Some(hr(12)));
        assert!(s.is_ready_empty());
    }
}