//! Sleep + wait-with-timeout timers.
//!
//! A min-heap of `(deadline, seq, reason)` entries lives on `SchedulerState`.
//! When an actor sleeps or starts a bounded wait (e.g. `mutex.lock()` with a
//! timeout), the runtime inserts an entry, marks the actor parked, and yields.
//! On every scheduler loop iteration the runtime pops all entries whose
//! deadline has passed and dispatches each according to its `Reason`:
//!
//!   - `Sleep`: unpark the actor.
//!   - `WaitTimeout`: call `on_timeout` on the registered target. The target
//!     (e.g. a `Mutex`) decides whether the actor was actually still waiting
//!     (timer fires first → unpark with error) or had already been granted
//!     what it was waiting for (lock granted first → no-op).
//!
//! `BinaryHeap` is a max-heap; entries are wrapped in `Reverse` to get
//! min-heap behaviour.
//!
//! Cancellation is selective. A `Sleep` / `WaitTimeout` entry is left in the
//! heap on a non-timer wakeup (lock granted before timeout): it is popped
//! eventually and no-ops because a stale unpark fails its epoch CAS — cheap
//! (~32 bytes per stale entry plus a few cycles on pop), bounded by one entry
//! per parked actor. A `Send` entry is different: running its thunk delivers a
//! real message, so a stale one is *not* inert. `send_after` therefore carries
//! true cancellation via the `armed` set keyed on the entry's `seq`; `pop_due`
//! fires a `Send` only while it is still armed, and `cancel` removes the arm.
//!
//! Stale pids (slot reused since the timer was inserted) are filtered on
//! pop by the scheduler — same convention as the run queue.

use crate::pid::Pid;
use std::cmp::Reverse;
use std::collections::BinaryHeap;
use std::sync::Arc;
use std::time::{Duration, Instant};

/// What to do when a timer entry's deadline arrives.
///
/// Held inside `Entry`, dispatched by the scheduler in `pop_due`.
pub enum Reason {
    /// `sleep(d)`. Wake `pid` via the epoch-matched unpark: if anything
    /// else (necessarily a terminal wake) already consumed the wait, the
    /// entry is stale and no-ops at the CAS.
    Sleep { epoch: u32 },
    /// A bounded wait (`Mutex::lock_timeout`, `Receiver::recv_timeout`,
    /// `select_timeout`). On expiry the scheduler calls
    /// `target.on_timeout(pid, epoch)`. The target then decides whether
    /// `pid` was actually still waiting (registration still present under
    /// its lock), and if so takes the registration and unparks via
    /// `unpark_at`. The epoch is the slot-word park-epoch — the runtime-wide
    /// wait identity — so a stale entry is doubly inert: the registration
    /// check misses, and even a racing unpark fails the word's epoch CAS.
    WaitTimeout {
        target: Arc<dyn TimerTarget>,
        epoch: u32,
    },
    /// `send_after`: deliver a message to an address at the deadline,
    /// cancellable. The destination (a `Pid<A>` / `Name<M>`) and the message
    /// are captured inside `fire`, which resolves the address through the
    /// registry and sends *when run* — so a target that died or, for a name,
    /// was restarted is observed at fire time, not arm time. A failed resolve
    /// or send is dropped (Erlang `erlang:send_after` semantics).
    ///
    /// Unlike `Sleep` / `WaitTimeout`, a stale `Send` is **not** inert — running
    /// the thunk delivers a real message — so these are the only timers that
    /// carry true cancellation (the `armed` set on [`Timers`], keyed by the
    /// entry's `seq`). `pop_due` fires the thunk only for an entry still armed.
    Send { fire: Box<dyn FnOnce() + Send> },
}

/// Opaque handle to an armed `send_after` timer, returned by
/// [`Timers::insert_send`] and consumed by [`Timers::cancel`]. The inner value
/// is the entry's insertion `seq`; callers must treat it as opaque so the
/// backing structure can change (e.g. a future hierarchical timing wheel) with
/// no API churn.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct TimerId(u64);

impl TimerId {
    /// Wrap a raw value. Crate-internal: the gen_server timer layer mints its
    /// own loop-local `TimerId`s (the public ids it hands out, decoupled from
    /// the per-re-arm substrate `seq`) and maps them to live substrate ids.
    /// These local ids are only ever resolved through that layer's registry —
    /// never passed back to [`Timers::cancel`] — so the two id roles do not mix.
    pub(crate) fn from_raw(v: u64) -> Self {
        TimerId(v)
    }
}

/// Callback the scheduler invokes when a `WaitTimeout` entry pops.
///
/// Implementors: do not touch `SchedulerState` other than via the public
/// `unpark` / channel APIs. The scheduler is mid-iteration when this fires.
pub trait TimerTarget: Send + Sync {
    fn on_timeout(&self, pid: Pid, epoch: u32);
}

pub struct Entry {
    pub deadline: Instant,
    /// Insertion order, used purely as a tiebreaker so `Entry: Ord` works
    /// without having to compare the `Reason` payload (which contains an
    /// `Rc<dyn TimerTarget>` and isn't `Ord`).
    seq: u64,
    pub pid: Pid,
    pub reason: Reason,
}

impl PartialEq for Entry {
    fn eq(&self, other: &Self) -> bool {
        self.deadline == other.deadline && self.seq == other.seq
    }
}
impl Eq for Entry {}

impl Ord for Entry {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        // Earlier deadline first; ties broken by insertion order so the
        // ordering is total. `Reason` and `Pid` deliberately don't
        // participate.
        self.deadline.cmp(&other.deadline).then_with(|| self.seq.cmp(&other.seq))
    }
}

impl PartialOrd for Entry {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

#[derive(Default)]
pub struct Timers {
    /// Reverse-wrapped so the smallest deadline is at the top.
    heap: BinaryHeap<Reverse<Entry>>,
    /// Monotonic counter for the tiebreaker `seq` field (and the `TimerId` of a
    /// `Send` timer — the two are the same value).
    next_seq: u64,
    /// Presence set of *live* `Send` timers, keyed by `seq`. Populated on
    /// `insert_send`, removed on fire (in `pop_due`) and on `cancel`. A `Send`
    /// entry fires only while present, so a `cancel` that lands before the
    /// entry pops prevents delivery; a `cancel` after it has fired finds
    /// nothing (the race signal). Bounded by armed-but-not-yet-resolved timers
    /// and self-collecting — no sweep. `Sleep` / `WaitTimeout` never touch it.
    armed: std::collections::HashSet<u64>,
}

impl Timers {
    pub fn new() -> Self {
        Self { heap: BinaryHeap::new(), next_seq: 0, armed: std::collections::HashSet::new() }
    }

    /// Insert a `Sleep` timer. Convenience for the common case.
    pub fn insert_sleep(&mut self, deadline: Instant, pid: Pid, epoch: u32) {
        self.insert(deadline, pid, Reason::Sleep { epoch });
    }

    /// Arm a cancellable `send_after` timer: run `fire` at `deadline` unless
    /// [`cancel`](Self::cancel)led first. `pid` is informational only (the
    /// destination, or who armed it — useful for introspection); it is *not*
    /// used to wake anyone, the delivery lives entirely inside `fire`. Returns
    /// a [`TimerId`] for cancellation.
    pub fn insert_send(
        &mut self,
        deadline: Instant,
        pid: Pid,
        fire: Box<dyn FnOnce() + Send>,
    ) -> TimerId {
        let seq = self.next_seq;
        self.next_seq = self.next_seq.wrapping_add(1);
        self.armed.insert(seq);
        self.heap.push(Reverse(Entry { deadline, seq, pid, reason: Reason::Send { fire } }));
        TimerId(seq)
    }

    /// Cancel an armed `send_after` timer. Returns `true` if the timer was
    /// still armed (delivery is now prevented), `false` if it had already
    /// fired or been cancelled. The heap entry, if still pending, is left to be
    /// discarded when its deadline passes — `pop_due` drops any `Send` entry
    /// whose `seq` is no longer armed.
    pub fn cancel(&mut self, id: TimerId) -> bool {
        self.armed.remove(&id.0)
    }

    /// Insert an arbitrary timer entry.
    pub fn insert(&mut self, deadline: Instant, pid: Pid, reason: Reason) {
        let seq = self.next_seq;
        self.next_seq = self.next_seq.wrapping_add(1);
        self.heap.push(Reverse(Entry { deadline, seq, pid, reason }));
    }

    pub fn is_empty(&self) -> bool {
        self.heap.is_empty()
    }

    /// Drop all pending entries. Called by the scheduler when it has decided
    /// no actor is live: any remaining timer is orphaned and exists only to be
    /// discarded so it can't keep the runtime alive.
    pub fn clear(&mut self) {
        self.heap.clear();
        self.armed.clear();
    }

    /// Soonest pending deadline, or `None` if the heap is empty.
    pub fn peek_deadline(&self) -> Option<Instant> {
        self.heap.peek().map(|r| r.0.deadline)
    }

    /// Pop every entry whose deadline is ≤ `now`, in deadline order.
    /// The scheduler dispatches each entry by inspecting `entry.reason`.
    ///
    /// A due `Send` entry is returned only if it is still armed; a cancelled
    /// one is silently dropped here (its `seq` was already removed from
    /// `armed` by [`cancel`](Self::cancel)). Returning it removes it from
    /// `armed`, so a later `cancel` of a fired timer reports `false`.
    pub fn pop_due(&mut self, now: Instant) -> Vec<Entry> {
        let mut out = Vec::new();
        while let Some(r) = self.heap.peek() {
            if r.0.deadline <= now {
                let entry = self.heap.pop().unwrap().0;
                if matches!(entry.reason, Reason::Send { .. }) && !self.armed.remove(&entry.seq) {
                    // Cancelled before it came due: discard, do not deliver.
                    continue;
                }
                out.push(entry);
            } else {
                break;
            }
        }
        out
    }
}

/// Wall-clock duration helper exposed for `sleep` and `lock_timeout`.
pub fn deadline_from_now(duration: Duration) -> Instant {
    Instant::now()
        .checked_add(duration)
        .unwrap_or_else(Instant::now)
}