//! 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,
    /// RFC 007 virtual time: the global delay ledger reading when this entry
    /// was (re-)queued. `pop_due` shifts the effective deadline by any delay
    /// injected since, so timers dilate together with the causally-delayed
    /// workload instead of firing early in virtual terms.
    #[cfg(feature = "smarm-causal")]
    delay_stamp: u64,
    /// RFC 007: a wall-anchored entry opts out of the virtual-time shift —
    /// its deadline is honoured in wall time regardless of injected delay.
    /// Used by the causal controller's own measurement/cooldown sleeps so
    /// experiment windows keep a fixed wall length; ordinary workload timers
    /// stay virtual (`false`).
    #[cfg(feature = "smarm-causal")]
    wall: bool,
}

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 {
    /// RFC 018: the scheduler coordination layer. Attached once at
    /// `RuntimeInner::new`; every insert notes its deadline (min-maintained
    /// snapshot for the busy-path due-check + the timekeeper re-arm wake)
    /// and every pop/clear re-anchors the snapshot to the heap minimum.
    /// All calls happen under the timers mutex — the serialization the
    /// coordinator's timer protocol mandates. `None` only in unit tests
    /// that construct a bare `Timers`.
    coord: Option<std::sync::Arc<crate::park::Coordinator>>,
    /// 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 {
            coord: None,
            heap: BinaryHeap::new(),
            next_seq: 0,
            armed: std::collections::HashSet::new(),
        }
    }

    /// Attach the scheduler coordination layer (RFC 018). Called once, at
    /// runtime construction, before any scheduler thread exists.
    pub(crate) fn attach_coordinator(&mut self, c: std::sync::Arc<crate::park::Coordinator>) {
        self.coord = Some(c);
    }

    /// 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 });
    }

    /// Insert a *wall-anchored* `Sleep` timer: fires at `deadline` in wall
    /// time even while causal profiling (feature `smarm-causal`) is injecting
    /// virtual delay — it never chases the delay ledger. Without the feature
    /// this is identical to [`insert_sleep`](Self::insert_sleep).
    ///
    /// Intended for measurement machinery (the causal controller's window and
    /// cooldown sleeps, TSC calibration) whose durations *define* wall time
    /// rather than participate in the workload. Workload code should use the
    /// ordinary virtual-anchored timers.
    pub fn insert_sleep_wall(&mut self, deadline: Instant, pid: Pid, epoch: u32) {
        self.push(deadline, pid, Reason::Sleep { epoch }, true);
    }

    /// 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 {
        self.armed.insert(self.next_seq);
        TimerId(self.push(deadline, pid, Reason::Send { fire }, false))
    }

    /// Arm a *wall-anchored* cancellable `send_after` timer (RFC 007): the
    /// same contract as [`insert_send`](Self::insert_send), but the entry
    /// opts out of the virtual-time shift and fires at its raw deadline
    /// regardless of injected delay — the `Send`-reason sibling of
    /// [`insert_sleep_wall`](Self::insert_sleep_wall). Without the
    /// `smarm-causal` feature this is identical to `insert_send`.
    pub fn insert_send_wall(
        &mut self,
        deadline: Instant,
        pid: Pid,
        fire: Box<dyn FnOnce() + Send>,
    ) -> TimerId {
        self.armed.insert(self.next_seq);
        TimerId(self.push(deadline, pid, Reason::Send { fire }, true))
    }

    /// 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 (virtual-anchored) timer entry.
    pub fn insert(&mut self, deadline: Instant, pid: Pid, reason: Reason) {
        self.push(deadline, pid, reason, false);
    }

    /// Common insertion path. `wall` selects the RFC 007 anchor (see
    /// [`insert_sleep_wall`](Self::insert_sleep_wall)); it is accepted — and
    /// ignored — without the `smarm-causal` feature so callers don't fork.
    /// Returns the entry's `seq`.
    fn push(&mut self, deadline: Instant, pid: Pid, reason: Reason, wall: bool) -> u64 {
        #[cfg(not(feature = "smarm-causal"))]
        let _ = wall;
        let seq = self.next_seq;
        self.next_seq = self.next_seq.wrapping_add(1);
        self.heap.push(Reverse(Entry {
            deadline,
            seq,
            pid,
            reason,
            #[cfg(feature = "smarm-causal")]
            delay_stamp: crate::causal::global_delay_cycles(),
            #[cfg(feature = "smarm-causal")]
            wall,
        }));
        // RFC 018: publish the (possibly new-minimum) deadline to the
        // busy-path snapshot and wake the timekeeper if it is parked
        // toward a later one. We hold the timers mutex — the mandated
        // serialization for both.
        if let Some(c) = &self.coord {
            c.note_deadline(deadline);
        }
        seq
    }

    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();
        if let Some(c) = &self.coord {
            c.refresh_deadline(None);
        }
    }

    /// 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`.
    ///
    /// RFC 007 virtual time (feature `smarm-causal`): before an entry fires,
    /// any global delay injected since it was (re-)queued is added to its
    /// deadline; an entry whose *effective* deadline hasn't passed is pushed
    /// back with the shifted deadline and a fresh stamp, so it keeps chasing
    /// delay injected while it waits. Consequences, both benign:
    /// [`peek_deadline`](Self::peek_deadline) may under-report (raw deadline
    /// earlier than effective), costing at most one spurious scheduler wake
    /// per injected chunk; and a shift never converts wall time — with zero
    /// debt the path is byte-identical to the featureless one. Wall-anchored
    /// entries ([`insert_sleep_wall`](Self::insert_sleep_wall)) are exempt
    /// from the shift and always fire at their raw deadline.
    pub fn pop_due(&mut self, now: Instant) -> Vec<Entry> {
        let mut out = Vec::new();
        #[cfg(feature = "smarm-causal")]
        let global = crate::causal::global_delay_cycles();
        while let Some(r) = self.heap.peek() {
            if r.0.deadline > now {
                break;
            }
            #[allow(unused_mut)]
            let mut entry = match self.heap.pop() {
                Some(e) => e.0,
                None => panic!("smarm: timer heap pop after peek returned None (core corrupt)"),
            };
            if matches!(entry.reason, Reason::Send { .. }) && !self.armed.contains(&entry.seq) {
                // Cancelled before it came due: discard, do not deliver.
                // (Checked before any shift so a cancelled entry is never
                // re-queued just to be discarded later.)
                continue;
            }
            #[cfg(feature = "smarm-causal")]
            if !entry.wall {
                let debt = global.saturating_sub(entry.delay_stamp);
                if debt > 0 {
                    let shifted = entry
                        .deadline
                        .checked_add(crate::causal::cycles_to_duration(debt))
                        .unwrap_or(entry.deadline);
                    if shifted > now {
                        // Not due in virtual time: re-queue at the shifted
                        // deadline, stamped, keeping `seq` (and thus `Send`
                        // cancellation identity) intact.
                        entry.deadline = shifted;
                        entry.delay_stamp = global;
                        self.heap.push(Reverse(entry));
                        continue;
                    }
                }
            }
            if matches!(entry.reason, Reason::Send { .. }) {
                self.armed.remove(&entry.seq);
            }
            out.push(entry);
        }
        // RFC 018: re-anchor the busy-path snapshot to the new heap minimum
        // (still under the timers mutex). A causal-shift re-queue above went
        // through `heap.push` directly, so this peek is the one place the
        // snapshot is guaranteed to catch up.
        if let Some(c) = &self.coord {
            c.refresh_deadline(self.peek_deadline());
        }
        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)
}