//! A stateful actor with typed request-reply messaging.
//!
//! ## What is a gen_server?
//!
//! Whenever you need mutable state shared between threads, the obvious tool is
//! `Arc<Mutex<State>>`. That works, but it scatters lock guards and error
//! handling everywhere, and offers no natural place to put the logic that
//! operates on the state.
//!
//! A gen_server is a cleaner alternative: a dedicated thread that *owns* the
//! state and handles one message at a time. Other threads talk to it by sending
//! typed messages and optionally waiting for a typed reply. Because messages are
//! serialised through a single inbox. The server is the only thing that ever
//! touches the state, so there are no lock guards at all.
//!
//! You write the logic by implementing [`GenServer`]. smarm runs the receive
//! loop, the reply plumbing, and the lifecycle.
//!
//! ## A first server
//!
//! Let's build a counter that can be incremented from any thread and queried for
//! its current value.
//!
//! ```ignore
//! use smarm::gen_server::{self, GenServer, GenServerRef};
//!
//! // State
//!
//! struct Counter {
//!     count: u64,
//! }
//!
//! // Message types
//!
//! // Calls expect a reply; casts do not.
//! enum CounterCall { Get }
//! enum CounterCast { Increment }
//!
//! // Behaviour
//!
//! impl GenServer for Counter {
//!     type Call  = CounterCall;
//!     type Reply = u64;
//!     type Cast  = CounterCast;
//!     type Info  = ();   // no out-of-band messages
//!     type Timer = ();   // no timers
//!
//!     fn handle_call(&mut self, request: CounterCall) -> u64 {
//!         match request {
//!             CounterCall::Get => self.count,
//!         }
//!     }
//!
//!     fn handle_cast(&mut self, request: CounterCast) {
//!         match request {
//!             CounterCast::Increment => self.count += 1,
//!         }
//!     }
//! }
//! ```
//!
//! Start the server and talk to it:
//!
//! ```ignore
//! let server: GenServerRef<Counter> = gen_server::start(Counter { count: 0 });
//!
//! server.cast(CounterCast::Increment).unwrap();
//! server.cast(CounterCast::Increment).unwrap();
//!
//! let n = server.call(CounterCall::Get).unwrap();
//! // n == 2
//! ```
//!
//! [`GenServerRef`] is cheaply clonable; hand copies to as many threads as you
//! like. They all share the same inbox; the server handles messages one at a
//! time, in arrival order.
//!
//! ## Client / server APIs
//!
//! In practice, callers should not have to know about `CounterCall` or
//! `CounterCast`. The recommended pattern is to wrap the message types in plain
//! functions on the same module:
//!
//! ```ignore
//! // Client API
//!
//! impl Counter {
//!     pub fn start() -> GenServerRef<Counter> {
//!         gen_server::start(Counter { count: 0 })
//!     }
//!
//!     pub fn increment(server: &GenServerRef<Counter>) {
//!         server.cast(CounterCast::Increment).unwrap();
//!     }
//!
//!     pub fn get(server: &GenServerRef<Counter>) -> u64 {
//!         server.call(CounterCall::Get).unwrap()
//!     }
//! }
//!
//! // Callers now just do:
//! let counter = Counter::start();
//! Counter::increment(&counter);
//! Counter::increment(&counter);
//! let n = Counter::get(&counter);   // n == 2
//! ```
//!
//! ## call vs cast
//!
//! `call` sends a request and blocks the calling thread until the server
//! replies. Use it when you need a return value, or when you need to know that
//! the server has processed the message before continuing.
//!
//! `cast` enqueues a message and returns immediately, without waiting for the
//! server to handle it. Use it for fire-and-forget updates where you don't need
//! confirmation.
//!
//! Both return `Err(ServerDown)` if the inbox is closed (the server is gone).
//! If you need to cap how long you wait for a reply, use
//! [`GenServerRef::call_timeout`], which additionally returns `Err(Timeout)` if
//! the deadline passes before a reply arrives.
//!
//! ## Lifecycle
//!
//! Two optional callbacks bracket the server's life:
//!
//! - [`GenServer::init`] runs once before the first message. Use it to start
//!   timers or set up monitors; see the [`GenServerCtx`] it receives.
//! - [`GenServer::terminate`] runs when the server is about to exit. It fires
//!   on every exit path (all `GenServerRef`s dropped, a handler panic, or an
//!   explicit [`GenServerRef::shutdown`]), not only on clean shutdown. Keep it
//!   short and non-blocking: if `terminate` panics while the server is already
//!   unwinding from a handler panic, the process aborts.
//!
//! ## When the server stops
//!
//! The server runs as long as at least one [`GenServerRef`] exists. When the last
//! one is dropped, the inbox closes and the loop exits gracefully. To stop a
//! server explicitly and wait for it to finish, call [`GenServerRef::shutdown`].
//!
//! If the server panics inside a handler, the panic unwinds the server thread.
//! Any caller currently waiting in `call` sees `Err(ServerDown)`: the reply
//! channel closes as the server unwinds, which wakes the caller.
//!
//! ## Going further
//!
//! Out-of-band messages: a server can receive messages from sources other
//! than `call`/`cast`, for example notifications from a background task. Pass
//! extra [`Receiver`] channels to [`GenServerBuilder::with_info`]; they are
//! dispatched to [`GenServer::handle_info`], always before inbox messages.
//!
//! Monitors: to watch another actor and be notified when it exits, clone a
//! [`Watcher`] from [`GenServerCtx::watcher`] during `init` and call
//! [`Watcher::watch`] with a [`Monitor`] from any handler. The loop dispatches
//! the resulting [`Down`] to [`GenServer::handle_down`].
//!
//! Timers: clone a [`TimerHandle`] from [`GenServerCtx::timer`] during `init`.
//! Use [`TimerHandle::arm_after`] for a one-shot and [`TimerHandle::tick_every`]
//! for a periodic; both fire into [`GenServer::handle_timer`].
//!
//! Idle detection: call [`GenServerCtx::idle_after`] during `init` to set a
//! quiet window: if no message of any kind is dispatched for that duration,
//! the loop calls [`GenServer::handle_idle`] and resets the window. This differs
//! from the client-side `call_timeout`. Idle measures silence on the *server*,
//! while `call_timeout` caps how long *one caller* waits.
//!
//! Names and registration: a server can be given a static name so other
//! actors can reach it without holding a `GenServerRef`. Use
//! [`GenServerBuilder::named`] to register on start, and the free functions
//! [`call`], [`cast`], and [`whereis_server`] to address it by name. Registered
//! servers are a natural fit for supervision; see `supervisor` for how to
//! build a tree that restarts servers on failure.
//!
//! ## Limitations
//!
//! The info-channel set is fixed at start; channels cannot be added or removed
//! while the server is running. Monitors are dynamic (they can be registered
//! from any handler via [`Watcher::watch`]) because monitors are inherently
//! created at runtime. The idle window is set once, in `init`.

use crate::channel::{channel, select, select_timeout, Receiver, RecvTimeoutError, Selectable, Sender};
use crate::monitor::{demonitor, monitor, Down, Monitor};
use crate::pid::Pid;
use crate::registry::{register_with, resolve_named_sender, RegisterError};
use crate::scheduler::{cancel_timer, request_stop, send_after_to, spawn, spawn_under};
use crate::timer::TimerId;
use std::cell::Cell;
use std::collections::HashMap;
use std::marker::PhantomData;
use std::sync::{Arc, Mutex};
use std::time::{Duration, Instant};

/// The behaviour you implement to make a type into a gen_server.
///
/// Implement this on your state struct. `handle_call` and `handle_cast` are
/// required; all other methods have no-op defaults and can be added as needed.
pub trait GenServer: Send + 'static {
    /// The request type sent by [`GenServerRef::call`]. Must produce a [`Reply`](Self::Reply).
    type Call: Send + 'static;
    /// The value returned to the caller by [`handle_call`](Self::handle_call).
    type Reply: Send + 'static;
    /// The request type sent by [`GenServerRef::cast`]. No reply is produced.
    type Cast: Send + 'static;
    /// Out-of-band message type delivered to [`handle_info`](Self::handle_info)
    /// from channels registered via [`GenServerBuilder::with_info`]. If you don't
    /// use info channels, set this to `()`.
    type Info: Send + 'static;
    /// Payload type for timers armed via [`TimerHandle`], delivered to
    /// [`handle_timer`](Self::handle_timer). Kept separate from [`Info`](Self::Info)
    /// so that timer fires (which your server schedules itself) stay distinct
    /// from external messages (which arrive from outside). Use `()` if unused.
    type Timer: Send + 'static;

    /// Runs once inside the server actor before any message is handled. The
    /// [`GenServerCtx`] is the loop's one runtime hook: clone whichever of
    /// [`ctx.watcher()`](GenServerCtx::watcher) (monitors) and
    /// [`ctx.timer()`](GenServerCtx::timer) (timers) you will need from later
    /// handlers into the state here. If you need neither, the unused ctx drops
    /// and the loop's system arm auto-closes (see module docs).
    fn init(&mut self, _ctx: &GenServerCtx<Self>)
    where
        Self: Sized,
    {
    }

    /// Handle a synchronous call and produce the reply sent back to the caller.
    fn handle_call(&mut self, request: Self::Call) -> Self::Reply;

    /// Handle a fire-and-forget cast.
    fn handle_cast(&mut self, request: Self::Cast);

    /// Handle an out-of-band message from one of the info channels. Default:
    /// drop it.
    fn handle_info(&mut self, _info: Self::Info) {}

    /// Handle a [`Down`] from a monitor handed to the loop via
    /// [`Watcher::watch`]. Default: drop it.
    fn handle_down(&mut self, _down: Down) {}

    /// Handle a fired timer armed through [`TimerHandle`]
    /// ([`arm_after`](TimerHandle::arm_after) /
    /// [`tick_every`](TimerHandle::tick_every)). Timer fires are delivered
    /// before info and inbox messages, so a heartbeat cannot be starved by
    /// application traffic. Default: drop the message.
    fn handle_timer(&mut self, _msg: Self::Timer) {}

    /// Handle a quiet-period notification: called when the server has gone the
    /// full idle window (set via [`GenServerCtx::idle_after`] in `init`) without
    /// dispatching any message. The window resets automatically after this
    /// fires, so it acts as a steady idle detector. To shut down after one idle
    /// period, call [`request_stop`](crate::scheduler::request_stop) here.
    /// Default: no-op.
    fn handle_idle(&mut self) {}

    /// Runs as the server actor exits, on any exit path (see module docs).
    fn terminate(&mut self) {}
}

/// What travels the server's single inbox channel: a synchronous call (with a
/// reply sender) or an asynchronous cast. Private — callers use [`GenServerRef`].
enum Envelope<G: GenServer> {
    Call(G::Call, Sender<G::Reply>),
    Cast(G::Cast),
}

/// A clonable handle to a running server. Cloning yields another sender to the
/// same inbox; the server lives until the last `GenServerRef` is dropped, at which
/// point its inbox closes and the loop exits normally.
pub struct GenServerRef<G: GenServer> {
    tx: Sender<Envelope<G>>,
    pid: Pid,
}

impl<G: GenServer> Clone for GenServerRef<G> {
    fn clone(&self) -> Self {
        GenServerRef { tx: self.tx.clone(), pid: self.pid }
    }
}

/// Returned by [`GenServerRef::call`] when the server is no longer reachable.
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub enum CallError {
    /// The server was already gone, or died before replying.
    ServerDown,
}

/// Returned by [`GenServerRef::call_timeout`].
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub enum CallTimeoutError {
    /// The server was already gone, or died before replying.
    ServerDown,
    /// The deadline passed before a reply arrived. The request is still in the
    /// server's inbox and will be handled — the reply is simply discarded
    /// because the reply channel was dropped when the timeout fired. Design
    /// calls that may time out to be idempotent, so a late reply causes no harm.
    Timeout,
}

/// Returned by [`GenServerRef::cast`] when the server is no longer reachable.
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub enum CastError {
    /// The server inbox was closed (the server is gone).
    ServerDown,
}

impl<G: GenServer> GenServerRef<G> {
    /// The server actor's pid — usable with `monitor`, `request_stop`, `link`.
    pub fn pid(&self) -> Pid {
        self.pid
    }

    /// Synchronous request-reply. Blocks (parking the calling actor) until the
    /// server replies, or returns [`CallError::ServerDown`] if the server is or
    /// becomes unreachable before a reply arrives.
    pub fn call(&self, request: G::Call) -> Result<G::Reply, CallError> {
        let (reply_tx, reply_rx) = channel::<G::Reply>();
        self.tx
            .send(Envelope::Call(request, reply_tx))
            .map_err(|_| CallError::ServerDown)?;
        reply_rx.recv().map_err(|_| CallError::ServerDown)
    }

    /// Like [`call`](Self::call), but gives up after `timeout` and returns
    /// [`CallTimeoutError::Timeout`] if no reply arrives in time.
    ///
    /// Note that "timeout" means two different things here and in
    /// [`GenServerCtx::idle_after`]: this one is a *client-side call deadline* —
    /// how long this particular caller waits. The idle timeout measures silence
    /// on the *server's* inbox. They are completely independent.
    ///
    /// On timeout, the server still handles the request; only the reply is
    /// discarded (the reply channel is dropped, so the server's send fails
    /// silently). Design timed-out calls to be idempotent.
    pub fn call_timeout(
        &self,
        request: G::Call,
        timeout: std::time::Duration,
    ) -> Result<G::Reply, CallTimeoutError> {
        let (reply_tx, reply_rx) = channel::<G::Reply>();
        self.tx
            .send(Envelope::Call(request, reply_tx))
            .map_err(|_| CallTimeoutError::ServerDown)?;
        reply_rx.recv_timeout(timeout).map_err(|e| match e {
            crate::channel::RecvTimeoutError::Disconnected => CallTimeoutError::ServerDown,
            crate::channel::RecvTimeoutError::Timeout => CallTimeoutError::Timeout,
        })
    }

    /// Fire-and-forget request. Returns once the message is enqueued; does not
    /// wait for the server to handle it. [`CastError::ServerDown`] if the inbox
    /// is already closed.
    pub fn cast(&self, request: G::Cast) -> Result<(), CastError> {
        self.tx
            .send(Envelope::Cast(request))
            .map_err(|_| CastError::ServerDown)
    }

    /// Stop the server and block until it has fully exited.
    ///
    /// Sends a cooperative stop signal to the server actor and waits for it to
    /// exit, so [`GenServer::terminate`] has run by the time this returns.
    /// Returns immediately if the server is already gone.
    ///
    /// This is the right teardown for a server kept alive by a registered
    /// [`GenServerName`], where dropping every external `GenServerRef` is not enough
    /// to close the inbox. Like all cooperative cancellation, it is best-effort:
    /// a server wedged in a tight loop with no observation point cannot be
    /// stopped this way. Panics if called outside `Runtime::run()`.
    pub fn shutdown(&self) {
        let mon = monitor(self.pid);
        request_stop(self.pid);
        // The Down lands when the server finalizes; an already-dead target makes
        // `monitor` deliver NoProc immediately, so this never blocks forever.
        let _ = mon.rx.recv();
        demonitor(&mon);
    }
}

/// Internal channel carrying control messages to the server loop. Monitors
/// and timer fires both need to land above the inbox in priority, so they
/// share one channel dispatched by variant. The arm stays open while any
/// [`Watcher`] or [`TimerHandle`] clone (or an in-flight timer fire) holds a sender.
enum Sys<G: GenServer> {
    /// A monitor handed in via [`Watcher::watch`]; pushed onto the loop's
    /// monitor set.
    Watch(Monitor),
    /// A fired one-shot timer: loop-local id (so the loop can retire the
    /// registry entry) plus the server's payload.
    Timer(crate::timer::TimerId, G::Timer),
    /// A fired periodic tick carrying its stable local id. The loop looks up
    /// the payload factory, dispatches it to [`GenServer::handle_timer`], and
    /// re-arms the next tick before returning.
    Tick(crate::timer::TimerId),
}

/// The server loop's runtime hook, passed to [`GenServer::init`]. Hands out the
/// loop's two clonable intake handles — the [`Watcher`] (monitors) and the
/// [`TimerHandle`] (timers) — plus the one-shot idle-window setter. All fields
/// are private so the struct can gain new hooks in future without breaking
/// existing implementations.
pub struct GenServerCtx<G: GenServer> {
    sys_tx: Sender<Sys<G>>,
    reg: Arc<Mutex<TimerReg<G>>>,
    /// The idle/receive-timeout window, set once via [`idle_after`](Self::idle_after)
    /// during `init` and read by the loop after `init` returns. Interior-mutable
    /// because `init` holds only `&ctx`; not `Send`, but `GenServerCtx` is only ever
    /// borrowed on the actor's own stack during `init`, never sent.
    idle: Cell<Option<Duration>>,
}

impl<G: GenServer> GenServerCtx<G> {
    /// A clonable handle to the loop's monitor intake. Store it in the state
    /// during `init` to watch monitors from later handlers.
    pub fn watcher(&self) -> Watcher<G> {
        Watcher { tx: self.sys_tx.clone() }
    }

    /// Shorthand for `ctx.watcher().watch(m)` when watching during `init`.
    pub fn watch(&self, m: Monitor) {
        self.watcher().watch(m)
    }

    /// A clonable handle to the loop's timer intake (the [`Watcher`] pattern,
    /// for time). Store it on the state during `init` to
    /// [`arm_after`](TimerHandle::arm_after) /
    /// [`tick_every`](TimerHandle::tick_every) /
    /// [`cancel`](TimerHandle::cancel) from any later handler.
    pub fn timer(&self) -> TimerHandle<G> {
        TimerHandle { sys_tx: self.sys_tx.clone(), reg: self.reg.clone() }
    }

    /// Set a quiet-period window: if the loop goes `after` without dispatching
    /// any message, it calls [`GenServer::handle_idle`] and resets the window.
    /// Call this once during `init`; the window is fixed for the server's
    /// lifetime.
    ///
    /// This is distinct from [`GenServerRef::call_timeout`], which caps how long a
    /// single caller waits for a reply. This one measures silence on the whole
    /// inbox.
    pub fn idle_after(&self, after: Duration) {
        self.idle.set(Some(after));
    }
}

/// Per-server timer bookkeeping, shared between the loop and every
/// [`TimerHandle`] clone. A gen_server actor is single-threaded — handlers
/// and the loop never run concurrently — so this `Mutex` is always
/// uncontended at runtime; `Arc<Mutex>` (rather than `Rc<RefCell>`) is used
/// only because `GenServer: Send` forces the handle (hence this shared state)
/// to be `Send`.
struct TimerReg<G: GenServer> {
    /// Monotonic minter for loop-local [`TimerId`]s — the ids handed to users,
    /// kept distinct from the substrate `seq` (which changes on every periodic
    /// re-arm). A local id is resolved only through this registry, never passed
    /// to [`cancel_timer`], so the two id roles never mix.
    next_local: u64,
    /// Live one-shot timers: local id → current substrate id. Present from
    /// [`arm_after`](TimerHandle::arm_after) until the loop retires it on fire
    /// or [`cancel`](TimerHandle::cancel) removes it.
    oneshots: HashMap<TimerId, crate::timer::TimerId>,
    /// Live periodic timers: stable local id → its bookkeeping. The stable id
    /// is minted once by [`tick_every`](TimerHandle::tick_every) and survives
    /// every re-arm; the entry lives until [`cancel`](TimerHandle::cancel)
    /// removes it (steady re-arm never self-removes).
    periodics: HashMap<TimerId, Periodic<G>>,
    /// A `Sys` sender held *only while ≥1 periodic is live*, used by the loop to
    /// re-arm the next tick. `Some` exactly when `periodics` is non-empty: this
    /// keeps the system arm open for a server whose only system use is a
    /// periodic, while leaving a non-timer server's arm to auto-close (the
    /// `unused_ctx` behaviour) since this stays `None` for it.
    rearm_tx: Option<Sender<Sys<G>>>,
}

/// One periodic timer's loop-side bookkeeping.
struct Periodic<G: GenServer> {
    /// Re-arm interval; each fire schedules the next at `now + every`.
    every: Duration,
    /// The currently-armed underlying timer for this periodic (changes on every
    /// re-arm); cancelled by [`cancel`](TimerHandle::cancel).
    live: crate::timer::TimerId,
    /// Produces a fresh payload for each tick. Built in `tick_every` as
    /// `move || msg.clone()` — so the `Clone` bound lives on that one method
    /// and never appears on `type Timer` or anywhere else in the loop.
    make: Box<dyn FnMut() -> G::Timer + Send>,
}

// Manual Default: deriving would demand `G: Default`, but a fresh registry is
// independent of the server type.
impl<G: GenServer> Default for TimerReg<G> {
    fn default() -> Self {
        TimerReg {
            next_local: 0,
            oneshots: HashMap::new(),
            periodics: HashMap::new(),
            rearm_tx: None,
        }
    }
}

impl<G: GenServer> TimerReg<G> {
    /// Mint the next loop-local id.
    fn mint(&mut self) -> TimerId {
        let id = TimerId::from_raw(self.next_local);
        self.next_local = self.next_local.wrapping_add(1);
        id
    }
}

/// Arms and cancels timers for a server loop, the time-side twin of [`Watcher`].
/// Handed out by [`GenServerCtx::timer`] in `init`; store it on the state so
/// handlers can arm timers without changing their signatures. Clonable; the
/// loop's timer arm stays open while any clone (or an in-flight fire) lives.
pub struct TimerHandle<G: GenServer> {
    sys_tx: Sender<Sys<G>>,
    reg: Arc<Mutex<TimerReg<G>>>,
}

// Manual Clone for the same reason as `Watcher`: no `G: Clone` needed.
impl<G: GenServer> Clone for TimerHandle<G> {
    fn clone(&self) -> Self {
        TimerHandle { sys_tx: self.sys_tx.clone(), reg: self.reg.clone() }
    }
}

impl<G: GenServer> TimerHandle<G> {
    /// Arm a one-shot timer: deliver `msg` to [`GenServer::handle_timer`] once,
    /// after `after`, unless [`cancel`](Self::cancel)led first. Returns a
    /// [`TimerId`] you can pass to `cancel`. Timer fires arrive before info and
    /// inbox messages.
    pub fn arm_after(&self, after: Duration, msg: G::Timer) -> TimerId {
        let mut reg = match self.reg.lock() {
            Ok(g) => g,
            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
        };
        let local = reg.mint();
        // The fire thunk carries the local id so the loop can retire the entry
        // on dispatch; `msg` is moved in (no `Clone` needed for one-shots).
        let sub = send_after_to(after, self.sys_tx.clone(), Sys::Timer(local, msg));
        reg.oneshots.insert(local, sub);
        local
    }

    /// Arm a periodic timer: deliver a fresh `msg` to
    /// [`GenServer::handle_timer`] every `every`, until
    /// [`cancel`](Self::cancel)led. The underlying timer is one-shot; the loop
    /// re-arms it after each fire to produce the repeating cadence, and exposes
    /// a single stable [`TimerId`] so one `cancel` stops both the pending
    /// instance and all future ones.
    ///
    /// Requires `G::Timer: Clone` because the same logical message is cloned
    /// fresh for each tick. This bound is on this method only; one-shot
    /// [`arm_after`](Self::arm_after) and the `type Timer` declaration are
    /// unconstrained.
    pub fn tick_every(&self, every: Duration, msg: G::Timer) -> TimerId
    where
        G::Timer: Clone,
    {
        let mut reg = match self.reg.lock() {
            Ok(g) => g,
            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
        };
        let local = reg.mint();
        // A live periodic must keep the system arm open so the next tick can be
        // re-armed; the first periodic installs the loop's re-arm sender.
        if reg.periodics.is_empty() {
            reg.rearm_tx = Some(self.sys_tx.clone());
        }
        let make: Box<dyn FnMut() -> G::Timer + Send> = Box::new(move || msg.clone());
        // First instance fires after `every`; the payload is produced loop-side
        // from `make` on fire, so the tick carries only the stable id.
        let sub = send_after_to(every, self.sys_tx.clone(), Sys::Tick(local));
        reg.periodics.insert(local, Periodic { every, live: sub, make });
        debug_assert!(reg.rearm_tx.is_some(), "rearm_tx must be Some while periodics is non-empty");
        local
    }

    /// Cancel an armed timer (one-shot or periodic). Returns the substrate's
    /// race signal — `true` if the cancel beat the fire, `false` if the timer
    /// had already fired, been cancelled, or is otherwise unknown. Most callers
    /// can ignore the return value: a fired-but-not-yet-dispatched one-shot is
    /// discarded by the loop on delivery (see the `Tick` arm in `server_loop`),
    /// and a cancelled periodic will not re-arm. For a periodic, `cancel` also
    /// stops re-arming: the entry is removed so the loop will not schedule
    /// another tick even if the pending one already escaped onto the channel.
    pub fn cancel(&self, id: TimerId) -> bool {
        let mut reg = match self.reg.lock() {
            Ok(g) => g,
            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
        };
        if let Some(sub) = reg.oneshots.remove(&id) {
            return cancel_timer(sub);
        }
        if let Some(p) = reg.periodics.remove(&id) {
            let beat = cancel_timer(p.live);
            if reg.periodics.is_empty() {
                reg.rearm_tx = None;
            }
            debug_assert!(
                reg.rearm_tx.is_some() != reg.periodics.is_empty(),
                "rearm_tx must be Some iff periodics is non-empty"
            );
            return beat;
        }
        false
    }
}

/// Hands [`Monitor`]s to a server loop; their [`Down`]s are dispatched to
/// [`GenServer::handle_down`]. Down arms outrank every other arm (a death
/// notice cannot be starved), and a delivered `Down` retires its arm —
/// monitors are one-shot.
pub struct Watcher<G: GenServer> {
    tx: Sender<Sys<G>>,
}

// Manual Clone: deriving would demand `G: Clone`, but the handle is clonable
// regardless of the server type (it clones only the inner sender).
impl<G: GenServer> Clone for Watcher<G> {
    fn clone(&self) -> Self {
        Watcher { tx: self.tx.clone() }
    }
}

impl<G: GenServer> Watcher<G> {
    /// Transfer `m` to the server loop. If the server is already gone the
    /// monitor is silently dropped (its queued `Down`, if any, with it) —
    /// there is no loop left to care.
    pub fn watch(&self, m: Monitor) {
        let _ = self.tx.send(Sys::Watch(m));
    }
}

/// Configure-then-start construction for servers that need more than a bare
/// [`start`]: info channels, a supervisor. Consumed by [`start`](Self::start).
///
/// ```ignore
/// let server = GenServerBuilder::new(state)
///     .with_info(events_rx)
///     .under(supervisor_pid)
///     .start();
/// ```
pub struct GenServerBuilder<G: GenServer> {
    state: G,
    infos: Vec<Receiver<G::Info>>,
    supervisor: Option<Pid>,
}

impl<G: GenServer> GenServerBuilder<G> {
    pub fn new(state: G) -> Self {
        GenServerBuilder { state, infos: Vec::new(), supervisor: None }
    }

    /// Add an out-of-band channel; messages arriving on it are dispatched to
    /// [`GenServer::handle_info`]. Repeatable; arm priority is call order
    /// (earlier = higher), and every info channel outranks the inbox.
    pub fn with_info(mut self, rx: Receiver<G::Info>) -> Self {
        self.infos.push(rx);
        self
    }

    /// Spawn the server under an explicit supervisor pid (via [`spawn_under`])
    /// so it slots into the supervision tree.
    pub fn under(mut self, supervisor: Pid) -> Self {
        self.supervisor = Some(supervisor);
        self
    }

    /// Spawn the server actor and hand back its [`GenServerRef`]. The server's
    /// lifetime is governed by its refs, not by joining, so the backing join
    /// handle is dropped.
    pub fn start(self) -> GenServerRef<G> {
        self.spawn_server()
    }

    /// Bind the server to a durable [`GenServerName`] as it starts. Switches to the
    /// fallible [`NamedGenServerBuilder::start`] (the name may already be held by a
    /// live server). Consumes the builder, carrying its `with_info` / `under`
    /// configuration through.
    pub fn named(self, name: GenServerName<G>) -> NamedGenServerBuilder<G> {
        NamedGenServerBuilder { builder: self, name: name.as_str() }
    }

    /// Private shared body behind [`start`](Self::start) and
    /// [`NamedGenServerBuilder::start`]: allocate the inbox, spawn the loop,
    /// return the ref. The named path additionally publishes the inbox sender
    /// under the name before returning.
    fn spawn_server(self) -> GenServerRef<G> {
        let (tx, rx) = channel::<Envelope<G>>();
        let GenServerBuilder { state, infos, supervisor } = self;
        let handle = match supervisor {
            Some(sup) => spawn_under(sup, move || server_loop::<G>(rx, state, infos)),
            None => spawn(move || server_loop::<G>(rx, state, infos)),
        };
        GenServerRef { tx, pid: handle.pid() }
    }
}

/// A typed, static name for a gen_server, used to address it through the
/// registry without holding a [`GenServerRef`]. Because a gen_server handles
/// multiple message types, the name is typed by the whole server (`G`) rather
/// than by a single message type. Declare it as a constant:
///
/// ```ignore
/// const COUNTER: GenServerName<Counter> = GenServerName::new("counter");
/// ```
///
/// Use [`GenServerBuilder::named`] to bind the name on start, and the free
/// functions [`call`], [`cast`], and [`whereis_server`] to address the server
/// by name. Under the hood the server's inbox sender is stored in the registry
/// keyed by name and message `TypeId`; `Envelope` is private, so only
/// `GenServerName<G>` can open that slot.
pub struct GenServerName<G> {
    name: &'static str,
    _marker: PhantomData<fn() -> G>,
}

impl<G> GenServerName<G> {
    /// Bind a static string as a server name. `const`, so names live as
    /// associated constants at call sites.
    #[inline]
    pub const fn new(name: &'static str) -> Self {
        Self { name, _marker: PhantomData }
    }

    /// The underlying registry key.
    #[inline]
    pub const fn as_str(self) -> &'static str {
        self.name
    }
}

impl<G> Copy for GenServerName<G> {}
impl<G> Clone for GenServerName<G> {
    fn clone(&self) -> Self {
        *self
    }
}

/// A [`GenServerBuilder`] that will bind a [`GenServerName`] as it starts. Reached via
/// [`GenServerBuilder::named`]; its [`start`](Self::start) is fallible because the
/// name may already be held by a live server. `with_info` / `under` stay
/// available so configuration can come before or after `named`.
pub struct NamedGenServerBuilder<G: GenServer> {
    builder: GenServerBuilder<G>,
    name: &'static str,
}

impl<G: GenServer> NamedGenServerBuilder<G> {
    /// Add an out-of-band info channel (see [`GenServerBuilder::with_info`]).
    pub fn with_info(mut self, rx: Receiver<G::Info>) -> Self {
        self.builder = self.builder.with_info(rx);
        self
    }

    /// Spawn under an explicit supervisor (see [`GenServerBuilder::under`]).
    pub fn under(mut self, supervisor: Pid) -> Self {
        self.builder = self.builder.under(supervisor);
        self
    }

    /// Spawn the server and bind its name in one step. Fallible: returns
    /// [`RegisterError::NameTaken`] if the name is already held by a different
    /// live server.
    ///
    /// The inbox sender is published under the name **from the parent side,
    /// before this returns**, so a by-name `call` / `cast` resolves the instant
    /// `start()` returns — no race with the server body. On a name clash the
    /// just-spawned server is wound down (its only ref is dropped, closing the
    /// inbox), so a failed bind leaks no actor.
    pub fn start(self) -> Result<GenServerRef<G>, RegisterError> {
        let NamedGenServerBuilder { builder, name } = self;
        let server = builder.spawn_server();
        match register_with::<Envelope<G>>(server.pid, name, server.tx.clone()) {
            Ok(()) => Ok(server),
            Err(e) => {
                drop(server); // inbox closes → loop exits gracefully
                Err(e)
            }
        }
    }
}

/// Resolve a [`GenServerName`] to a [`GenServerRef`] when you want a handle to hold or
/// pass on rather than resolve per call. Rebuilds the ref from the registry's
/// stored inbox sender; `None` if no live server holds the name.
///
/// Panics if called outside `Runtime::run()`.
pub fn whereis_server<G: GenServer>(name: GenServerName<G>) -> Option<GenServerRef<G>> {
    resolve_named_sender::<Envelope<G>>(name.as_str()).map(|(pid, tx)| GenServerRef { tx, pid })
}

/// Synchronous request-reply to the server currently registered under `name`,
/// resolving through the registry on every call (so a server restarted under
/// the same name is reached transparently). [`CallError::ServerDown`] if no live
/// server holds the name, or if it dies before replying.
///
/// Panics if called outside `Runtime::run()`.
pub fn call<G: GenServer>(name: GenServerName<G>, request: G::Call) -> Result<G::Reply, CallError> {
    match whereis_server(name) {
        Some(server) => server.call(request),
        None => Err(CallError::ServerDown),
    }
}

/// Fire-and-forget to the server registered under `name`, resolving per cast.
/// [`CastError::ServerDown`] if no live server holds the name.
///
/// Panics if called outside `Runtime::run()`.
pub fn cast<G: GenServer>(name: GenServerName<G>, request: G::Cast) -> Result<(), CastError> {
    match whereis_server(name) {
        Some(server) => server.cast(request),
        None => Err(CastError::ServerDown),
    }
}

/// Terminate the server registered under `name` and block until it is down (see
/// [`GenServerRef::shutdown`]). A no-op if no live server holds the name.
///
/// Panics if called outside `Runtime::run()`.
pub fn shutdown<G: GenServer>(name: GenServerName<G>) {
    if let Some(server) = whereis_server(name) {
        server.shutdown();
    }
}

/// Spawn `state` as a server under the current actor (via [`spawn`]). Returns a
/// [`GenServerRef`]. Shorthand for `GenServerBuilder::new(state).start()`.
pub fn start<G: GenServer>(state: G) -> GenServerRef<G> {
    GenServerBuilder::new(state).start()
}

/// Like [`start`], but spawns the server under an explicit supervisor pid (via
/// [`spawn_under`]) so it slots into the supervision tree.
pub fn start_under<G: GenServer>(supervisor: Pid, state: G) -> GenServerRef<G> {
    GenServerBuilder::new(state).under(supervisor).start()
}

fn server_loop<G: GenServer>(
    rx: Receiver<Envelope<G>>,
    state: G,
    mut infos: Vec<Receiver<G::Info>>,
) {
    // Drop guard — owns the server state and the timer registry.
    //
    // Why a guard rather than code after the loop:
    //   - `terminate()` must fire on *every* exit path: clean inbox close,
    //     a handler panic, and cooperative `request_stop`. A guard's `Drop`
    //     covers all three; code after the loop only covers the clean path.
    //   - The timer drain must run *before* `terminate()`: terminate may
    //     inspect state but must not arm new timers into a dead loop. By
    //     owning the registry here the drain and the terminate call are
    //     sequenced correctly and can never be reordered.
    //   - A debug_assert pins the post-drain invariant cheaply.
    struct Terminate<G: GenServer>(G, Arc<Mutex<TimerReg<G>>>);
    impl<G: GenServer> Drop for Terminate<G> {
        fn drop(&mut self) {
            {
                let mut reg = match self.1.lock() {
                    Ok(g) => g,
                    Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
                };
                for (_, sub) in reg.oneshots.drain() {
                    cancel_timer(sub);
                }
                for (_, p) in reg.periodics.drain() {
                    cancel_timer(p.live);
                }
                reg.rearm_tx = None;
                // A fired-but-gone send already no-ops on the closed Sys channel;
                // this guards the armed-but-unfired re-arming case specifically.
                debug_assert!(
                    reg.oneshots.is_empty() && reg.periodics.is_empty(),
                    "an armed timer survived gen_server loop exit"
                );
            }
            self.0.terminate();
        }
    }

    // Unwrap an envelope and hand it to the right handler. For calls, a failed
    // reply send means the caller went away (e.g. cancelled while parked) —
    // that is the caller's problem, not the server's.
    fn dispatch<G: GenServer>(state: &mut G, env: Envelope<G>) {
        match env {
            Envelope::Call(request, reply_tx) => {
                let reply = state.handle_call(request);
                // The caller may have gone away (e.g. cancelled while parked);
                // a failed reply send is not the server's problem.
                let _ = reply_tx.send(reply);
            }
            Envelope::Cast(request) => state.handle_cast(request),
        }
    }

    // Shared timer bookkeeping: the loop keeps one clone (to retire one-shots on
    // fire, re-arm periodics, and read the idle window); the guard holds another
    // to drain on exit; every TimerHandle the state stores holds more.
    let reg = Arc::new(Mutex::new(TimerReg::default()));
    let mut guard = Terminate(state, reg.clone());

    // The system intake arm: Watchers feed Monitors and armed timers feed fires
    // to the loop through it. The ctx (and with it the loop's own sender) drops
    // right after init — a state that cloned no Watcher/TimerHandle closes the
    // arm, the first select observes the closure, and the loop falls back to the
    // plain-inbox park.
    let (sys_tx, sys_rx) = channel::<Sys<G>>();
    // Bind the ctx so the idle window set during init can be read back, then
    // drop it — that drops the loop's own Sys sender, so a state that cloned no
    // Watcher/TimerHandle lets the arm auto-close (the unused-ctx behaviour).
    let ctx = GenServerCtx { sys_tx, reg: reg.clone(), idle: Cell::new(None) };
    guard.0.init(&ctx);
    let idle = ctx.idle.get();
    drop(ctx);

    let mut monitors: Vec<Monitor> = Vec::new();
    let mut sys_open = true;
    // The receive-timeout deadline, live only when an idle window is set. Reset
    // to `now + idle` after any dispatched message and after handle_idle fires
    // (steady detector); `None` disables the timeout entirely.
    let mut idle_deadline: Option<Instant> = idle.map(|d| Instant::now() + d);
    let reset_idle = |dl: &mut Option<Instant>| {
        if let Some(d) = idle {
            *dl = Some(Instant::now() + d);
        }
    };

    loop {
        if monitors.is_empty() && !sys_open && infos.is_empty() {
            // Fast path: no extra arms, no select overhead — park directly on
            // the inbox. Mirrors the inbox arm of the select path below; any
            // change there must be applied here too.
            match idle_deadline {
                Some(dl) => {
                    let wait = dl.saturating_duration_since(Instant::now());
                    match rx.recv_timeout(wait) {
                        Ok(env) => {
                            dispatch(&mut guard.0, env);
                            reset_idle(&mut idle_deadline);
                        }
                        // Quiet for the whole window → the idle event.
                        Err(RecvTimeoutError::Timeout) => {
                            guard.0.handle_idle();
                            reset_idle(&mut idle_deadline);
                        }
                        // All ServerRefs dropped → inbox closed → shutdown.
                        Err(RecvTimeoutError::Disconnected) => break,
                    }
                }
                None => match rx.recv() {
                    Ok(env) => dispatch(&mut guard.0, env),
                    Err(_) => break,
                },
            }
        } else {
            // Slow path: one or more extra arms live — build the arm slice and
            // select. Arm order encodes priority: downs → system → infos →
            // inbox. The slice is rebuilt each iteration because the monitor
            // and info sets shrink/grow. Mirrors the fast-path inbox park
            // above; keep them in sync.
            let nd = monitors.len();            // monitor band:  [0,      nd)
            let nw = sys_open as usize;         // system arm:    [nd,     nd+nw)
                                                // info band:     [nd+nw,  nd+nw+ni)
                                                // inbox arm:     [nd+nw+ni]
            let sel = {
                let mut arms: Vec<&dyn Selectable> =
                    Vec::with_capacity(nd + nw + infos.len() + 1);
                for m in &monitors {
                    arms.push(&m.rx);
                }
                if sys_open {
                    arms.push(&sys_rx);
                }
                for r in &infos {
                    arms.push(r);
                }
                arms.push(&rx);
                match idle_deadline {
                    Some(dl) => {
                        select_timeout(&arms, dl.saturating_duration_since(Instant::now()))
                    }
                    None => Some(select(&arms)),
                }
            };
            let i = match sel {
                Some(i) => i,
                // No arm ready for the whole window → idle, then re-arm steady.
                None => {
                    guard.0.handle_idle();
                    reset_idle(&mut idle_deadline);
                    crate::check!();
                    continue;
                }
            };
            if i < nd {
                // Monitor band: a Down retires its arm either way (one-shot)
                // or closes without delivering (defensive; shouldn't happen).
                let m = monitors.remove(i);
                if let Ok(Some(down)) = m.rx.try_recv() {
                    guard.0.handle_down(down);
                    reset_idle(&mut idle_deadline);
                }
            } else if i < nd + nw {
                match sys_rx.try_recv() {
                    // Control intake, not a dispatched message: no idle reset.
                    Ok(Some(Sys::Watch(m))) => monitors.push(m),
                    Ok(Some(Sys::Timer(id, msg))) => {
                        // The one-shot fired: retire its registry entry so the
                        // live set tracks only still-pending timers, then
                        // dispatch.
                        match reg.lock() {
                            Ok(mut g) => { g.oneshots.remove(&id); }
                            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
                        }
                        guard.0.handle_timer(msg);
                        reset_idle(&mut idle_deadline);
                    }
                    Ok(Some(Sys::Tick(id))) => {
                        // A periodic fired. Under the lock: if it is still live
                        // (cancel didn't beat us here), produce its payload and
                        // re-arm the next tick at now + every *before* dispatch,
                        // so the period is measured from fire-handling and a
                        // handler that cancels stops the just-armed instance.
                        let msg = {
                            let mut g = match reg.lock() {
                                Ok(g) => g,
                                Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
                            };
                            let r = &mut *g;
                            if let Some(p) = r.periodics.get_mut(&id) {
                                let every = p.every;
                                let msg = (p.make)();
                                let tx = match r.rearm_tx.as_ref() {
                                    Some(tx) => tx.clone(),
                                    None => panic!(
                                        "smarm: live periodic without rearm_tx (logic bug)"
                                    ),
                                };
                                p.live = send_after_to(every, tx, Sys::Tick(id));
                                Some(msg)
                            } else {
                                // Cancelled between fire and dispatch: discard.
                                None
                            }
                        };
                        if let Some(msg) = msg {
                            guard.0.handle_timer(msg);
                            reset_idle(&mut idle_deadline);
                        }
                    }
                    // Single-receiver: nothing can drain the arm between
                    // select's ready and our try_recv.
                    Ok(None) => debug_assert!(false, "ready system arm was empty"),
                    // Every Watcher/TimerHandle gone: stop selecting on the arm.
                    Err(_) => sys_open = false,
                }
            } else if i < nd + nw + infos.len() {
                // Info band.
                let j = i - nd - nw;
                match infos[j].try_recv() {
                    Ok(Some(info)) => {
                        guard.0.handle_info(info);
                        reset_idle(&mut idle_deadline);
                    }
                    Ok(None) => debug_assert!(false, "ready info arm was empty"),
                    // Senders all gone: drop the arm, keep serving. `remove`
                    // (not swap_remove) — order is priority.
                    Err(_) => {
                        infos.remove(j);
                    }
                }
            } else {
                // Inbox arm (mirrors the fast-path park above).
                match rx.try_recv() {
                    Ok(Some(env)) => {
                        dispatch(&mut guard.0, env);
                        reset_idle(&mut idle_deadline);
                    }
                    Ok(None) => debug_assert!(false, "ready inbox was empty"),
                    Err(_) => break,
                }
            }
        }
        // Observation point so a server whose arms are never empty stays
        // preemptible and cancellable.
        crate::check!();
    }
}