//! Asynchronous signals: a word of bits, a FIFO of waiters and at most one
//! bound thread. See docs/ipc.md.

use types::{
    badge::Badge,
    generation::Generation,
    id::{NotificationId, ProcessId, ThreadId},
};

use crate::{endpoint::wait_queue::WaitQueue, generations::SlotGenerations};

#[cfg(test)]
#[path = "tests.rs"]
mod tests;

#[cfg(kani)]
#[path = "proofs.rs"]
mod proofs;

pub const MAX_NOTIFICATIONS: usize = 128;

/// What the kernel must do after a signal.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SignalEffect {
    /// Wake this blocked `Wait`er with these bits.
    WakeWaiter(ThreadId, usize),
    /// Wake the bound thread out of its `Receive` with these bits.
    WakeBound(ThreadId, usize),
    /// Nobody to wake; the bits were stored.
    Stored,
}

pub struct Notification {
    id: NotificationId,
    owner: ProcessId,
    bits: usize,
    waiters: WaitQueue,
    bound: Option<ThreadId>,
}

impl Notification {
    pub fn new(id: NotificationId, owner: ProcessId) -> Self {
        Self {
            id,
            owner,
            bits: 0,
            waiters: WaitQueue::new(),
            bound: None,
        }
    }

    pub fn id(&self) -> NotificationId {
        self.id
    }

    pub fn owner(&self) -> ProcessId {
        self.owner
    }

    pub fn bound(&self) -> Option<ThreadId> {
        self.bound
    }

    /// Delivers a signal. `bound_is_receiving` says whether the bound
    /// thread is currently blocked in `Receive` and can be woken.
    pub fn signal(&mut self, badge: Badge, bound_is_receiving: bool) -> SignalEffect {
        let bits = if badge.is_none() { 1 } else { badge.raw() };
        if let Some(waiter) = self.waiters.pop() {
            return SignalEffect::WakeWaiter(waiter, bits);
        }
        if let (Some(bound), true) = (self.bound, bound_is_receiving) {
            return SignalEffect::WakeBound(bound, bits);
        }
        self.bits |= bits;
        SignalEffect::Stored
    }

    /// Returns and clears the pending bits.
    pub fn take_bits(&mut self) -> usize {
        core::mem::take(&mut self.bits)
    }

    pub fn has_bits(&self) -> bool {
        self.bits != 0
    }

    pub fn is_waiter(&self, thread: ThreadId) -> bool {
        self.waiters.contains(thread)
    }

    pub fn add_waiter(&mut self, thread: ThreadId) -> bool {
        self.waiters.push(thread).is_some()
    }

    pub fn bind(&mut self, thread: ThreadId) -> bool {
        if self.bound.is_some() {
            return false;
        }
        self.bound = Some(thread);
        true
    }

    pub fn unbind(&mut self, thread: ThreadId) {
        if self.bound == Some(thread) {
            self.bound = None;
        }
    }

    /// Forgets `thread` as waiter and as bound thread.
    pub fn remove_thread(&mut self, thread: ThreadId) {
        self.waiters.remove_thread(thread);
        self.unbind(thread);
    }

    /// Drains every waiter, e.g. when the notification is destroyed.
    pub fn pop_waiter(&mut self) -> Option<ThreadId> {
        self.waiters.pop()
    }
}

pub struct NotificationTable {
    slots: [Option<Notification>; MAX_NOTIFICATIONS],
    generations: SlotGenerations<MAX_NOTIFICATIONS>,
}

impl NotificationTable {
    pub fn new() -> Self {
        Self {
            slots: [const { None }; MAX_NOTIFICATIONS],
            generations: SlotGenerations::new(),
        }
    }

    pub fn create(&mut self, owner: ProcessId) -> Option<NotificationId> {
        let idx = (0..MAX_NOTIFICATIONS)
            .find(|&i| self.slots[i].is_none() && self.generations.is_usable(i))?;
        let id = NotificationId::from_raw(idx as u16);
        self.slots[idx] = Some(Notification::new(id, owner));
        Some(id)
    }

    pub fn get(&self, id: NotificationId) -> Option<&Notification> {
        self.slots.get(id.raw() as usize)?.as_ref()
    }

    pub fn get_mut(&mut self, id: NotificationId) -> Option<&mut Notification> {
        self.slots.get_mut(id.raw() as usize)?.as_mut()
    }

    pub fn generation(&self, id: NotificationId) -> Option<Generation> {
        self.get(id)?;
        self.generations.current(id.raw() as usize)
    }

    pub fn revoke(&mut self, id: NotificationId) {
        if self.get(id).is_some() {
            self.generations.advance(id.raw() as usize);
        }
    }

    pub fn destroy(&mut self, id: NotificationId) -> Option<Notification> {
        let n = self.slots.get_mut(id.raw() as usize)?.take()?;
        self.generations.advance(id.raw() as usize);
        Some(n)
    }

    pub fn ids_owned_by(&self, owner: ProcessId) -> impl Iterator<Item = NotificationId> + '_ {
        self.slots
            .iter()
            .flatten()
            .filter(move |n| n.owner == owner)
            .map(|n| n.id)
    }

    pub fn remove_thread_from_all(&mut self, thread: ThreadId) {
        for n in self.slots.iter_mut().flatten() {
            n.remove_thread(thread);
        }
    }
}

impl Default for NotificationTable {
    fn default() -> Self {
        Self::new()
    }
}