#[cfg(feature = "metrics")]
use std::sync::atomic::Ordering;

use std::{
    cell::{Cell, UnsafeCell},
    ptr::NonNull,
    sync::{Arc, Barrier},
    time::Duration,
};

#[cfg(feature = "metrics")]
use werkzeug::CachePadded;

use crate::{
    channel::Receiver,
    context::{Context, Message},
    heartbeat::OwnedHeartbeatReceiver,
    job::{Job2 as Job, JobQueue as JobList, SharedJob},
    queue,
    util::DropGuard,
};

pub struct WorkerThread {
    pub(crate) context: Arc<Context>,
    pub(crate) receiver: queue::Receiver<Message>,
    pub(crate) queue: UnsafeCell<JobList>,
    pub(crate) heartbeat: OwnedHeartbeatReceiver,
    pub(crate) join_count: Cell<u8>,

    #[cfg(feature = "metrics")]
    pub(crate) metrics: CachePadded<crate::metrics::WorkerMetrics>,
}

thread_local! {
    static WORKER: UnsafeCell<Option<NonNull<WorkerThread>>> = const { UnsafeCell::new(None) };
}

impl WorkerThread {
    pub fn new_in(context: Arc<Context>) -> Self {
        let heartbeat = context.heartbeats.new_heartbeat();

        Self {
            receiver: context.queue.new_receiver(),
            context,
            queue: UnsafeCell::new(JobList::new()),
            heartbeat,
            join_count: Cell::new(0),
            #[cfg(feature = "metrics")]
            metrics: CachePadded::new(crate::metrics::WorkerMetrics::default()),
        }
    }
}

impl WorkerThread {
    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace", skip_all, fields(
        worker = self.heartbeat.index(),
    )))]
    pub fn run(self: Box<Self>, barrier: Arc<Barrier>) {
        let this = Box::into_raw(self);
        unsafe {
            Self::set_current(this);
        }

        let _guard = DropGuard::new(|| unsafe {
            // SAFETY: this is only called when the thread is exiting
            Self::unset_current();
            Self::drop_in_place(this);
        });

        #[cfg(feature = "tracing")]
        tracing::trace!("WorkerThread::run: starting worker thread");

        barrier.wait();
        unsafe {
            (&*this).run_inner();
        }

        #[cfg(feature = "metrics")]
        unsafe {
            eprintln!("{:?}", (&*this).metrics);
        }

        #[cfg(feature = "tracing")]
        tracing::trace!("WorkerThread::run: worker thread finished");
    }

    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace", skip_all))]
    fn run_inner(&self) {
        loop {
            if self.context.should_exit() {
                break;
            }

            match self.receiver.recv() {
                Message::Shared(shared_job) => {
                    self.execute(shared_job);
                }
                Message::Finished(werkzeug::util::Send(ptr)) => {
                    #[cfg(feature = "tracing")]
                    tracing::error!(
                        "WorkerThread::run_inner: received finished message: {:?}",
                        ptr
                    );
                }
                Message::Exit => break,
                Message::ScopeFinished => {}
            }
        }
    }
}

impl WorkerThread {
    /// Checks if the worker thread has received a heartbeat, and if so,
    /// attempts to share a job with other workers. If a job was popped from
    /// the queue, but not shared, this function runs the job locally.
    pub(crate) fn tick(&self) {
        if self.heartbeat.take() {
            #[cfg(feature = "metrics")]
            self.metrics.num_heartbeats.fetch_add(1, Ordering::Relaxed);
            #[cfg(feature = "tracing")]
            tracing::trace!(
                "received heartbeat, thread id: {:?}",
                self.heartbeat.index()
            );

            self.heartbeat_cold();
        }
    }

    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace", skip_all))]
    fn execute(&self, job: SharedJob) {
        unsafe { SharedJob::execute(job, self) };
        // TODO: maybe tick here?
    }

    /// Attempts to share a job with other workers within the same context.
    /// returns `true` if the job was shared, `false` if it was not.
    #[cold]
    fn heartbeat_cold(&self) {
        if let Some(job) = self.pop_back() {
            #[cfg(feature = "tracing")]
            tracing::trace!("heartbeat: sharing job: {:?}", job);

            #[cfg(feature = "metrics")]
            self.metrics.num_jobs_shared.fetch_add(1, Ordering::Relaxed);

            if let Err(Message::Shared(job)) =
                self.context
                    .queue
                    .as_sender()
                    .try_anycast(Message::Shared(unsafe {
                        job.as_ref().share(Some(self.receiver.get_token()))
                    }))
            {
                unsafe {
                    SharedJob::execute(job, self);
                }
            }
        }
    }
}

impl WorkerThread {
    pub fn pop_back(&self) -> Option<NonNull<Job>> {
        unsafe { self.queue.as_mut_unchecked().pop_back() }
    }

    pub fn push_back<T>(&self, job: *const Job<T>) {
        unsafe { self.queue.as_mut_unchecked().push_back(job.cast()) }
    }
    pub fn push_front<T>(&self, job: *const Job<T>) {
        unsafe { self.queue.as_mut_unchecked().push_front(job.cast()) }
    }

    pub fn pop_front(&self) -> Option<NonNull<Job>> {
        unsafe { self.queue.as_mut_unchecked().pop_front() }
    }
}

impl WorkerThread {
    pub fn current_ref<'a>() -> Option<&'a Self> {
        unsafe { (*WORKER.with(UnsafeCell::get)).map(|ptr| ptr.as_ref()) }
    }

    unsafe fn set_current(this: *const Self) {
        WORKER.with(|cell| {
            unsafe {
                // SAFETY: this cell is only ever accessed from the current thread
                assert!(
                    (&mut *cell.get())
                        .replace(NonNull::new_unchecked(
                            this as *const WorkerThread as *mut WorkerThread,
                        ))
                        .is_none()
                );
            }
        });
    }

    unsafe fn unset_current() {
        WORKER.with(|cell| {
            unsafe {
                // SAFETY: this cell is only ever accessed from the current thread
                (&mut *cell.get()).take();
            }
        });
    }

    unsafe fn drop_in_place(this: *mut Self) {
        unsafe {
            // SAFETY: this is only called when the thread is exiting, so we can
            // safely drop the thread. We use `drop_in_place` to prevent `Box`
            // from creating a no-alias reference to the worker thread.
            core::ptr::drop_in_place(this);
            _ = Box::<core::mem::ManuallyDrop<Self>>::from_raw(this as _);
        }
    }
}

pub struct HeartbeatThread {
    ctx: Arc<Context>,
    num_workers: usize,
}

impl HeartbeatThread {
    const HEARTBEAT_INTERVAL: Duration = Duration::from_micros(100);

    pub fn new(ctx: Arc<Context>, num_workers: usize) -> Self {
        Self { ctx, num_workers }
    }

    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace", skip(self)))]
    pub fn run(self, barrier: Arc<Barrier>) {
        #[cfg(feature = "tracing")]
        tracing::trace!("new heartbeat thread {:?}", std::thread::current());
        barrier.wait();

        let mut i = 0;
        loop {
            let sleep_for = {
                // loop {
                //     if self.ctx.should_exit() || self.ctx.queue.num_receivers() != self.num_workers
                //     {
                //         break;
                //     }

                //     self.ctx.heartbeat.park();
                // }
                if self.ctx.should_exit() {
                    break;
                }

                self.ctx.heartbeats.notify_nth(i);
                let num_heartbeats = self.ctx.heartbeats.len();

                if i >= num_heartbeats {
                    i = 0;
                } else {
                    i += 1;
                }

                Self::HEARTBEAT_INTERVAL.checked_div(num_heartbeats as u32)
            };

            if let Some(duration) = sleep_for {
                std::thread::sleep(duration);
            }
        }
    }
}

impl WorkerThread {
    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace", skip(self)))]
    pub fn wait_until_shared_job<T: Send>(&self, job: &Job<T>) -> std::thread::Result<T> {
        loop {
            match self.receiver.recv() {
                Message::Shared(shared_job) => unsafe {
                    SharedJob::execute(shared_job, self);
                },
                Message::Finished(send) => {
                    break unsafe { *Box::from_non_null(send.0.cast()) };
                }
                Message::Exit | Message::ScopeFinished => {}
            }
        }
    }

    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace", skip_all))]
    pub fn wait_until_recv<T: Send>(&self) -> std::thread::Result<T> {
        loop {
            match self.receiver.recv() {
                Message::Shared(shared_job) => unsafe {
                    SharedJob::execute(shared_job, self);
                },
                Message::Finished(send) => break unsafe { *Box::from_non_null(send.0.cast()) },
                Message::Exit | Message::ScopeFinished => {}
            }
        }
    }
}