//! The real executor: only polls tasks that were *woken*.
//!
//! * `tasks`:      all live tasks, by id
//! * `task_queue`: ids of tasks ready to run. Wakers push onto it -- possibly
//!                 from an interrupt handler, so it is a lock-free queue and
//!                 pushing never allocates.
//! * `waker_cache`: one Waker per task, created once and reused.
//!
//! When nothing is ready, the CPU sleeps with `hlt` until the next interrupt.

use super::{Task, TaskId};
use alloc::{collections::BTreeMap, sync::Arc, task::Wake};
use core::task::{Context, Poll, Waker};
use crossbeam_queue::ArrayQueue;

pub struct Executor {
    tasks: BTreeMap<TaskId, Task>,
    task_queue: Arc<ArrayQueue<TaskId>>,
    waker_cache: BTreeMap<TaskId, Waker>,
}

impl Executor {
    pub fn new() -> Self {
        Executor {
            tasks: BTreeMap::new(),
            task_queue: Arc::new(ArrayQueue::new(100)),
            waker_cache: BTreeMap::new(),
        }
    }

    pub fn spawn(&mut self, task: Task) {
        let task_id = task.id;
        if self.tasks.insert(task.id, task).is_some() {
            panic!("task with same ID already in tasks");
        }
        self.task_queue.push(task_id).expect("queue full");
    }

    /// Run tasks forever (the kernel's main loop).
    pub fn run(&mut self) -> ! {
        loop {
            self.run_ready_tasks();
            self.sleep_if_idle();
        }
    }

    /// Run until every task has finished (used by tests).
    pub fn run_until_done(&mut self) {
        while !self.tasks.is_empty() {
            self.run_ready_tasks();
            self.sleep_if_idle();
        }
    }

    fn run_ready_tasks(&mut self) {
        // Destructure `self` so we can borrow the fields separately.
        let Self { tasks, task_queue, waker_cache } = self;

        while let Some(task_id) = task_queue.pop() {
            let task = match tasks.get_mut(&task_id) {
                Some(task) => task,
                None => continue, // task already finished (stale wake-up)
            };
            let waker = waker_cache
                .entry(task_id)
                .or_insert_with(|| TaskWaker::new(task_id, task_queue.clone()));
            let mut context = Context::from_waker(waker);
            if let Poll::Ready(()) = task.poll(&mut context) {
                tasks.remove(&task_id);
                waker_cache.remove(&task_id);
            }
        }
    }

    /// `hlt` until the next interrupt, but only if the queue is still empty.
    /// Interrupts are disabled for the check: otherwise an interrupt could
    /// wake a task *between* the check and `hlt`, and we'd sleep with work
    /// waiting. `enable_and_hlt` is `sti; hlt`, which the CPU executes
    /// atomically (interrupts are only recognised after the `hlt` starts).
    fn sleep_if_idle(&self) {
        use x86_64::instructions::interrupts::{self, enable_and_hlt};
        if crate::thread::scheduler_started() {
            // Block this thread until a waker fires (e.g. from the keyboard
            // interrupt): other threads get the CPU meanwhile.
            EXECUTOR_IDLE.wait_until(|| (!self.task_queue.is_empty()).then_some(()));
            return;
        }
        // No scheduler (some tests): just halt until the next interrupt.
        interrupts::disable();
        if self.task_queue.is_empty() {
            enable_and_hlt();
        } else {
            interrupts::enable();
        }
    }
}

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

/// Waking = pushing the task id back onto the ready queue.
struct TaskWaker {
    task_id: TaskId,
    task_queue: Arc<ArrayQueue<TaskId>>,
}

impl TaskWaker {
    fn new(task_id: TaskId, task_queue: Arc<ArrayQueue<TaskId>>) -> Waker {
        // `Wake` trait + Arc -> Waker conversion from the alloc crate.
        Waker::from(Arc::new(TaskWaker { task_id, task_queue }))
    }

    fn wake_task(&self) {
        self.task_queue.push(self.task_id).expect("task_queue full");
        EXECUTOR_IDLE.wake_all();
    }
}

/// Where an idle executor thread blocks until a task is woken.
static EXECUTOR_IDLE: crate::thread::WaitQueue = crate::thread::WaitQueue::new();

impl Wake for TaskWaker {
    fn wake(self: Arc<Self>) {
        self.wake_task();
    }
    fn wake_by_ref(self: &Arc<Self>) {
        self.wake_task();
    }
}