rstubs/threading/
sema.rs

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use arraydeque::ArrayDeque;

use super::{scheduler::APPS, Scheduler};

/// Semaphore used for the synchronization of threads.
///
/// Other than the spinlock, this semaphore can put threads to sleep and wake them up.
#[derive(Debug)]
pub struct Semaphore {
    counter: usize,
    waiting: ArrayDeque<usize, APPS>,
}

impl Semaphore {
    // Initialize the `counter` with provided value
    pub const fn new(counter: usize) -> Self {
        Self {
            counter,
            waiting: ArrayDeque::new(),
        }
    }
    /// Get the current value of the counter
    pub fn count(&self) -> usize {
        self.counter
    }
    /// Wait for access to the critical area
    pub fn wait(&mut self, scheduler: &mut Scheduler) {
        if let Some(c) = self.counter.checked_sub(1) {
            self.counter = c;
        } else {
            let tid = scheduler.active().expect("No running threads");
            self.waiting.push_back(tid).unwrap();
            scheduler.resume(false);
        }
    }
    /// Leave the critical area
    pub fn signal(&mut self, scheduler: &mut Scheduler) {
        if let Some(thread) = self.waiting.pop_front() {
            scheduler.ready(thread);
        } else {
            self.counter += 1;
        }
    }
}