#[cfg(test)]
mod tests;

use crate::fmt;
use crate::sync::{Condvar, Mutex};

/// 屏障使多个线程能够同步某些计算的开始。
///
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Barrier};
/// use std::thread;
///
/// let mut handles = Vec::with_capacity(10);
/// let barrier = Arc::new(Barrier::new(10));
/// for _ in 0..10 {
///     let c = Arc::clone(&barrier);
///     // 相同的消息将一起打印。
///     // 您将看不到任何交错。
///     handles.push(thread::spawn(move|| {
///         println!("before wait");
///         c.wait();
///         println!("after wait");
///     }));
/// }
/// // 等待其他线程完成。
/// for handle in handles {
///     handle.join().unwrap();
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Barrier {
    lock: Mutex<BarrierState>,
    cvar: Condvar,
    num_threads: usize,
}

// 双重屏障的内部状态
struct BarrierState {
    count: usize,
    generation_id: usize,
}

/// 当 [`Barrier`] 中的所有线程都汇合时，[`Barrier::wait()`] 将返回 `BarrierWaitResult`。
///
///
/// # Examples
///
/// ```
/// use std::sync::Barrier;
///
/// let barrier = Barrier::new(1);
/// let barrier_wait_result = barrier.wait();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct BarrierWaitResult(bool);

#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for Barrier {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Barrier").finish_non_exhaustive()
    }
}

impl Barrier {
    /// 创建一个新的屏障，该屏障可以阻止给定数量的线程。
    ///
    /// 屏障将阻塞调用 [`wait()`] 的 `n - 1` 个线程，然后在第 n 个线程调用 [`wait()`] 时立即唤醒所有线程。
    ///
    ///
    /// [`wait()`]: Barrier::wait
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Barrier;
    ///
    /// let barrier = Barrier::new(10);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn new(n: usize) -> Barrier {
        Barrier {
            lock: Mutex::new(BarrierState { count: 0, generation_id: 0 }),
            cvar: Condvar::new(),
            num_threads: n,
        }
    }

    /// 阻塞当前线程，直到所有线程都在此处集合为止。
    ///
    /// 所有线程集合一次后，屏障可以重新使用，并且可以连续使用。
    ///
    /// 从该函数返回时，单个 (arbitrary) 线程将接收从 [`BarrierWaitResult::is_leader()`] 返回 `true` 的 [`BarrierWaitResult`]，而所有其他线程将接收从 [`BarrierWaitResult::is_leader()`] 返回 `false` 的结果。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Barrier};
    /// use std::thread;
    ///
    /// let mut handles = Vec::with_capacity(10);
    /// let barrier = Arc::new(Barrier::new(10));
    /// for _ in 0..10 {
    ///     let c = Arc::clone(&barrier);
    ///     // 相同的消息将一起打印。
    ///     // 您将看不到任何交错。
    ///     handles.push(thread::spawn(move|| {
    ///         println!("before wait");
    ///         c.wait();
    ///         println!("after wait");
    ///     }));
    /// }
    /// // 等待其他线程完成。
    /// for handle in handles {
    ///     handle.join().unwrap();
    /// }
    /// ```
    ///
    ///
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn wait(&self) -> BarrierWaitResult {
        let mut lock = self.lock.lock().unwrap();
        let local_gen = lock.generation_id;
        lock.count += 1;
        if lock.count < self.num_threads {
            // 我们需要一个 while 循环来防止虚假唤醒。
            // https://en.wikipedia.org/wiki/Spurious_wakeup
            while local_gen == lock.generation_id {
                lock = self.cvar.wait(lock).unwrap();
            }
            BarrierWaitResult(false)
        } else {
            lock.count = 0;
            lock.generation_id = lock.generation_id.wrapping_add(1);
            self.cvar.notify_all();
            BarrierWaitResult(true)
        }
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for BarrierWaitResult {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("BarrierWaitResult").field("is_leader", &self.is_leader()).finish()
    }
}

impl BarrierWaitResult {
    /// 如果此线程是调用 [`Barrier::wait()`] 的 "leader thread"，则返回 `true`。
    ///
    /// 只有一个线程从其结果返回 `true`，所有其他线程将返回 `false`。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Barrier;
    ///
    /// let barrier = Barrier::new(1);
    /// let barrier_wait_result = barrier.wait();
    /// println!("{:?}", barrier_wait_result.is_leader());
    /// ```
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn is_leader(&self) -> bool {
        self.0
    }
}