use std::{
    sync::{mpsc, Arc, Mutex},
    thread,
};

pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}

type Job = Box<dyn FnOnce() + Send + 'static>;

impl ThreadPool {
    /// 创建出一个新的 ThreadPool。
    ///
    /// 其中的 size 为线程池中线程的数目。
    ///
    /// # 终止运行
    ///
    /// 这个 `new` 函数将在 size 为零时终止运行。
    pub fn new(size: usize) -> ThreadPool {
        assert! (size > 0);

        let (sender, receiver) = mpsc::channel();

        let receiver = Arc::new(Mutex::new(receiver));

        let mut workers = Vec::with_capacity(size);

        for id in 0..size {
            workers.push(Worker::new(id, Arc::clone(&receiver)));
        }

        ThreadPool { workers, sender }
    }

    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);

        self.sender.send(job).unwrap();
    }
}

struct Worker {
    id: usize,
    thread: Option<thread::JoinHandle<()>>,
}

impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();

            println! ("Worker {id} 获取到一项作业；执行中。");

            job();
        });

        Worker {
            id,
            thread: Some(thread),
        }
    }
}

impl Drop for ThreadPool {
    fn drop(&mut self) {
        for worker in &mut self.workers {
            println! ("关闭 worker {}", worker.id);

            if let Some(thread) = worker.thread.take() {
                thread.join().unwrap();
            }
        }
    }
}