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

pub struct Threadpool {
    pub sender: Sender<Option<Box<dyn FnOnce() + Send + 'static>>>,
    workers: Vec<Worker>,
}

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

impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<Receiver<Option<Box<dyn FnOnce() + Send + 'static>>>>>,
    ) -> Worker {
        let handle = Some(thread::spawn(move || loop {
            match receiver.lock().unwrap().recv() {
                Ok(Some(job)) => {
                    println!("Worker {} got a job; executing.", id);
                    job();
                }
                Ok(None) => {
                    println!("Worker {} was told to terminate.", id);
                    break;
                }
                Err(_) => {
                    println!("Worker {} disconnected; shutting down.", id);
                    break;
                }
            }
        }));
        Worker { id, handle }
    }
}

impl Threadpool {
    pub fn new(size: usize) -> Threadpool {
        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 { sender, workers }
    }

    pub fn execute(&self, job: Box<dyn FnOnce() + Send + 'static>) {
        self.sender.send(Some(job)).unwrap();
    }
}

impl Drop for Threadpool {
    fn drop(&mut self) {
        println!("Sending termination message to all workers.");
        for _ in &self.workers {
            self.sender.send(None).unwrap();
        }
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);
            if let Some(handle) = worker.handle.take() {
                handle.join().unwrap();
            }
        }
    }
}