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

trait FnBox {
  fn call_box(self: Box<Self>);
}

impl<F: FnOnce()> FnBox for F {
    fn call_box(self: Box<F>) {
        (*self)();
    }
}

type Job = Box<dyn FnBox + Send + 'static>;

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

enum Message {
  NewJob(Job),
  Terminate,
}

impl Worker {
    pub fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Message>>>) -> Worker {
        Worker {
            id,
            thread: Some(thread::spawn(move || {
                loop {
                    if let Ok(job) = receiver.lock().expect("unrecoverable poisened panicked thread state").recv() {
                        match job {
                            Message::NewJob(job) => {
                                println!("such busy, so worker #{}!", id);
                                job.call_box();
                            }
                            Message::Terminate => {
                                println!("such sad, so dead #{}!", id);
                                break;
                            }
                        }
                    } else {
                        // TODO respawn? (unless in terminate mode)
                        // or why would the recv fail? already terminated?
                        // thread panic not handled?
                        break;
                    }
                }
            })),
        }
    }
}

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

impl Drop for ThreadPool {
    fn drop(&mut self) {
        // Each worker will only receive Terminate once
        for _ in &mut self.workers {
            self.sender.send(Message::Terminate).unwrap();
        }

        // Each worker will join once complete
        for worker in &mut self.workers {
            println!("Shutting down worker {} ...", worker.id);

            if let Some(thread) = worker.thread.take() {
                thread.join().unwrap();
                println!("Killed {}!", worker.id);
            }
        }
    }
}

impl ThreadPool {
    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.max = size;
        ThreadPool {
            workers,
            sender,
        }
    }

    pub fn run<F: FnOnce() + Send + 'static>(&self, task: F)
    {
        let task = Box::new(task);
        self.sender.send(Message::NewJob(task)).unwrap();
    }
}