use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;

enum Message {
    NewJob(Job),
    Terminate,
}

pub struct ThreadPool {
    jobs: mpsc::Sender<Message>,
    threads: Vec<Worker>,
}

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>;

impl ThreadPool {
    pub fn new(thread_count: usize) -> Self {
        let (jobs, receiver) = mpsc::channel();
        let receiver = Arc::new(Mutex::new(receiver));

        let mut threads: Vec<Worker> = Vec::new();
        for id in 1..thread_count {
            threads.push(Worker::new(id, Arc::clone(&receiver)));
        }

        ThreadPool { jobs, threads }
    }

    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.jobs.send(Message::NewJob(job)).unwrap();
    }
}

impl Drop for ThreadPool {
    fn drop(&mut self) {
        for _ in &mut self.threads {
            self.jobs.send(Message::Terminate).unwrap();
        }

        for worker in &mut self.threads {
            if let Some(thread) = worker.thread.take() {
                thread.join().unwrap();
            }
        }
    }
}

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

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

            match message {
                Message::NewJob(job) => job.call_box(),
                Message::Terminate => {
                    break;
                }
            }
        });

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