use std::sync::Arc;
use std::sync::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<FnBox + Send + 'static>;

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

impl ThreadPool {
    pub fn new(size: usize) -> Result<ThreadPool, &'static str> {
        if size <= 0 {
            return Err("failed to create pool");
        }

        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))?);
        }

        Ok(ThreadPool {
            workers,
            sender,
        })
    }

    pub fn spawn<F, T>(f: F) -> thread::JoinHandle<T>
        where
            F: FnOnce() -> T + Send + 'static,
            T: Send + 'static
    {
        thread::spawn(f)
    }

    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: thread::JoinHandle<()>,
}

impl Worker {
    pub fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>)
        -> Result<Worker, &'static str>
    {
        let thread = thread::spawn(move || { 
            loop {
                let job = receiver
                    .lock().expect("lock poisoned!")
                    .recv().unwrap();
                println!("worker {} got job; executing...", id);

                job.call_box();
            }
        });

        Ok(Worker {
            id,
            thread,
        })
    }
}