webbington 0.1.0

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

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

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 {
            workers,
            sender: Some(sender),
        }
    }

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

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

impl Drop for ThreadPool {
    fn drop(&mut self) {
        drop(self.sender.take());

        for worker in &mut self.workers.drain(..) {
            println!("Shutting down worker {}", worker.id);

            worker.thread.join().unwrap();
        }
    }
}

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

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

impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        Worker {
            id,
            //For this basic implementation, this is fine.
            //But thread::spawn panics if
            //the OS can't create a thread due to system resource
            //issues
            //We should preferably use std::thread::builder
            thread: thread::spawn(move || loop {
                let message = receiver.lock().unwrap().recv();

                match message {
                    Ok(job) => {
                        println!("Worker {id} got a job; executing.");
                        job();
                    }
                    Err(_) => {
                        println!("Worker {id} disconnected; shutting down.");
                        break;
                    }
                };
            }),
        }
    }
}