pub mod handlers;

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

/// Result returns custom error
pub type Result<T> = std::result::Result<T, PoolCreationError>;

pub struct PoolCreationError;

impl fmt::Display for PoolCreationError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Must have at least one thread to create pool")
    }
}

/// Holds workers inside a `Vec` so we retain access to them
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Message>,
}

/// Type alias to shorten up the long type definition
type Job = Box<dyn FnOnce() + Send + 'static>;

/// A worker knows what to do based on what this enum provides
enum Message {
    NewJob(Job),
    Terminate,
}

impl ThreadPool {
    /// Create a new ThreadPool.
    ///
    /// The size is the number of threads in the pool.
    ///
    /// # Errors
    ///
    /// Returns a `PoolCreationError` if there are no worker available in the
    /// pool, so the caller can decide what to do with it
    pub fn new(size: usize) -> Result<ThreadPool> {
        if size <= 0 {
            return Err(PoolCreationError);
        }

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

        self.sender.send(Message::NewJob(job)).unwrap();
    }
}

impl Drop for ThreadPool {
    fn drop(&mut self) {
        println!("Sending terminate message to all workers.");

        for _ in &self.workers {
            self.sender.send(Message::Terminate).unwrap();
        }

        println!("Shutting down all workers.");

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

            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) => {
                    println!("Worker {} got a job; executing.", id);

                    job();
                }
                Message::Terminate => {
                    println!("Worker {} was told to terminate.", id);

                    break;
                }
            }
        });

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