//! Small and simple ThreadPool implementation from the rust book.
//!
//! Provides an abstraction to manage multiple threads.
//! Using this abstraction will give you following advantages:
//! - Customizable pool size
//! - Graceful shutdown of the threads
use std::error::Error;
use std::fmt;
use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;

/// Error types
#[derive(Debug)]
pub enum ThreadPoolError {
    PoolCreationError,
}

impl fmt::Display for ThreadPoolError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            ThreadPoolError::PoolCreationError => f.write_str("ThreadPoolCreationError"),
        }
    }
}

impl Error for ThreadPoolError {
    fn description(&self) -> &str {
        match *self {
            ThreadPoolError::PoolCreationError => "Could not create a thread pool",
        }
    }
}

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

enum Message {
    NewJob(Job),
    Terminate,
}

/// The thread pool containing all workers.
///
/// # Examples
/// ```
/// use poolth::ThreadPool;
///
/// //creates a thread pool holding 4 threads.
/// let pool = ThreadPool::new(4).unwrap();
/// pool.execute(println("Hello World"))
/// ```
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Message>,
}

impl ThreadPool {
    /// Construct a new ThreadPool using a custom size.
    ///
    /// # Examples
    /// ```
    /// use poolth::ThreadPool;
    ///
    /// //creates a thread pool holding 4 threads.
    /// let pool = ThreadPool::new(4).unwrap();
    /// ```
    ///
    /// # Errors
    /// Creating a new ThreadPool will return an [ThreadPoolError::PoolCreationError] if the size <= 0.
    /// ```
    /// use poolth::ThreadPool;
    ///
    /// // panics due the `unwrap()` call
    /// let pool = ThreadPool::new(0).unwrap();
    /// // => thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: PoolCreationError'
    /// ```
    ///
    pub fn new(size: usize) -> Result<ThreadPool, ThreadPoolError> {
        if size <= 0 {
            return Err(ThreadPoolError::PoolCreationError);
        }

        let (sender, receiver) = mpsc::channel();
        let receiver = Arc::new(Mutex::new(receiver));
        let mut workers: Vec<Worker> = Vec::with_capacity(size);
        for id in 0..size {
            workers.push(Worker::new(id, Arc::clone(&receiver)))
        }
        Ok(ThreadPool { workers, sender })
    }
    /// Run a closure in a worker of the pool.
    /// # Examples
    /// ```
    /// use poolth::ThreadPool;
    ///
    /// let pool = ThreadPool::new(1).unwrap();
    /// pool.execute(|| println!("Hello World"))
    /// ```
    /// Output: `"Hello World"`
    ///
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(Message::NewJob(job)).unwrap();
    }
}

/// For a graceful shutdown of the thread pool.
impl Drop for ThreadPool {
    /// Sends a terminate message and wait until the worker is done.
    fn drop(&mut self) {
        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),
        }
    }
}